Here we will review briefly those parts of the animal body that provide the life-support system for the parts we are really interested in - the things we can sell for lots of money. But the bits and pieces we remove in the abattoir to produce a dressed carcass are important. Not much gets thrown away, and the animal by-product industry is also a vital part of the overall economic system of animal agriculture and must be included in any consideration of animal growth or meat science. In many countries, the by-products of animal production and slaughtering are an important source of fuel and fertilizers, and a source of materials for clothing, bedding, and building (McDowell, 1977). Abattoir technology includes all the methods used to produce a dressed carcass from a live animal and is an important subject with regard to animal welfare. From a scientific viewpoint as well, it is not possible to ignore all other body systems except those that contribute directly to the commercial carcass.

  11. References

Some descriptive terms

A number of anatomical terms are needed to describe the relative positions of structures within the body.

Anterior = towards the head

Posterior = towards the tail

Dorsal = towards the upper part or back of the standing animal

Ventral = towards the lower part or belly of the standing animal

Medial = towards the midline plane that separates right and left sides of the body

Lateral = towards the sides of a standing animal

Proximal = towards the body in a limb of the animal

Distal = away from the body in a limb of the animal

The names for different types of farm animals also may be unfamiliar to some readers. The adjectives that relate to cattle, sheep and pigs are bovine, ovine and porcine, respectively. The first of these may be used elliptically so that bovine may stand for bovine animal. For cattle, sheep, pigs and poultry, the sire or father is called a bull, a ram, a boar or a cock (tom in turkeys), respectively, while the dam or mother is called a cow, a ewe, a sow or a hen, respectively. A heifer is an immature female bovine, and a gilt is an immature female pig. A hogget is a yearling sheep. The neonates or new-born of cattle, sheep and pigs are called calves, lambs or piglets, respectively. For pigs, the process of birth or parturition is called farrowing. Newly hatched chickens, turkeys, ducks and geese are called chicks, poults, ducklings or goslings, respectively. For cattle, sheep, pigs and poultry, a castrated male is called a steer, a wether, a barrow or a caponxv , respectively.


The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus. Associated with the alimentary tract are the following accessory organs; teeth, tongue, salivary glands, liver and pancreas.

Mouth, pharynx and esophagus

The jaw and tongue muscles are an important source of low grade meat from cattle, pigs and lambs. The mouth is lined by a slippery mucous membrane which, in cattle and sheep (ruminants), has posteriorly directed tags of tissue known as papillae. Bovine papillae are stiffened by a core of keratin in their axis (Steflik et al., 1983). Lips, cheeks and teeth are, of course, absent in poultry. Whereas mammals have a secondary palate that separates the mouth from the nasal cavity, the nostrils of poultry open directly into the roof of the mouth. Thus, when drinking, a bird must use its long neck to keep its head in a horizontal position.

After the mouth, the alimentary tract leads to the pharynx. The pharynx is a complicated junction because the posterior nares from the nasal cavity open into it, as well as the eustachian tubes balancing the air pressure behind the ear drums, the larynx which tops the windpipe from the lungs and, finally, the esophagus which continues the alimentary canal. When a bolus of food is swallowed, the pharyngeal muscles contract to force it down the esophagus.

The esophagus is a long muscular tube that runs to the stomach. It is located dorsally to the trachea so that it appears behind the trachea when the throat is opened ventrally in the abattoir. At this point in the slaughter of ruminants it is desirable to tie off the esophagus to minimize the spread of ruminal contents onto the carcass. In the meat trade, the esophagus is known as the gullet or weasand. Weasands from beef and lamb carcasses may be used as sausage casings after they have been cleaned and scraped.

In poultry, just before the esophagus enters the thoracic cavity, there is a large sack-like expansion on the right side known as the crop. The crop is a temporary storage area for feed. The muscular wall of the mammalian esophagus starts with two oblique or spiral layers that then develop into inner circular, and outer longitudinal layers farther down. In ruminants, all the muscle tissue of the esophagus is striated. Smooth muscle replaces striated muscle at the level of the diaphragm in pigs. Smooth muscle occurs along the remainder of the alimentary tract and in other organs such as the uterus. Microscopically, smooth muscle is formed from thick layers of elongated cells, each with a single nucleus. The contractile elements of smooth muscle cells do not form microscopically striated fibrils as they do in heart and skeletal muscles.


The stomach differs in structure between pigs, ruminants, and poultry. Pigs have a relatively simple, single-chambered stomach (monogastric). Cattle and sheep have three additional chambers before the true stomach. Poultry have a second chamber after the true stomach.

This is the stomach of a pig on the floor of our abattoir, with some of the small intestines showing in the top right corner. In pigs, the entrance of the esophagus into the stomach is controlled by a sphincter. This region is called the cardia (not to be confused with the cardiac gland region farther into the stomach). The esophageal region of the stomach (Figure 1-1) receives incoming food and is lined by stratified squamous epithelium. An epithelium is a sheet of cells; squamous cells are flattened in shape; stratified tissues have more than one layer of cells. In the adjacent cardiac gland region, the epithelium is supplemented by simple and by compound tubular glands which produce mucus to keep the food sliding through. Tubular glands are formed from a layer of cells rolled into a tube. Each cell secretes its products into the lumen of the tube which then opens into the stomach. Compound tubular glands are formed by branching tubes.

This shows the fundic gland region of the porcine stomach, with the lumen of the stomach towards the top of the window. Simple (unbranched) tubular glands open into pits in the stomach wall.

.This diagram shows a greatly simplified view of one of these glands. The rings at the bottom are where the tubular part of the gland has coiled around like a hosepipe and been cut into rings in the plane of sectioning of the diagram. Cells in the neckx or towards the opening of these fundic glands produce mucus. Parietal cells in the body of the gland produce hydrochloric acid. Other cells called chief or zymogen cells produce pepsinogen which is split by hydrochloric acid to release the digestive enzyme pepsin. The zymogen cells of milk-fed young animals produce rennin which initiates the digestion of milk. The pyloric glands are deeper and more branched, and they produce a small amount of protease and a lot of mucus.

In cattle and sheep, instead of opening directly into a glandular stomach where digestion begins , the esophagus leads to a series of three extra compartments, the rumen, the reticulum and the omasum. These compartments are lined with stratified squamous epithelium. In young lambs and calves that are still drinking milk, the rumen and reticulum may be by-passed. The presence of the milk is detected by sensory nerve endings in the mouth and pharynx. Reflex activity brings heavy muscular folds in the walls of the rumen and reticulum together forming an esophageal groove that leads directly from the cardia to the omasum. The rumen or paunch is a very large muscular bag on the left side of the body, extending from the diaphragm back to the pelvis. The smooth muscle of the rumen wall consists of two layers; a superficial layer from anterior to posterior, and an inner layer running transversely to form muscular pillars. The reticulum is lined by thin, wall-like ridges arranged in a honeycomb pattern. The reticulum is posterior to the heart and diaphragm (Figure 1-3). The rumen and reticulum contain countless microorganisms whose metabolic activity greatly enhances the nutritive value of typical ruminant feed.

The omasum is almost spherical in shape and is filled with muscular plates hanging from the dorsal roof. These plates or laminae are studded with short, blunt papillae whose function is to grind roughage. The trade name of the omasum is the manyplies or book-bag. The true glandular stomach or abomasum is located ventrally to the omasum (Figure 1-4). The epithelium of the abomasum is glandular with many mucus cells. Prenatally, the glands increase in number rapidly towards the end of gestation and are mature at birth (Asari et al., 1985). In a typical lean beef steer, the emptied weight of the rumen, reticulum, omasum and abomasum comprises about 2.5% of the live weight. The growth of the rumen and reticulum in calves is very rapid but the abomasum grows more slowly (Godfrey, 1961). The gut fill is extremely variable, but is often around 15% of the live weight.

A number of different types of animals have the ability to digest cellulose with the help of symbiotic bacteria and ciliates in modified parts of the alimentary canal, but the ruminant system has a number of superior features that account for its great efficiency. Chewing the cud (the repeated regurgition and mastication of feed) would not be possible unless the main fermentation chamber, the rumen, was situated before the true stomach or abomasum. Thus, ruminants are able to achieve a very high efficiency of feed grinding, unlike the horse in whose feces may be seen quite large particles of intact plant material. Another advantage of the ruminant system is that a long length of intestine is available for absorption after the point at which fermentation occurs.

Rumen microorganisms themselves are very effective - they synthesize protein from low-grade nitrogenous materials such as ammonium salts and urea added to the feed. Equally important, they utilize sulfate to produce the essential amino acids cysteine and methionine, and they also synthesize B group vitamins, particularly vitamin B12. Rumen microorganisms obtain their own energy anaerobically with only a relatively low energy yield. Thus, the ruminant is able to take the residual energy from products of fermentation such as acetic, propionic, and butyric acids. The acidity of these substances is buffered by sodium bicarbonate from the saliva.

This shows how, in poultry, the stomach is divided into two chambers (Figure 1-5). As is often the case, there is a fair amount of fat or adipose tissue covering both chambers. The first chamber, the proventriculus or glandular stomach seen on the left in the color frame, secretes pepsin and hydrochloric acid. The second chamber, the gizzard seen on the right in the color frame, is thick and muscular with a horny internal epithelium and a high collagen content (Gabella, 1985). In the color frame, the collagen can be seen as a blue sheen, radiating across the gizzard. The gizzard grinds the feed and mixes it with the enzyme mixture from the proventriculus. This is the reverse of the sequence found in the omasum and abomasum of ruminants where grinding of the feed takes place before it is exposed to the enzymes from the true stomach.

Small intestine

No mistaking this lot - a pile of small intestines from the pig. In beef animals, slightly over 2% of the live weight is from the emptied weight of the intestines. The small intestine is composed of three regions, the duodenum, the jejunum and the ileum. The duodenum receives the hepatic and pancreatic ducts and is lined by a simple epithelium of columnar cells together with numerous goblet cells that produce mucus. The surface area of the epithelium is greatly expanded by two structural features. Firstly, individual epithelial cells have a brush border facing into the lumen of the duodenum. The brush border forms a surface like a carpet with short bristles, but this is only just visible by light microscopy and is best seen by electron microscopy. Secondly, the epithelial surface area is expanded by large numbers of finger-like villi projecting into the lumen.

Villi are quite large structures (approximately 0.5 to 1 mm long and 0.2 mm thick) covered with columnar cells, and are seen here like fingers pointing towards the lumen of the small intestine towards the top of the window.. In the axis of each villus is a tubular lymph vessel called a lacteal. The intestinal glands of Lieberkuhn are tightly packed glands with a simple tubular structure. Their main products are (1) mucus; (2) enzymes that attack peptides, fats and carbohydrates; and (3) enterokinase that activates trypsinogen from the pancreas to produce the digestive enzyme trypsin. The duodenal or Brunner's glands are tubuloalveolar glands (composed of tubes and thin walled vesicles) that produce mucus to protect the duodenal mucosa. In the intestinal wall are nodules or aggregations of lymphatic cells called Peyer's patches.

Large intestine

The mammalian large intestine consists of the caecum and the colon. A caecum is a sac that opens into the alimentary tract.

This is the caecum of a pig.

The colon is divided into ascending, transverse and descending parts, and it terminates at the rectum and anus. Poultry have two caeca (Figure 1-5) just before the rectum. In poultry, but not in cattle, sheep or pigs, the inner surface area of the large intestine is expanded by villi.

In poultry, the equivalent aperture to the anus is part of a compound structure called the cloaca. The cloaca is divided into three regions but these are difficult to distinguish. The rectum enters the cloaca at the coprodeum, the urinary and genital ducts enter at the urodeum and the opening to the exterior is called the proctodeum. Dorsal to the proctodeum is a region of lymphoidal tissue called the bursa of Fabricius.


Some parts of the alimentary canal have a considerable commercial value as natural casings. After extensive cleaning and preparation, they are used to contain different types of sausages and processed meat products. When first taken from the animal and emptied, natural casings typically have five layers. From lumen to exterior these are; (1) the mucosa, composed of epithelial, glandular and vascular components; (2) the submucosa composed of connective tissues that strengthen the gut wall; (3) a circular layer of smooth muscle cells; (4) a longitudinal layer of smooth muscle cells; and (5), an irregular layer of visceral fat covering the outside. The fat is removed manually and by machine brushing. The intestinal contents are squeezed out and washed away in a process called stripping. Finally, the layers of muscle and mucosa are removed as the intestine passes between a pair of rollers in a process called sliming. This leaves the strong connective tissues of the submucosa as the sausage casing.

The commercial properties of casings originate from the high collagen content of the submucosa, together with smaller amounts of elastin. Casings often are turned inside-out to faciliate processing. Clean casings are preserved with dry sodium chloride prior to sale. With approximate metric lengths given in parentheses, the commonly used beef casings are the weasand (0.6 m), rounds from the small intestine (32 m), the bung or caecum (1.8 m), and middles from the large intestine (8 m). Beef casing are tough and strong, and are usually removed before consumption of the product - like skinning a slice of salami. The commonly used hog casings are rounds or small casings from the small intestine (18 m), the cap from the caecum (0.4 m), middles from the middle part of the large intestine (1 m), and the bung from the terminal end of the large intestine (1 m). Being intermediate in tenderness, they either may be eaten or removed before conumption of the product. The small intestine from sheep provides 27 m of tender casing that gives natural-casing wieners their "snap".


The embryological formation of teeth starts by the cooperative action of two types of tissue. The dental lamina tucks downwards from the surface epithelium of the mouth to meet a dental papilla growing towards the surface. Tooth enamel is formed by the ameloblast cells of an enamel organ formed from the dental lamina. Dentine is formed by the outer cells of the dental papilla, and pulp is formed by the inner cells of the dental papilla. Cementum is a type of modified bone that develops around the roots of a tooth.

Brachydont teeth have a simple structure and do not grow very tall. The crown of a brachydont tooth is made of enamel, the inner core is made of dentine, and the root is covered by cementum. All the teeth of humans and pigs, and the incisor teeth of cattle and sheep are of this type. The premolar and molar teeth of pigs have a grinding surface covered by rounded bumps or tubercles; this is called a bunodont type of tooth (Figure 1-6). Bunodont teeth may be used for grinding and crushing a variety of feeds. However, they would soon be worn away by the constant grinding of tough fibrous feeds such as those consumed by foraging ruminants.

Ruminants have a special type of tooth distinguished by a greater height (hypsodont) and complex curved ridges of enamel (selenodont). Hypsodont teeth are long-lasting because they develop a greater depth to be worn down. Selenodont ridges are composed of alternating layers of enamel, dentine and cementum, and they exert a powerful grinding action when top and bottom teeth are moved horizontally against each other.

Dental formulae may be used to describe the patterns of teeth in meat animals. The four types of teeth are indicated by a letter notation; I for incisors or biting teeth, C for canine or tearing teeth, P for premolars or anterior grinding teeth, and M for molars or posterior grinding teeth. The numerator and denominator of a fraction are used to indicate upper and lower numbers of teeth, respectively. Left and right sides of the jaws are not written separately but are indicated by the initial factor, x2. A prefix, D, is used to denote deciduous teeth that are present in the young animal but replaced in the older animal.

Calf and lamb = 2 x (DI 0/4, DC 0/0, DP 3/3)

Mature cattle and sheep = 2 x (I 0/4, C 0/0, P 3/3, M 3/3)

Young pig = 2 x (DI 3/3, DC 1/1, DP 4/4)

Mature pig = 2 x (I 3/3, C 1/1, P 4/4, M 3/3)

The transition from deciduous to permanent dentition follows a rather complex pattern between 1.5 to 4 years in cattle, 0.25 to 4 years in sheep, and 8 to 20 months in pigs. Most commercially reared meat animals will, therefore, be at an intermediate stage between deciduous and permanent dentition when slaughtered.

The fate of the missing canine teeth in ruminants is quite interesting. The teeth of the upper jaw are inserted into two bones, the premaxilla and the maxilla. Many anatomists define the upper canine tooth as the tooth that is immediately posterior to the suture between the premaxilla and the maxilla. The lower canine is then defined as the tooth that articulates immediately anterior to the upper canine. Thus, the fourth incisor in the lower jaw may be claimed as a canine tooth (Andrews, 1981). In embryonic ruminants, the control system that causes the most anterior three pairs of teeth to become shaped as incisors appears to spread posteriorly and to take control over the developing canine tooth (Osborn, 1978). Although poultry do not have any teeth, the genetic information for tooth formation may still be present in an unexpressed form. When grafted onto jaw tissue from mice, the dental epithelium from chicks may develop ameloblasts that may synthesize enamel (Kollar and Fisher, 1980).

Salivary glands

This shows the histological structure of a very small part of a salivary gland. Salivary glands are compound tubuloalveolar, which simply means they are formed from tubes and balloons. The two things that look like doughnuts at the top are two sections through ducts or tubes. The rest of the picture is filled with pale alveoli, the balloons where the saliva is produced. The nuclei (pale blue) of the salivary cells lining the alveoli can be seen.

Salivary glands have a yellowish color and occur at three major locations. The sublingual glands are located under the tongue and between the lower jaw bones, and they have a multiple duct system that drains saliva into the mouth. The submaxillary glands are located at the angle of the lower jaw and have large ducts that open onto the floor of the mouth, beneath the tip of the tongue. Beneath each ear, is a parotid salivary gland with a duct that opens into the mouth, near to the molar teeth. The salivary glands of ruminants are extremely productive since they must produce much of the fluid with which the feed is mixed to form a slurry. The salivary glands of a steer, for example, might secrete well over 100 liters of saliva per day.


The liver is a large organ, about 1.5% of the live weight in beef cattle. In mammals, it is located in the anterior part of the visceral cavity, just posterior to the diaphragm.

As you may be able to see in this pig liver, the pig has four equally large lobes plus a small caudate lobe on the right side. Apart from a small caudate lobe on the right side, the bovine liver is not subdivided into lobes. The liver in sheep is similar to that in the bovine, but there is something of a fissure in the main lobe.

The liver stores and processes newly digested nutrients that are brought to the liver by the blood vessels of the hepatic portal system. The liver receives oxygenated blood for its extensive metabolic activities from the hepatic artery. Processed blood is returned via the hepatic vein to the general circulation. The liver also produces bile which is emptied into the intestine to aid in the digestion of fats. Histologically, the liver is divided into lobules (Figure 1-7).

This is part of a liver lobule, and you may see part of one lobule filling the lower part of the frame, so that the central vein is the space just under half way down, slightly to the right.

Here we are within another liver lobule, so we cannot see the edges of the lobule, only the central vein. This example shows how the liver cells or hepatocytes tend to form plates or layers, with spaces or sinusoids between them.

Livers are condemned if they are infected by trematode flukes such as Fasciola hepatica in ruminants, or by nematodes such as Ascaris suum in pigs.

Here is the liver of a hen, with yellow yolks of eggs being formed at the bottom of the window.


The pancreas is a pale yellow gland located between the stomach and the small intestine in mammals, and in a loop of the duodenum in poultry. It has one or two ducts that convey pancreatic juice to the duodenum. The external secretions of the pancreas are controlled by the nervous sytem (vagus nerve) and the endocrine system (the hormone secretin from the duodenum). The three major constituents of the pancreatic juice are trypsin (for the hydrolysis of proteins when in conjunction with enterokinase present in the small intestine), amylase (for initial digestion of starch), and lipase (for the digestion of fats).


Heart and Blood Vessels

The right ventricle (Figure 1-8) pumps blood into the pulmonary arteries and then to the lungs. Oxygenated blood returns to the left atrium in the pulmonary veins, the atrium fills the left ventricle, and oxygenated blood is then pumped through the aorta to the body tissues. The aorta branches to form the major arteries. These branch again many times and eventually give rise to arterioles and, finally, to capillaries. Blood is collected from the body tissues by the venous system, and eventually returns to the right atrium via the anterior vena cava and the posterior vena cava for another cycle through the lungs. Thus, relative to other arteries, the pulmonary artery is unusual because it contains de-oxygenated blood. And, relative to other veins, the pulmonary vein is unusual because it contains oxygenated blood.

In the arterial system of meat animals, the aorta bends to the left side of the body (Figure 1-9-1), and then runs posteriorly in the midline, ventral to the vertebral column. The right forelimb is supplied from the right brachial (Figure 1-9-2) which, like the common carotids (Figure 1-9-3) to the head, branches from the brachiocephalic trunk (Figure 1-9-4). The left brachial (Figure 1-9-5) to the left forelimb originates directly from the aorta. As it passes under the vertebral column in the ribcage, the aorta gives rise to a series of small intercostal arteries (Figure 1-9-6) before the first main branch to the viscera, the coeliac (Figure 1-9-7). In cattle, the coeliac artery has five main branches; (1) the hepatic to the liver, pancreas and nearby structures, (2) left and (3) right ruminals to the rumen, (4) an omaso-abomasal artery to the omasum and abomasum, and (5) the splenic to the spleen. Proceeding posteriorly, the remaining major branches from the aorta are the anterior mesenteric (Figure 1-9-8), right and left renals (Figure 1-9-9), posterior mesenteric (Figure 1-9-10), a spermatic or utero-ovarian artery (depending on the sex of the animal, Figure 1-9-11), and the external (Figure 1-9-12) and internal iliacs (Figure 1-9-13) to the hindlimb and rump.

In the arterial system of poultry, the aorta swings to the right side of the body after leaving the heart (Figure 1-10-1). The arteries to the head are the common carotids (Figure 1-10-2) which divide into external (Figure 1-10-3) and internal (Figure 1-10-4) branches. The shoulder and wing are supplied by the brachial (Figure 1-10-5), the large flight muscles covering the sternum by the pectoral (Figure 1-10-6), and the ribs by the subclavian (Figure 1-10-7). The first visceral branch of the aorta is the coeliac (Figure 1-10-8) to the proventriculus, gizzard, spleen, liver, and pancreas. The anterior mesenteric (Figure 1-10-9) supplies the intestine. After the renals (Figure 1-10-10) to the kidneys are two major pairs of arteries to the leg, the external iliacs (Figure 1-10-11) and sciatics (Figure 1-10-12). The intestine also is supplied from the posterior mesenteric (Figure 1-10-13). The internal iliacs (Figure 1-10-14) and caudal (Figure 1-10-15) supply the cloaca and tail, respectively.

If poultry develop heavy muscling, the thin-walled right ventricle may be unable to cope with the increased cardiovascular demands and the ventricular wall may thicken as it adapts to the situation. But this may prevent the atrioventricular valve from functioning properly and the back pressure to the liver may cause ascites, an accumulation of fluid in the abdomen that may kill the bird (Julian, 1993).

In the venous system of meat animals, the anterior vena cava (Figures 1-11-1) receives blood from the external (Figure 1-11-2) and internal jugulars (Figure 1-11-3) that drain the head, and from the brachials (Figure 1-11-4) that drain the forelimb. The posterior vena cava passes through the liver where it receives blood from the hepatic vein (Figure 1-11-5). The mesenterics (Figure 1-11-6) collect blood with digested nutrients from the intestines and take it to the liver in the hepatic portal vein (Figure 1-11-7) to be processed before being returned to the general circulation. The remaining major inputs to the posterior vena cava are the renals (Figure 1-11-8) from the kidneys, the ovarian or spermatic veins (depending on the sex of the animal, Figure 1-11-9)), the external iliacs (Figure 1-11-10) from the hindlimb, and the internal iliacs (Figure 1-11-11) from the penis or udder. The vena azygos (Figure 1-11-12) is a single vein on the right side of the body, running posteriorly along the vertebral column, but in the diagram it has been deflected so that it does not look as if it runs from the liver.

In the venous system of poultry, the anterior vena cava (Figure 1-12-1) receives blood from the head in the jugulars (Figure 1-12-2). Left and right jugulars are linked by the jugular anastomosis which provides an alternative flow if one side of the neck becomes constricted. The pectorals (Figure 1-12-3), brachials (Figure 1-12-4) and subclavians (Figure 1-12-5) drain the shoulder, wing and pectoral region while the rib region is drained by the internal thoracics (Figure 1-12-6). Poultry have an hepatic portal vein (Figure 1-12-8) that allows blood-borne nutrients from the gut to be processed by the liver before entering the general circulation in the hepatic vein (Figure 1-12-9). The venous return of blood from the leg and pelvic regions is through a complex pattern of veins in and around the kidney. The main components are the renals (Figure 1-12-10), femorals (Figure 1-12-11), sciatics (Figure 1-12-12) and internal iliacs (Figure 1-12-13).

Cardiovascular function

There are three different types of muscle tissue in the body -smooth, cardiac and skeletal (Figure 1-13). Smooth muscle occurs in the digestive and reporoductive tracts, cardiac muscle is only found in the heart, while skeletal muscle forms all the meat of the commercial carcass.

Most cardiac muscle cells are mononucleate. They are arranged in rows to form branching fibers, but individual cells are separated by intercalated discs. Cardiac muscles have a striated appearance due to the precise alignment of sliding filaments in their contractile fibrils, but skeletal muscles also are striated.

Cardiac muscle cells are continuously pumping out sodium ions through their membranes. This causes the inside of the cell to have an electrical charge of approximately -90 mV with respect to the outside of the cell. This is called a resting potential. Extrinsic factors such as electrical activity (ionic movements) in adjacent cells may decrease the resting potential towards zero. When it reaches a value of approximately -65 mV, the threshold potential, the decrease in electrical potential accelerates, and it shoots past the zero value so that for a brief instant (about one tenth of a second) the membrane potential is positive. This sudden reversal of electrical charges is called an action potential. Action potentials are propogated into the interior of cardiac muscle cells by transverse tubules. In each cell, the transverse tubular system is an extensive series of finger-like indentations of the surface membrane.

The arrival of an action potential in the interior of the cardiac muscle cell causes the release of calcium ions from the sarcoplasmic reticulum. The sarcoplasmic reticulum is a series of membrane-bounded vesicles in the interior of the cell. Unlike the transverse tubular system, the sarcoplasmic reticulum does not open to the surface of the muscle cell. Units of the sarcoplasmic reticulum surround the contractile fibrils in the interior of cardiac muscle cells. The sarcoplasmic reticulum sequesters and stores calcium ions, but it releases them again when prompted to do so by the transverse tubular system. Calcium ions activate the system of sliding protein filaments which is responsible for muscle contraction (Chapter 5).

The intrinsic rhythm of heart contraction originates from a group of cells at the sinu-atrial node. The membranes of these cells behave as if they had a sodium ion leak. Thus, at regular intervals their resting potentials drop to their threshold values, and they initiate action potentials. Action potentials then spread in a coordinated wave through right and left atria. The atria then contract and pump blood into the ventricles. However, the ventricles also are capable of filling themselves as they expand after pumping out their previous fill of blood. Under normal conditions, atrial contraction contributes to the overall cardiovascular efficiency, but its contribution may become vital when the heart is weakened by disease. In the medial wall of the heart, at the junction between the atria and ventricles, is a sensitive group of cells forming the atrioventricular node. This node is connected to a conduction system called the bundle of His that runs down the medial wall separating left and right ventricles. The atrioventricular node is activated by contraction of the atrial cells, and the bundle of His conducts the action potential wave to the base of the ventricles. From this point, a wave of contraction spreads upwards through the ventricles so that the the blood that has just filled the ventricles is now pumped out of the heart. The intrinsic heart rate is determined by the rate at which the sinu-atrial cells "leak" or depolarize, by the value of their threshold potentials, and by their resting potential. The flow of blood through the heart is directed by the heart valves (Figure 1-8). Mitral and tricuspid valves make a "lub" sound and the semilunar valves make a "dup" sound.

Coordinated electrical activity of cardiac muscle cells generates an electrical signal that may be detected on the surface of the fore flank as an electrocardiogram (Figure 1-14). The P wave is due to atrial excitation, PQ is the delay as the action potential passes down the bundle of His, QRS is due to ventricular contraction or systole, and T is caused by repolarization of the ventricles. Activity of the heart is greatly influenced by its ionic environment. Isotonic sodium chloride plus calcium ions tend to stop the heart in systole (contracted) while isotonic sodium chloride plus potassium ions tend to stop the heart in diastole (relaxed).

The nervous system also has an effect on heart rate. The thoracic nerve of the sympathetic nervous system releases catecholamines that increase the heart rate (tachycardia) while the vagus nerve of the parasympathetic nervous system releases acetylcholine that slows the heart (bradycardia). The neural regulation of cardiac activity is a reflex response to inputs from blood pressure receptors or baroreceptors, and from chemoreceptors that monitor the concentration of carbon dioxide in the blood. When the heart contracts, it works against the resistance to blood flow created by the peripheral blood vessels in the body tissues. Thus, if the peripheral blood vessels decrease their diameter (vasoconstriction), the blood pressure tends to rise. Conversely, if peripheral blood vessels are dilated (vasodilation), the blood pressure tends to drop.


The nasal cavity of the skull contains the turbinate bones. Left and right turbinate bones have a shape that resembles a loosely rolled sheet of paper. This creates a large surface area for the nasal epithelium. The nasal cavity opens into the pharynx (shared with the alimentary canal), and then opens into the larynx. The larynx has a cartilagenous skeleton with muscles that support and stretch the vocal cords. In poultry, however, sound is produced by a separate organ, the syrinx, which is located farther down the respiratory system. The epiglottis is a spout-shaped cartilage that protects the entrance to the larynx. The larynx leads to the trachea or windpipe.

The trachea is a flexible tube held open by rings of cartilage. The continuity of each ring of cartilage is broken by a small dorsal gap. The trachea divides into two bronchi at a "Y" fork (Figure 1-15). The bronchi connect with the right and left lungs, where they branch into progressively smaller ducts called bronchioles. The trachea, bronchi and bronchioles are lined with ciliated epithelium and mucous glands. Cilia are extremely fine whip-like hairs on the lumenal surfaces of cells. A complex system of mobile protein strands along the length of each cilium provides the motive power for movements that appear whip-like. Millions of cilia beat in a coordinated manner so that they can propel a continuous stream of mucus from the lungs to the nasal cavity. Thus, any small particles that have entered the lungs, despite the protective filtering of incoming air by the turbinate bones, can be removed. Gaseous exchange between inhaled air and the blood in the lungs takes place across the moist surfaces of alveoli or alveolar sacs. In mammals, the alveoli are the final blind-ending branches of the air duct system. Beneath the moist epithelium which lines each alveolus is an extensive meshwork of lung capillaries.

Oxygen is taken up by the blood in a loose combination with the hemoglobin of red blood cells or erythrocytes. There are three ways in which carbon dioxide may be carried in the blood; (1) in solution, (2) combined with blood proteins, or (3) as bicarbonate. Carbon dioxide is more soluble and diffuses faster than oxygen. The ratio of bicarbonate to carbonic acid determines the pH or acidity of the blood. This ratio is regulated by the rate of escape of carbon dioxide from the blood in the lungs: loss of carbon dioxide increases pH (decreases acidity). Gaseous exchange does not occur across the walls of the major air ducts that lead into the lungs. Thus, the last fraction of air that is inhaled becomes the first fraction to be exhaled, and the oxygen it contains is not utilized. Typical resting rates of respiration are 12 to 18 breaths per minute in cattle, 12 to 20 in sheep and 10 to 18 in pigs.

When the lungs are removed from the body, slippery pleural membranes may be seen covering both the inner surface of the thoracic cavity and the lung surface. Pleural membranes prevent friction between the lungs and the body wall. Inspiration and expiration are caused by movements of the intercostal muscles, the ribs, the diaphragm and, sometimes, the abdominal muscles. The diaphragm resembles a strong drumskin that divides the thoracic and abdominal cavities, but it is thickened by muscle where it joins the body wall. In a dressed carcass, the muscular part of the diaphragm remains as a flap of muscle running diagonally across the inside of the ribcage. The rate of respiration is controlled by the medulla oblongata in the posterior part of the brain. The medulla responds primarily to the pH and the carbon dioxide content of the blood; it increases the rate of respiration if the blood becomes acidic with a high level of carbon dioxide.

The proteins of the lungs, together with those of the rumen and spleen, may be recovered by alkaline extraction followed by reacidification (Swingler and Lawrie, 1979). These proteins can be isolated as a powder (Levin, 1970) or texturized to form fibers (Swingler and Lawrie, 1978). Lungs may be processed to isolate heparin, an anticoagulant for medical use (Levin, 1970).

The avian respiratory system is quite different from that found in mammals (Figure 1-15). There is no diaphragm separating thoracic from abdominal cavities. Instead of being drawn into the lungs and then exhaled, air is drawn through the lungs and into air sacs outside the lungs. On exhalation, the air passes back through the lungs to the exterior. In poultry, therefore, the gaseous exchange between air and blood takes place as the air is moving through the lungs. The lungs of poultry are much smaller than those of mammals. Instead of occupying almost the whole of the thoracic cavity, they are located under the vertebral column where they are shaped to fit between the deep arches of the ribs where they meet the vertebral column. The lungs of poultry are usually removed with a suction tube during commercial slaughter procedures. In meat animals, the lungs are removed together with the trachea, bronchi and heart, as "plucks".

The extensive system of air sacs in poultry extends between many of the viscera and even into certain bones (Hogg, 1984). The interclavicular air sac (Figure 1-16-1) is a single structure in the midline but the other air sacs are paired (right and left). The cervical (Figure 1-16-2) extends towards the neck, the axillary (Figure 1-16-3) is within the body at the junction with the wing, and the anterior thoracic (Figure 1-16-4), posterior thoracic (Figure 1-16-5) and abdominal (Figure 1-16-6) sacs are in the body cavity. The humeral (Figure 1-16-7) is located within the humerus as a branch of the axillary sac. Air sacs have extremely thin walls and, when poultry are dissected, they should be identified while the viscera are in a relatively undisturbed condition. Special techniques for the demonstration and dissection of air sacs are described by Goodchild (1970).


The urinary system has two major functions, (1) to remove waste products from the blood stream, and (2) to regulate the amount of water present in the body. In mammals, the kidneys are ventral to the vertebral column in the anterior lumbar region. The kidneys of pigs and sheep are oval in shape while the kidneys of cattle are each divided into approximately 20 lobules. Pork kidneys are flatter and paler than lamb kidneys. In healthy well-fed animals, the kidneys are usually surrounded by perirenal fat. This is called leaf fat in the pork carcass. Each kidney has a depression or hilum where the renal artery enters the kidney, and where the renal vein and ureter leave the kidney (Figure 1-17). The ureter from each kidney carries urine to the bladder.

When a kidney is cut open, a pale inner medulla is seen surrounded by a dark red cortex (Figure 1-18). The wide entrance to the ureter is called the pelvis of the kidney. Running through the medulla, towards the pelvis of the kidney, are many small collecting tubules. Each of these terminates at a small conical mound called the pyramic, so that the pyramids project into the pelvis of the kidney. Urine is produced from the blood by a functional unit of the kidney called a nephron. There are large numbers of nephrons in each kidney. Urine leaves the bladder in a single tube, the urethra, that runs to the penis or to the vagina.

The main parts of a nephron are shown in Figure 1-19. Ultrafiltration occurs in the malpighian corpuscle so that blood plasma passes from the capillary glomerulus to the bowman's capsule. Large molecules cannot leave the blood because the pores of the filter are too small. In the proximal convoluted tubule, sodium chloride together with useful substances such as glucose, amino acids, proteins and ascorbic acid may be absorbed back into the blood stream. The descending and ascending loops of henle dip into an osmotic gradient in the fluid between the cells which is maintained by sodium ion movements. When urine eventaully flows down the collecting tubule, water may be lost from the tubule into the surrounding tissue (since the concentration is higher outside the tubule near the bottom of the loops). The final concentration of the urine is controlled by antidiuretic hormone (ADH) from the posterior lobe of the pituitary gland beneath the brain. ADH makes the wall of the collecting tubule more permeable, and water can leave more readily by osmosis. Thirsty animals produce a lot of ADH and only a small volume of urine. Their urine is more concentrated. The distal convoluted tubule is the principal site of acidification of the urine, and this is where potassium, hydrogen and ammonium ions enter the urine. Nitrogen is excreted from the body as urea.

In poultry, the kidneys are pressed closely against the ventral surface of the vertebral column, posterior to the lungs. Urine from each kidney leaves in a ureter but passes directly to the cloaca (Figure 1-20). Here, the urine rapidly loses water and the main component of nitrogenous excretion, uric acid, is precipitated as a sludge.



The paired testicles or testes of male mammals are located in a muscular bag called the scrotum. Here they can be maintained several degrees below body temperature for the efficient production of spermatozoa or sperm. Each testis can be raised by a detached strip of the obliquus abdominis internus muscle (Chapter 4, Group 5 muscles) called the cremaster externus. Evidence of cremaster muscles may appear in carcasses of sows and gilts and so cannot be used to identify male carcasses. The connective tissue surrounding the testis is called the tunica albugenia and it is white in color with a good blood supply. Spermatozoa are produced in seminiferous tubules that are tightly packed into the oval shape of the testis. The many seminiferous tubules in each testis open into a labyrinth of tubes called the rete testis. Immature spermatozoa from the rete testis pass in a number of efferent ducts to a further tubular system, the epididymis, located on the surface of the testis (Figure 1-21). Spermatozoa become mature during storage in the epididymis. They are carried to the urethra during mating by peristalsis of the vas deferens. The urethra is located ventrally in the penis. Seminal fluid to carry the spermatozoa is produced by the paired seminal vesicles, by the prostate gland, and by the paired bulbo-urethral glands (= Cowper's glands). The glands are located along the urethra, near to the bladder. In boars, the length of the bulbo-urethral glands is correlated with the androstenone content of the carcass (Forland et al., 1980). As discussed briefly towards the end of Chapter 2, androstenone is associated with the level of boar taint in a carcass.

The penis contains a sigmoid flexure or S-shaped bend along its length. The sigmoid flexure is straightened out when the penis is extended for mating. This occurs when a pair of muscles, the ischio-cavernosus muscles, compress the veins which drain the blood from the penis. Arterial blood pressure then expands the volume of vascular tissue in the penis. The ischio-cavernosus muscles are attached to the ischium (the posterior bone of the pelvis) and the trimmed stump of the muscle may be seen on dressed sides of beef as a pizzle eye. The pizzle eye is poorly developed in steer carcasses and is larger and darker in bull carcasses.

During the embryonic development of both mammals and birds, the testes are formed from tissue located near to the kidneys. In male mammals, the testes normally move to the scrotum outside the body cavity, and they pass through the body wall in the inguinal canal. The testes are attached to the inside of the scrotum by the gubernaculum which is contractile in fetal animals and is responsible for pulling the testis through the inguinal canal. The layers of connective tissue that cover each testis are the (1) tunica vaginalis communis and (2) the inner layer formed by the tunica vaginalis propria. Both layers are derived from modified layers of peritoneum gathered by the testes during their migration. The inner layer supports the blood vessels and nerves to the testis.

In cryptorchid pigs or ridgelings, movement of the testes along the inguinal canal is incomplete and they do not reach the scrotum. This abnormality causes infertility, but an older cryptorchid pig may still develop boar taint like a normal boar. The cause of the condition is not fully known but the normal mechanism of testicular movement appears to involve the action of testosterone on the genito-femoral nerve which then produces a peptide that activates the gubernaculum (Egan, 1993).

In male poultry, the testes remain in their original position near the kidneys, and a highly coiled vas deferens links each testis separately to the urodeum of the cloaca. The testes of cockerels may be removed through an incision made posterior to the ribs and anterior to the pelvis. Capons produced in this way are less aggressive in their behavior and they tend to deposit fat more readily than entire males. Before genetic advances in poultry growth rates made capon production uncompetitive, capons also used to be produced by administering the female hormone, estrogen. In poultry, some of the male hormone testosterone is converted to estrogen in the central nervous system and this estrogen is used to control testosterone production. Thus, increased estrogen levels thereby lower testosterone levels to produce a capon with tender meat and a high fat content (Wilson et al., 1983). Except for ducks, male poultry have no functional penis.


Female mammals have a pair of ovaries located posteriorly and dorsally in the abdominal cavity. Ova develop in the cortex (outer layer) of the ovary. Each ripe ovum is enclosed in a fluid-filled follicle. At estrus, ova are released into a ciliated funnel or infundibulum at the end of each oviduct (fallopian tube). The oviduct on each side leads into a horn of the uterus where embryonic development takes place. At birth, the offspring emerge through the dilated cervix and vagina.

The mammary glands are derived from highly modified sweat gland of the skin. The udders of sheep and goats are divided into right and left halves, each with a teat. The cow's udder has four quarters so that there are two teats on each side. Most sows have seven pairs of mammary glands and a total of 14 teats. Milk is produced in glandular alveoli, and it collects in the cistern of the teat. The bovine udder is supported by medial and lateral suspensory ligaments which are dominated by elastin and collagen fibers, respectively.

In female poultry there is only a single ovary since the ovary and oviduct of the right side do not normally develop. In poultry, the ovary usually contains a cluster of ova in different stages of development. The ova in the most advanced state of development appear as full-sized egg yolks. A large infundibulum (ostium) leads to a thick glandular region of the oviduct where egg albumen is formed, then to a narrower isthmus where shell membranes are added, and finally to a wide uterus where a calcareous shell is formed. The vagina opens into the cloaca and forms mucus to facilitate egg-laying.



The nervous system is a single integrated system composed of distinct regions. The central nervous system is composed of the brain and spinal cord. The peripheral nervous system is composed of nerves that radiate from the central nervous system to all parts of the body. The peripheral nervous system includes cranial nerves that radiate from the base of the brain, the spinal nerves that radiate from the spinal cord, and the autonomic nervous system. The autonomic nervous system is subdivided into the sympathetic nervous system which originates from the thoracic and lumbar regions of the spinal cord, and the parasympathetic nervous system which originates from the brain and the sacral part of the spinal cord. The salivary glands are innervated by the parasympathetic system. The heart, lungs, alimentary canal and bladder receive dual innervation from both the parasympathetic and the sympathetic nervous systems.

A ganglion is a bead-like swelling along a nerve, and it contains the cell bodies of certain nerve cells (neurons). The autonomic nerve fibers which radiate from the central nervous system to the ganglia of the autonomic nervous system are called preganglionic nerves. The autonomic nerve fibers that continue on from the ganglia to the organs which they innervate are called postganglionic fibers. The ganglia of the sympathetic nervous system are mostly located under the vertebral column so that their preganglionic nerves are short and their postganglionic nerves are long. The ganglia of the parasympathetic nervous system are mostly located close to or within the innervated organs so that their preganglionic fibers are long and their postganglionic fibers are short. The peripheral nervous system innervates muscle and skin while the autonomic nervous system innervates glands and viscera.


The surface of the brain is covered by a delicate membrane (pia mater) that carries a network of small blood vessels supplying the brain. The pia mater is covered by another thin membrane called the arachnoid membrane. On top of this membrane is a layer of tough tissue, the dura mater, that adheres to the inner surface of the skull. The surface of the brain in meat animals (Figure 1-22) is increased in area by folds (gyri) and grooves (sulci). The cerebrum is composed of left and right cerebral hemispheres separated by a deep fissure. If the brain is exposed by cutting through the skull with a band saw, the outer layers of the brain appear gray in color while the inner parts appear white. The gray areas are dominated by nerve cell bodies while the white areas are dominated by axons. Axons are cable-like extensions of the nerve cell body (Figure 1-23), and they are electrically insulated by a sheath of myelin. The gyri and sulci on the surface of the brain allow large numbers of nerve cell bodies to connect with the bundles of axons which carry information within the central nervous system. The function of the cerebrum is to regulate higher forms of nervous activity such as recognition, learning, communication and behavior. The cerebellum is posterior to the cerebrum, and is formed from a middle lobe called the vermis and two lateral hemispheres. The cerebellum coordinates muscle movements during locomotion and in the maintenance of posture. The thalamus is the region of the brain located ventrally to the cerebrum. It links the cerebrum to the rest of the central nervous system. The hypothalamus is located ventrally to the thalamus and connects the major regulatory gland of the endocrine system, the pituitary, to the brain. The most posterior region of the brain, where it tapers down to the diameter of the spinal cord, is called the medulla oblongata. This region controls the heart rate via elements of the autonomic nervous system described earlier. Cavities or ventricles filled with cerebrospinal fluid run through the brain and extend down the spinal cord as a small canal.

Spinal cord

The spinal cord is an extension of the brain. It emerges from the skull through the foramen magnum, and extends posteriorly to the first (cattle and sheep) or third (pigs) sacral vertebrae in the sirloin region. At regular intervals, pairs of dorsal and ventral roots enter and leave the spinal cord (Figure 1-23). Dorsal and ventral roots unite outside the spinal cord to form the nerves of the peripheral nervous system. Sensory neurons with their cell bodies in the dorsal root ganglia at the side of the spinal cord carry incoming sensory information from the skin, muscles and tendons. Sensory axons terminate on a variety of different types of neurons in the spinal cord. These neurons may relay information up the spinal cord to the brain, or to other regions of the spinal cord, or to nearby motor neurons. Motor neurons have their cell bodies in the central gray region of the spinal cord (dorsal and ventral horns, Figure 1-23). Their axons leave the spinal cord in ventral roots and innervate muscle fibers in the carcass muscles. Motor axons do not branch much before they reach their muscles but, once inside a muscle, they branch extensively to innervate a large group of muscle fibers called a motor unit. Myelinated axons which ascend and descend the spinal cord are located outside the central gray areas so that, unlike the situation in the brain, the white matter is placed outside the gray matter. The spinal cord is located within the vertebral column. It is separated from the inner bony surfaces of the vertebrae by an epidural space.


Many different types of neurons contribute to the complex circuitry of the central nervous system. Two relatively simple types of neurons are sensory neurons, with their cell bodies located part-way along their axons outside the spinal cord, and motor neurons, with their cell bodies located within the spinal cord. Dendrites are small root-like branches that provide the input to motor neuron cell bodies and to the axons of sensory neurons. Information is communicated along axons by waves of ionic activity called action potentials. Information is communicated between neurons by the release and reception of chemical transmitters. A synapse is a junction between two neurons, or between a neuron and a muscle fiber.

Action potentials

The cell bodies, dendrites and axons of a neuron are bounded by a cell membrane that is able to pump sodium ions outwards. This allows the concentration of potassium ions to build up inside the neuron. Because of the unequal distribution of these and other ions, the neuronal membrane carries an electrical charge of 50 to 70 millivolts, with the negative charge on the inner face of the membrane. If the membrane is briefly short circuited by a change in its ionic permeability, sodium ions rush inwards and potassium ions rush outwards for a brief instant. This rapid movement of ions short circuits an adjacent region of the membrane so that the cycle is propogated along the membrane. This self-propogating ionic and electrical change is called an action potential. Once an action potential has passed a region of a membrane, an equilibrium is restored ready for the next action potential. During this brief restoration period, called the refractory period, the membrane does not respond to any further stimuli. Action potentials normally are carried in only one direction, away from the origin of the action potential. All action potentials are identical once they are underway, and information is coded by their number and frequency pattern.


Communication between cells and organs within the body is essential for the efficient control of body metabolism. There are probably many modes of cellular communication yet to be discovered: nerve impulses and hormones are the two most conspicuous and best known types of communication. Although nerve cells can communicate rapidly by the transmission of action potentials, they rely on chemical transmitters for the final step of the journey to their destination. Endocrine glands have made this last step their whole journey, and they release chemical transmitters or hormones directly into the blood stream to act on cells at remote destinations. Unlike exocrine glands that release their secretions onto the skin or into the alimentary canal, the endocrine glands do not need a duct for the removal of their secretions. The differences between nerves and endocrine glands may not be as well defined as elementary textbooks might suggest. Examples can be found where neural communication is sometimes slow, diffuse and imprecise. In a few cases, endocrine substances act rapidly, or are sharply localized within restricted circulatory pathways. During the course of evolution, endocrine glands may have become specialized for the production of substances that normal tissues produce in only small amounts.

The systems which regulate animal growth are not yet fully known. When reading Chapter 6, keep in mind the nerve-like properties of some endocrine systems and the endocrine-like properties of some nerve cells. It might also be useful to note that the name hormone was adapted from the Greek, hormaein, to excite. This makes an appropriate contast with the term chalone, an internal secretion which depresses or inhibits activity, from the Greek, chalinos, to curb (Henderson et al., 1966). Jenkin (1970) gives some wise advice concerning the definition of a hormone, "... accept the simpler statement that many members of the animal kingdom have been able in the course of evolution to turn to good physiological use a very wide range of chemical messengers, which cannot at all easily be contained within the limits of any one man made category." Abbreviations for the more readily identifiable hormones are given in Table 1-1.

Table 1-1. Abbreviations for hormones


ACTH = adrenocorticotropic hormone

ADH = anitdiuretic hormone = vasopressin

CRH = corticotropin releasing hormone

FSH = follicle stimulating hormone

GnRH = gonadotropin releasing hormone

ICSH = interstitial-cell stimulating hormone

LH = luteinizing hormone

LTH = luteotropic hormone

MSH = melanocyte stimulating hormone

PTH = parathyroid hormone

STH = somatotropic hormone

TRH = thyrotropin releasing hormone

TSH = thyroid-stimulating hormone


Pituitary gland

The pituitary gland or hypophysis is a small round gland located ventrally to the brain. Embryologically, it is formed from the conjunction of an outgrowth from the floor of the brain (neurohypophysis or posterior pituitary) and a detached upgrowth from the roof of the mouth (adenohypophysis or anterior pituitary). A simplified list of the parts of the hypophysis, their products and their functions is given in Table 1-2.

Table 1-2. Pituitary (hypothalmo-hypophyseal system) components.


Component Product released Result


Hypothalamus CRH ------------- release of ACTH

GnRH ------------ release of LTH, FSH & LH TRH ------------- release of TSH

Posterior pituitary

neurohypophysis ADH ------------ water retention in

(outgrowth of kidney

brain) OXYTOCIN ------- uterine contraction

and milk release

Anterior pituitary

adenohypophysis STH --------------- stimulates growth

pars distalis LTH ------------- stimulates mammary glands

ACTH ------------ stimulates adrenal cortex

TSH ------------- activates thyroids and

adipose tissue lipase

FSH ------------- activates testes or

prepares ovarian follicles

LH -------------- completes spermatogenesis

and stimulates androgen

secretion OR follicle growth,

estrogen secretion, ovulation,

formation of corpus luteum and

progesterone secretion

adenohypophysis MSH --------------- may stimulate pigment

pars intermedia cells


Pineal gland

The pineal gland is a neurosecretory gland whose evolutionary origin may be traced back to the third eye found in the skull roof of certain fossil fishes. It is innervated by sympathetic nerves and is located deep in the brain, anterior to the cerebellum. It releases the hormone melatonin which acts on the ovaries to inhibit the estrus cycle. Melatonin also has wider effects on other neuroendocrine control systems. Melatonin synthesis is inhibited by nerve impulses to the pineal gland; the frequency of impulses is inversely related to the amount of visible light reaching the retinas of the eyes. In poultry, it has been shown that the pineal gland is probably responsible for circadian rhythms (24-hour cycles) in physiological activity (Binkley et al. 1977).

Thyroid gland

The thyroid gland is located around the trachea, near to the larynx in mammals. Left and right thyroid glands are joined ventrally in pigs; in cattle and sheep the junction is restricted to a narrow connecting isthmus. In poultry, left and right thyroid glands are deep red in color instead of pale brown, and they are completely separated at the base of the neck. The thyroid glands receive an abundant supply of blood from which they are able to capture iodine. Iodine is used for the synthesis of hormones which contain three or four iodine atoms, triiodothyronine and thyroxine. Thyroid hormones regulate oxidative metabolism and heat production in the body. Some cells in the thyroid also produce the hormone calcitonin.

Parathyroid glands

In mammals, two pairs of very small parathyroid glands are located in or near the thyroid glands. Their position is somewhat variable and difficult to identify in the abattoir. In poultry, there is a small parathyroid gland at the posterior end of each thyroid. The parathyroid hormone produced by the parathyroid glands forms one circuit of a double feedback system that regulates calcium levels in blood and bone. The other circuit is mediated by the hormone calcitonin from the thyroid gland. Parathyroid hormone causes the mobilization of calcium from bone. Calcitonin causes the inhibition of calcium release from bone.

Thymus gland

The thymus is a large gland, particularly in young animals, and it is located anteriorly to the heart and has lateral extensions into the neck. Thymus glands are sold for human consumption as sweetbreads. The thymus gland is composed of lymphoidal tissue and has a vital immunological function in young animals. It produces hormones, not yet completely characterized, which act on other cellular elements of the immune system.

The animal's immune system, with which it defends itself against invading microorganisms, exhibits two types of responses. Humoral antibody responses include: (1) the production of circulating antibodies, (2) the binding of antibodies to antigens, (3) the facilitation of phagocytic ingestion, and (4) the activation of certain blood proteins (the complement) to aid in the destruction of antigens. Cell mediated responses occur when specialized types of cells directly attack, or encourage macrophages to attack diseased cells bearing the target antibodies. The body contains two types of lymphocytes - the B cells responsible for humoral responses, and the T cells responsible for cell-mediated responses.

Both types of lymphocytes originate from hemopoietic stem cells of the fetal liver or adult bone marrow. Early in their development, immature lymphocytes migrate from their source into the circulation. In both mammals and poultry, immature T cells collect in the thymus where they undergo further development before they migrate out to peripheral lymphoid tissues (such as the spleen, the lymph nodes or Peyer's patches in the intestinal wall) to become mature T cells. In poultry, the B cells migrate to the Bursa of Fabricius for a period of development before they are released. In mammals, however, there is no Bursa or equivalent structure, and B cells are retained in the bone marrow for this period of their development.

Adrenal glands

Left and right adrenal glands are located anteriorly to the kidneys; in cattle they are roughly triangular, in sheep and poultry they are oval, and in pigs they are elongated. Each adrenal gland is composed of two distinct endocrine glands. In mammals, the adrenal cortex is wrapped around the adrenal medulla, although the two glands are mingled in poultry. Table 1-3 gives a simplified list of their products and functions. Table 1-3. Adrenal components and some of their functions.


Component Control Product Function



multiformis - Na & K - mineralocorticoids homeostasis of

ions deoxycorticosterone extracellular

aldosterone electrolytes

fasciculata - ACTH - the following to facilitate

+ CRH glucocorticoids: gluconeogenesis,

reticularis cortisone proteolysis and

hydrocortisone release of

corticosterone fatty acids

from adipose


- neural - epinephrine rapid response

norepinephrine to stress



The islets of langerhans are microscopic areas of the pancreas with an important endocrine function. The islets contain alpha cells that produce glucagon, and beta cells that produce insulin. Insulin facilitates the uptake and utilization of blood glucose by body cells. Thus, insulin deficiency causes the elevated blood sugar levels that occur in diabetes. Pancreas glands may be collected in abattoirs for the commercial isolation of insulin. Insulin concentration is highest in the tail end of the pancreas, and the tissue must be kept dry before being frozen since insulin is water soluble. The action of glucagon is the opposite to that of insulin.

There are also several other organs of the body that produce hormones in addition to their other activities. The testes produce testosterone. The ovaries produce estrogen, progesterone and relaxin. During gestation, the uterus and placenta secrete chorionic gonadotropin. The stomach wall secretes gastrin. The kidney produces the hormone renin. The liver produces somatomedins (Chapter 8).


Animal skin is composed of three basic layers. From outside to inside these layers may be called the (1) epidermis, (2) the dermis, and (3) the hypodermis. The epidermis is formed by layers of flat cells composing a stratified squamous epithelium. New cells originate in the lowest layer and become keratinized as they are pushed to the surface. Keratin is a fibrous protein that also forms the substance of hair, horns and hoofs. At the ultrastructural level it is deposited in a fibrillar form which then may be incorporated into a granular form.

Each hair follicle develops from an inpushing of the epidermis down into the dermis (Figure 1-24). Hair is formed by epithelial cells of a papilla at the base of the follicle. There is considerable variation in the rate of hair growth in meat animals. For example, the average length of bovine hair may reach a maximum between 6 and 24 months, and then may decrease (Camek, 1920). The underlying sequence of events in hair growth is due to the periodic shedding of hairs from their follicles. The bulb at the base of the hair eventually becomes hard and clublike. This holds the hair in its follicle for some time, but no further growth is possible. Eventually the hair is released when a new hair starts to form in the base of the follicle. This cycle determines the average external hair length and is influenced by factors such as climate, age, nutrition and breed. Chemical analysis of animal hairs may be used to measure the nutritional status of an animal, but the method is not very precise (Combs, 1987).

Most mammalian hairs and bristles have three layers that appear as concentric rings in a cross section through the hair shaft (Figure 1-24-B). From outside to inside these are: (1) a thin cuticle, (2) the cortex, and (3) the large cells of the medulla. Many of the wavy wool fibers of a sheep's fleece lack a medulla but, like strong straight pig bristles, they are still composed of keratin. The high tensile strength and low solubility of keratin in hair and wool fibers is caused by the cross-linking of protein chains by disulfide bonds, hence, dietary sulfur is important for wool production in sheep (Fraser, 1969). In sheep, the sebaceous glands that open into the wool follicles produce an oily secretion called lanolin. Pelt removal is more difficult for older lambs than for young lambs (Andersen et al., 1991).

In meat animals, most of the sweat glands open near the entrance of hair follicles. Although less conspicuous than the sweat glands of human skin, they still make an important contribution to thermoregulation in meat animals (McDowell et al., 1961). It has been suggested that hair follicles exert some control over the development of surrounding adipose tissue (Hausman and Martin, 1982).

Feathers are also formed in follicles. The follicles are grouped in feather tracts that are readily visible on the skin of the eviscerated carcass. In the spaces between the tracts, the follicles produce only filoplumes with a rudimentary feather vane at the end of a hair-like shaft. The arrangement of feather follicles is governed by waves of morphogenetic activity that move across the skin of the embryonic chick like ripples on a pond (Davidson, 1983). The large feathers of the wings are called remiges while those of the tail are called retrices. The contour feathers provide the main covering of the body and are interspersed with filoplumes. Young birds have large numbers of down feathers. The structure of the vane of a typical feather resembles a hollow quill that has been obliquely sliced and unrolled. Thus, when it is formed within the follicle it is like a hollow cylinder. The lateral branches or barbs of the vane are held together by hooked anterior barbules that catch on the saw-like edges of adjacent posterior barbules. The skin of poultry is dry and does not produce its own oil. In poultry, there is an oil gland located dorsally to the stumpy tail of the bird. The oil is distributed when the feathers are kept in order as the bird preens itself.

Pigment cells or melanocytes are located in the deepest layers of the epidermis or in the underlying dermis. Melanin is a pigment formed in organelles called melanosomes. Melanin is passed from melanocytes to skin cells by cytocrine secretion. Melanin is formed from the oxidation of tyrosine by tyrosinase. Absence of this enzyme results in an albino animal. Variation in the color of farm animals is caused by variations in the amount and distribution of melanin. Melanin may be extracted with an aqueous solution of sodium hydroxide and then recovered by acidification. Melanin is a polymer based on indole monomers, but there is also a protein component involved that makes precise determination of its structure difficult (Thomson, 1962). The distribution of melanin over the animals' skin is determined prenatally by an interaction between the migration patterns of melanocytes and the diffusion patterns of the messenger substances that either activate or suppress the synthesis of melanin (Bard, 1981). A single dominant gene determines the belt pattern marking that runs over the shoulders and forelimbs of some breeds of pigs (Donald, 1951).

The epidermis is supported on the ridged surface of the underlying dermis. The upper region of the dermis, often called the papillary layer of the dermis, is a tightly woven network of collagen fibers with some elastin fibers. After the tanning of a hide to make leather, the papillary layer becomes the top surface of the leather. With a hand lens, the openings where the hair follicles once penetrated the dermis are easily visible (Figure 1-24-C). When the leather is turned over, the much looser coarse fibrous weave of the lower dermis is evident. In pigskin, the follicles of the strong bristles are rooted at the lowest level of the dermis so that many of the follicles almost perforate the leather.

When the hide is removed from the carcass, the separation is made through the deepest layer of the integument - the hypodermis. Fat is often deposited in the hypodermis and, particularly in sheep, may even infiltrate the dermis. Numerous blood vessels run through the hypodermis to reach the extensive vascular bed (for heat dissipation) in the dermis. The hide weight of a typical lean steer is about 7% of the live weight, but there is considerable seasonal variation (Dowling, 1964) with colder climates inducing heavier hides and there are also differences between types of cattle. Terry et al. (1991) found that Bos indicus cattle had relatively heavy hides while Holstein cattle had relatively light hides.

Beef hides are graded on their cleanliness and degree of damage due to branding or warble fly larvae. If beef hides have been processed with a high standard of hygiene, the collagen of the inner layer of the hide may be used for processed food products such as sausage casings. Green hides (those from recently slaughtered animals) are treated with sodium chloride prior to tanning. The hides are trimmed, split into left and right sides, and soaked for several days in water. Then the hides are dehaired in a calcium hydroxide solution that contains sodium or calcium hydrosulfide. The conversion of a hide to leather occurs when it is TANNED, originally with a tree bark extract but now usually with sodium dichromate. Hair remnants are physically forced from the hair follicles (scudding) prior to deliming in sulfuric acid. Elastin fibers are removed enzymatically before the hides are pickled in sodium chloride acidified with sulfuric acid.


Strictly speaking, an abattoir is a place where cattle are slaughtered, but other species of meat animals are often slaughtered in the same building. The methods used to slaughter meat animals have a profound effect on meat quality. The scientific basis of this relationship is considered again in Chapter 9 but, at this early point in a long and complicated story, only the general principles of slaughter methods need to be introduced. Slaughter methods are important, but so is the expertise with which they are applied (Weeding et al., 1993).

Preparation for slaughter

The optimum amount of rest required by meat animals before they are slaughtered depends on the climate, the distance they have traveled, their method of transport and their general health. In some countries, where animals are auctioned at stock yards before they are taken to an abattoir, the rest periods are sometimes inadequate. This creates a commercial problem that is difficult to evaluate. On one hand, animals lose weight during transport and in holding pens, and it is undesireable to use pens and labor to prolong a rest period that confers no immediately obvious commercial advantage. On the other hand, stressed or weary animals sometimes produce meat with an unacceptable appearance or water holding capacity, and this may create economic losses later on. Animals lose about 0.2% per hour of their live weight once feeding has ceased, but this is very variable. For beef cattle, losses in 48 hours of fasting may range from less than 1% to 8% (Warriss, 1990). About half the live weight loss shows up as a loss in carcass weight. However, improvements may be gained by electrolyte therapy, allowing animals free access to drink electrolytes during lairage (Schaefer et al., 1992).

Lairage, the holding of animals at an abattoir before slaughter, is currently being improved in many countries since lairage designs that reduce animal stress and discomfort generally improve meat quality. Apart from eliminating electric prods and the like, the newer designs may incorporate features such as the automation of animal movement and color coordination, as in the moving gate system and green decor of the Danish automated pig lairage system. Spray cooling of pigs may be used to improve meat quality (Weeding et al., 1993).

It is traditionally maintained that bleeding or exsanguination is less complete when animals are extremely fatigued at the time of slaughter, and that bacteria from the gut more readily enter the blood stream and contaminate meat from exhausted animals. Any residual blood in meat is often regarded as a good medium for bacterial growth that may cause meat spoilage. In some situations, a rest period of one day for cattle and two or three days for pigs is considered to be optimum. However, such rest periods may be counter productive if the animals fight among themselves. Animals are not fed in the 24 hour period prior to slaughter.

Stunning methods

There are several criteria for a good slaughter method: (1) animals must not be treated cruelly, (2) animals must not be unecessarily stressed, (3) exsanguination must be as rapid and as complete as possible, (4) damage to the carcass must be minimal, and the method of slaughter must be (5) hygienic, (6) economical and (7) safe for abattoir workers.

To avoid the risk of cruelty, animals must be stunned or rendered unconscious before they are actually killed by exsanguination. When religious reasons do not allow stunning, extra care is needed to ensure that exsanguination causes the minimum of distress to the animal. In the Kosher method of killing, conscious cattle are suspended with the head stretched back, and then the throat and its major blood vessels are severed. Drugs cannot be used in the meat industry to induce unconsciousness in animals for slaughter since unacceptable residues would remain in the meat.

Animals can be effectively stunned by concussion. Concussion may be induced by a bullet or a bolt that penetrates the cranium, or by the impact of a fast-moving knocker on the surface of the cranium. In modern abattoirs, the primitive pole-axe has been replaced by devices which use expanding gas, either from an air-compressor or from a blank ammunition cartridge. First, the animal is restrained in a narrow pen or knocking box in order to to minimize its head movements. The concussion instrument is then accurately located at a point on the midline of the skull, above the level of the brow ridges of the eye sockets. Concussion stunning should not be applied on the neck or posterior part of the skull (Lambooy and Spanjaard, 1981).

The knocker is a heavy instrument held with both hands. There is a safety catch on the handle, but the actual trigger protrudes from the head of the knocker and is activated as the knocker is tapped against the animal's head (Figure 1-25-B). The captive bolt pistol (Figure 1-25-A) resembles a heavy hand gun, but a blank cartridge is used to propel a cylindrical bolt rather than a bullet into the skull. After penetration, the bolt is withdrawn into the barrel of the pistol and the pistol is reloaded. Steers, heifers and cows are normally stunned with a knocker or a heavy captive bolt pistol, but bulls and boars which have massive skulls are sometimes shot with a rifle bullet. Pigs and lambs may be stunned with a light-weight captive bolt pistol.

Meat animals may be stunned by passing an alternating electric current through the brain. The method is widely used for stunning pigs and poultry, and more recently, for cattle (Gregory, 1993). Unconsciousness is induced by a wide range of voltages, from about seventy volts to several hundred volts. The length of time that the current is passed through the brain may be reduced to only one or two seconds if abattoir workers are waiting to shackle the pig's hindlimb with a chain, and then to exsanguinate the animal immediately. With a simple, hand-held electric stunner, the current is applied to the pig's head with two electrodes that protrude from an insulated handle. The electrodes must be cleaned at frequent intervals to ensure good electrical contact with the pig. The transformer which supplies the current is usually mounted on a nearby wall. However, large automated stunning systems are used in most commercial abattoirs and these may have one of a variety of different patterns of electrodes. Some designs have a current flow through the chest to stop the heart. While this may be acceptable for pigs, neck to brisket stunning may not be for cattle (Cook et al., 1991). Even for pork, high voltage head to back stunning intended to stop the heart may cause vertebral fractures and blood splashes in the meat if the system is not carefully operated and monitored. Essentially, if the rear electrode is moved forward there is less damage to the carcass, but also a reduced probability of stopping the heart (Wotton et al., 1992). High frequency stunning of pigs, which was pioneered by van der Wal and Mazee (1974), also may be used to reduce carcass damage (Anil and McKinstry, 1992).

Pigs may be stunned by placing them in an atmosphere which contains 65% carbon dioxide. Carbon dioxide is heavier than air and is trapped in a pit or deep tunnel into which the pigs are conveyed. After about one minute, the pigs are withdrawn in a cage or on a conveyer belt, and are then exsanguinated as rapidly as possible. Carbon dioxide stunning may also be used for turkeys (Fleming et al., 1991).

Meat animals are usually stunned, shackled and exsanguinated, in that order. However, poultry may be shackled or hooked by their feet as soon as they are unloaded from the crate. Getting poultry into crates prior to transport is a major commercial problem. Live birds are easily bruised or more seriously damaged: this causes suffering to the birds and creates carcasses with an unattractive appearance. Electrical stunning is very effective for poultry and is highly automated. It improves exsanguination and facilitates the subsequent removal of the feathers, and there is a trend towards using higher voltages (200 V) that are more effective and humane (Dickens and Lyon, 1993). Concussion from a hammer-blow still is commonly used to stun ducks.


Cattle and pigs are usually exsanguinated by a puncture wound which opens the major blood vessels at the base of the neck, not far from the heart (Figure 1-26). The trade name for this process is sticking. In sheep, lambs and small calves, the major blood vessels may be severed by a transverse cut across the throat, near to the head. Poultry can be exsanguinated with a diagonal cut from the corner of the jaw towards the ear on the other side, or by a knife thrust through the roof of the mouth to severe the brain and its major blood vessels. For poultry, the cut may be made on the side of the head if the head is later to be removed automatically by machine.

If the sticking wound is inaccurately placed, exsanguination may be too slow, and it may be almost halted by the formation of large blood clots. The formation of blood clots is accelerated when large areas of tissue are damaged by repeated inaccurate punctures. If the trachea is severed by the sticking wound, blood may be drawn into the lungs as the animal breathes. Later in the slaughter procedure, this may necessitate the trimming of blood clots from the pleural membranes after they have been inspected. If the esophagus is severed, the vascular system may be contaminated by the entry of food particles into the venous system. If the connective tissues of the shoulder are opened, blood may seep into the shoulder region to form blood clots between the muscles.

Incomplete exsanguination increases the amount of residual blood in the carcass. The lean meat may then appear unduly dark and the fat may become streaked with blood. On the surface of incompletely exsanguinated poultry, the skin may appear dark and bloody over the breast, neck, shoulders and wings. The microscopic tissue damage that may later be caused by the freezing and thawing of poultry enables residual blood to leak from skin capillaries. Thus, the results of incomplete exsanguination are often more noticeable to the consumer than to the producer.

The exsanguination or sticking of meat animals in an abattoir is usually performed by severing the carotid arteries and the jugular vein at the base of the neck. In poultry, these vessels may be cut only on one side of the neck (Gregory and Wotton, 1986). The sticking knife must be kept clean otherwise bacteria might be introduced into the venous system and spread through the otherwise relatively sterile muscles of the carcass (Gill, 1979). Once exsanguination has started, the pulse and mean blood pressure rapidly decline because of the reduced stroke volume of the heart. Blood pressure changes are monitored physiologically by baroreceptors in the carotid sinuses(Booth et al., 1966). During exsanguination, respiratory movements of the thorax may be stimulated, and neurogenic and hormonal mechanisms attempt to restore the blood pressure by increasing the peripheral resistance by vasoconstriction. The heart keeps beating for some time after the major blood vessels are emptied, but rapidly stops if exposed and cooled (Thurston et al., 1978). Electrical stunning of pigs may terminate cardiac activity so that, at the start of exsanguination, the blood escapes by gravity rather than being pumped out. In pigs, cardiac arrest does not affect the rate and extent of exsanguination (Warriss and Wotton, 1981). After exsanguination has started, the heart usually re-starts and attempts to pump, until it runs out of energy (Swatland, 1982). Thus, in many cases, there is no reason why animals such as pigs and sheep cannot be killed by electrocution rather than being merely electrically stunned (Gregory and Wotton, 1984a; Wotton and Gregory, 1986). In cattle stunned by concussion, more or less complete exsanguination may be obtained without ventricular pumping (Vimini et al., 1983a). Similarly, normal exsanguination is obtained in poultry that have been killed by electrocution rather than by being electrically stunned (Griffiths et al., 1985). In meat animals, "head to back" stunning may be used to stop the heart (Gregory and Wotton, 1984b).

Blood loss as a percentage of body weight differs between species (Ostertag, 1907; Dickens and Lyon, 1993),

cows 4.2 to 5.7%

calves 4.4 to 6.7%

sheep 4.4 to 7.6%

pigs 1.5 to 5.8%

chickens 2.6 to 2.9%

Blood content as a percentage of live weight may decrease in heavier animals since the growth of blood volume does not keep pace with growth of live weight (Hansard et al., 1953). Approximately 60% of blood is lost at sticking, 20-25% remains in the viscera, while a maximum of 10% may remain in carcass muscles (Warriss, 1977). Different stunning methods may modify the physiological conditions at the start of exsanguination (Leach and Warrington, 1976) and, also, the neural responses to exsanguination (Kollai et al., 1973). Electrically stunned sheep lose more blood than those stunned with a captive bolt (Warriss and Leach, 1978), but they also have more blood splashes in their carcasses (Kirton et al., 1981).

Reduction of blood flow to the kidneys causes the release of a proteolytic enzyme, renin, which acts on a plasma protein to produce a polypeptide, angiotensin I. This polypeptide is converted enzymatically to angiotensin II which then causes widespread vasoconstriction. Vasoconstriction is important because it decreases the retention of blood in meat (Warriss, 1978). Angiotensin II vasoconstriction is operative in both conscious and anesthetised animals (Miller et al., 1979). Catecholamines and ADH may also enhance vasoconstriction during exsanguination. Speed of exsanguination may modify the balance between neural and hormonal vasoconstrictive mechanisms, with hormonal vasoconstriction predominating in rapid exsanguination (Hall et al., 1976). However, asphyxia prior to exsanguination may result in vasoconstriction due to the activity of the sympathetic nervous system (Weissman et al., 1978).

It is traditionally maintained that poor bleeding leads to dark meat with poor keeping qualities due to microbial spoilage and rancidity. However, there is little scientific evidence in support of this view (Warriss, 1977), and it may be false, even in animals which retain massive amounts of blood in their carcasses (Roberts, 1980). Delayed exsanguination of cattle may lead to a slight reduction in the amount of blood removed so that the carcass and spleen are slightly heavier. The effects on meat quality, however, are negligible (Vimini et al., 1983b).

Factors that regulate the balance between extracellular and intracellular fluid compartments in meat are poorly understood. Fluid is delivered to living muscles by arteries, but it may return to the heart by either of two routes, in the venous system or in the lymphatic system. The route taken by intercellular fluid depends primarily on the extent to which fluid is taken up by capillaries and then passed to the venous system. In living animals, the venous return is far greater than the lymphatic return. The lymphatic capillaries which drain skeletal muscles are mostly located in the connective tissue around bundles of muscle fibers (Korneliussen, 1975). The small amount of lymph that drains from muscles is increased after neural stimulation, and its LDH content (LDH is an enzyme from within the muscle fiber) increases dramatically following muscle damage (Bach and Lewis, 1973). In sheep, the flow of lymph from lymph nodes increases within 15 minutes of stress due to pain (Shannon et al., 1976). Hemorrhage may, (Lundvall and Hillman, 1978) or may not (Johnson, 1972) cause absorption of intercellular fluid into the blood stream, depending on the degree of vasoconstriction and consequent hydrostatic pressure in the vasculature.

Blood to the brain

The effects of exsanguination on conscious sheep and cattle are reviewed by Baldwin (1971). Arterial blood to the brain is evenly distributed by a circular pattern of arteries called the circle of willis. The circle of Willis receives blood from the intracranial carotid rete (a rete is a meshwork of blood vessels). In sheep, the external carotid arteries supply the intracranial carotid rete, via the internal maxillary arteries since the internal carotid arteries are absent in adults. However, blood may also reach the intracranial carotid rete from vertebral arteries via the occipito-vertebral anastomosis (an anastomosis is a communicating link between two vessels). The situation in cattle is similar, but with an additional supply to the intracranial carotid rete from vertebral and occipital arteries. The extent to which intact vertebral arteries might prolong a supply of oxygenated blood to the brain once an animal's throat has been cut is difficult to assess. In sheep, consciousness may persist for 65 to 85 seconds (Newhook and Blackmore, 1982). In pigs, the delay between exsanguination and termination of EEG activity is approximately 20 seconds following proper stunning (Hoenderken, 1978). However, anoxia causes the dilation of cerebral blood vessels (Zeuthen et al., 1979) so that their storage capacity may be increased. An important point to bear in mind in considering studies on this topic is the difference between severing the carotid arteries (as in the Jewish Shechita method) and in ligation of the carotids (as in experiments attempting to simulate Shechita conditions). In the former case there is a rapid loss of blood supply to the brain whereas, in the latter case, the blood supply may be maintained (Gerlis, 1987).

Utilization of blood

The recovery of animal blood for utilization in food products for human consumption should be attempted. The main problems are to prevent the contamination of collected blood by bacteria from the skin, and to keep the blood of different animals separate until their carcasses have passed veterinary inspection for human consumption. Blood may be collected hygienically with a hollow knife. Coagulation of the blood can be prevented by the addition of anticoagulants such as citric acid or sodium citrate. Alternatively, the fibrin which binds blood clots together can be removed by stirring with a paddle (Pals, 1970). When utilized for human food or pet food, blood contains easily assimilated iron. Blood proteins have a high nutritional value and a high water binding capacity in processed products. The red blood cells burst if water is added to blood. If they are kept intact, red blood cells can be removed by centrifugation in order to prepare plasma. Plasma is a yellow liquid, rather like egg-white, and it may be dried to a powder for use in human food. If blood is discharged into the abattoir effluent instead of being utilized, it increases the biological oxygen demand (BOD) of the effluent. Chemical oxygen demand is another index of the pollution load of the abattoir effluent: it can be measured in several hours rather than in the 5 days required for BOD determinations.

Carcass dressing

Slaughter procedures are extremely variable, and common methods in one locality may be unheard of in another. The introduction of equipment, such as a hide puller, may lead to a reorganization of the slaughter line. The main objectives of a slaughter procedure are to get the job done neatly, hygienically and fast. One possible method for beef carcasses is described below. In this case, the carcass has already been suspended on an overhead rail in a manner that enables the removal of the distal parts of the hindlimbs.

(1) skin the head and remove the skull and lower jaw, leaving the whole of the neck and the skin of the head hanging on the carcass,

(2) remove each foot and the distal part of each limb by cutting through the joint immediately proximal to the long cannon bone,

(3) make a long incision through the hide in the midline of the chest and abdomen, and continue the incision along the medial face of each of the limbs,

(4) remove the hide altogether if suitable equipment is available, or just remove it from the ventral part of the body and leave it temporarily hanging from the animal's back,

(5) open the thoracic cavity with a midventral saw-cut through the breast bone or sternum,

(6) open the abdomen with a long mid-ventral incision, and remove the penis or udder tissue, and any loose fat in the abdominal cavity,

(7) split the pelvic girdle with a mid-ventral knife-cut or saw-cut through the cartilage that separates the pelvic bones in the midline,

(8) cut around the anus and close it off with a plastic bag,

(9) skin out the tail (if this was not done earlier),

(10) separate the esophagus (which takes food to the stomach) from the trachea (which takes air to the lungs), by pulling the esophagus through a metal ring; close off the esophagus by knotting it,

(11) eviscerate the carcass by pulling out the bladder (and uterus if present),intestines and mesenteries,rumen and other parts of the stomach,liver; after cutting through the diaphragm, remove the plucks (heart, lungs and trachea),

(12) separate the left and right sides of the carcass by sawing down the midline of the carcass, through the vertebral column,

(13) trim and weigh the carcass to obtain its HOT WEIGHT,

(14) wash the carcass and pin a shroud over it to smooth the subcutaneous fat.

The other species of meat animals are treated in a corresponding manner, except for the head, feet and hide. With calves, the skin may be left on until the eviscerated carcass has been chilled. Beef carcasses are first shackled with a chain around the foot, but before the feet are removed, the carcasses are re-suspended from a hook under the Achilles tendon at each hock. However, the feet are usually left on pork carcasses. After being shackled during exsanguination, usually by one hindlimb, pork carcasses are re-suspended from a hooked bar or gambrel. This is inserted beneath tendons that have been freed underneath the hind-feet. When pigs are shackled by one hindlimb during exsanguination, differences in meat tenderness may be created between left and right hams (Cagle and Henrickson, 1970). In some abattoirs, carcasses are skinned while they are on a metal cradle which holds them off the floor.

Washing of the dressed carcass is more complex than it might first appear. Apart from considerations relating to water purity and waste treatment, consideration must be given to sanitizing factors such as chlorine, organic acids and high temperature (Dickson and Anderson, 1992). Sanitizing agents may greatly reduce the levels of surface bacteria when the carcass is washed, but at the risk of hiding poor sanitation at earlier stages of processing. There is much to commend the philosophy of preventing initial contamination rather then removing it once it is present.

In lambs and sheep, the forelimb metacarpal cannon bone is removed at its distal extremity at the break or spool joint. Force is applied to this joint and it is loosened with a knife. In relatively young animals, the epiphyseal growth plate (see Chapter 2) fractures to give a "break joint". In older animals, the end of the bone is fused to the shaft of the bone so that the joint breaks to reveal a "spool joint". When a spool joint is revealed, the animal is classified as yearling mutton or mutton. The time at which a spool joint is apparent is quite variable, ranging from 9 to 21 months depending on the animal (Field et al., 1990).

Hairs and bristles must be removed from pork carcasses when the skin is to be left on the carcass. After exsanguination, otherwise intact pork carcasses may be immersed in a scalding tank that contains water at about 60oC. Under normal conditions, with normal pigs, there is little or no heat penetration into the underlying musculature so that meat quality is unaffected (van der Wal et al., 1993). Scalding at this temperature for longer than six minutes damages the skin (Mowafy and Cassens, 1975). Lime salts or a depilator such as sodium borohydride are added to the water to facilitate loosening of the hair. After five or six minutes, the carcass is lifted out of the tank and is placed in a dehairing machine, where it is repeatedly slapped by strong rubber paddles with metal edges. Loosening of the hair by hot water also may be accomplished by the action of steam on carcasses hanging vertically from an overhead rail. Microbial contamination is minimized but costs due to energy and water are increased. Other possibilities include scraping loose hairs from the skin with a jet of fast-moving ice particles, as currently being tested in Denmark.

After removal of the hoof from each toe, the pork carcass is re-suspended from the overhead rail. Then the carcass is quickly singed with a gas flame that burns all the fine hairs which have escaped the dehairing machine. The carcass is shaved with a sharp knife until it is clean. However, this often damages the skin and it may spoil the skin for leather production. Sometimes it is almost impossible to remove the stumps of strong bristles from the skin, particularly in the early months of the winter.

In some abattoirs, pork carcasses are skinned like beef carcasses. This enables better quality leather to be made from the skin and, in the long run, is less expensive than scalding (Judge et al., 1978). In this procedure, the skin is manually detached in the ventral region of the head and body, and on the medial faces of the limbs. Then the skin is removed from the dorsal part of the carcass with an air-knife (Figure 1-25) or with a hide-puller. The hide-puller is driven by a powerful motor or hydraulic piston and it simply rips the skin off the carcass. Ideally, the vertebral axis of the animal should be temporarily strengthened by brief electrical stimulation to tighten the muscles, otherwise some hide puller may cause a separation of the vertebrae, particularly in younger cattle (Cooke, 1987). However, this usually displaces several kilograms of fat from the edible carcass to the inedible hide, with a consequent loss in revenue (Rust, 1974). The kidney and pelvic fat of beef carcasses may be removed before chilling to facilitate lard rendering operations. There is no deleterious effect on the underlying meat (De Felicio et al., 1982).

Usually poultry are scalded to facilitate the removal of their feathers. The ease with which feathers may be removed is related to the temperature and duration of scalding. However, high temperatures (> 58oC) cause the skin to become dark, sticky and easily invaded by bacteria. Consequently, hard scalding (at 70 to 80oC) is only used for low grade poultry destined for immediate use in processed products. For broilers, the appearance of the skin is unharmed by about thirty seconds of semi-scalding in water at 50 to 54oC. Both temperature and duration are precisely controlled, depending on the age and condition of the birds. After the feathers have been loosened, they are removed by machines that have thousands of rubber fingers mounted on rotating drums. However, many of the strong pin feathers on the tail and wings may survive this treatment and must be removed manually.

The feathers on the carcasses of ducks and geese are difficult to remove. Following scalding and the mechanical removal of as many feathers as possible, ducks and geese may be quickly dipped in hot wax. After the birds have been removed and cooled, the wax sets hard and can be pulled off together with large numbers of feathers. The wax is melted and recycled, and the birds are picked bare manually.

Methods for the evisceration of poultry are even more variable than those for meat animals and many of the operations for poultry evisceration have been successfully automated. Poultry usually are suspended on some type of moving overhead rail. Sometimes they are suspended by their feet, sometimes by their heads, and sometimes by both, so that the vent or cloaca bulges downwards. One possible method for the evisceration of poultry is as follows:

(1) after stunning and exsanguination, the bird is suspended from its head, and the oil gland at the base of the tail is removed,

(2) an incision is made through the skin along the back of the neck, from the head to the shoulders,

(3) the crop and the trachea are removed,

, (4) the bird is re-suspended by its feet, and an incision is made through the skin, around the cloaca and towards the sternum,

(5) the viscera and the intact cloaca are pulled out and inspected for signs of disease,

(6) the liver is removed and the green gall bladder is discarded, without contaminating the carcass with bile,

(7) the muscular wall of the gizzard is slit open so that the inner lining and the contents can be discarded,

(8) the heart is removed from the hanging viscera and trimmed,

(9) the remaining viscera are removed and discarded, and the lungs, kidneys and ovary or testes are removed from under the vertebral column with a suction tube,

(10) the head, neck and feet are removed,

(11) the carcass is chilled in a mixture of ice and water,

(12) after chilling, the giblets (neck, gizzard wall, liver and heart) are packed into the carcass.

Although mass produced poultry are now almost all eviscerated prior to distribution to retail outlets, intact poultry carcasses keep quite well if their viscera are left in place. Growth of intestinal bacteria is minimal below 7oC and, at temperatures below 4oC, uneviscerated carcasses may be stored for at least as long as eviscerated carcasses (Barnes and Impey, 1975).

Although automated evisceration of poultry has been around for years, it is only recently that the major engineering problems associated with eviscerating larger animals have been solved. Both New Zealand and Australia have developed large-scale systems, for lamb and beef, respectively. If successful they are destined to have a dramatic impact on the meat industry where labor costs in slaughtering have always been a major factor in locating abattoirs in relation to meat producing regions. Recent developments include automated equipment for removal of the brain (either as an edible byproduct or for extraction of pharmaceuticals), brisket cutting, evisceration, removal of muscles from the neck and between the ribs, removal of shoulder and chine bones, and inspection stamping (MIRINZ, 1992).

Meat Inspection

Meat inspection involves the examination of live animals (ante mortem inspection), carcasses and viscera (post mortem inspection), and finished products. The buildings and equipment of the abattoir must conform to a prescribed standard of hygiene, and abattoir workers must be properly trained. The main objectives of meat inspection are (1) to ensure that consumers receive only wholesome products for consumption, (2) to ensure that by-products are properly treated so as to cause no direct hazard to health, and (3) to provide a warning of the presence of serious contagious diseases among farm livestock. The purpose of ante mortem inspection is to identify injured animals that must be slaughtered before the others, and to identify sick animals that must be slaughtered separately or subjected to special post mortem examination.

Man has exhibited a fear and dislike of contaminated meat throughout recorded history. The Mosaic food laws with a religious basis have survived to the present day, while Greek and Roman civil laws have slowly evolved into modern civil legislation. Many European countries have a long history of legislation relating to meat hygiene and, by 1707, these laws had reached Canada (Heagerty, 1928). However, it was not until the early years of the present century that modern meat inspection started to develop with the science of veterinary microbiology as its basis (1906 in the USA, and 1907 in Canada). The general principle of commercial responsibility in meat inspection should be that the party responsible for a condemnation must bear the financial loss. In many cases, only a relatively small part of the carcass is condemned. Diseases and conditions that may be attributed to the producer include items such as abscesses, antiobiotic residues, parasitic infections, hernias, and a range of bacterial and viral diseases. Conditions such as bruises, bone fractures, frostbite and pneumonia may be attributed to the producer or to the packer, depending on when they are found. In Canada, they are the responsibility of the producer if they are found within 24 hours of leaving the farm. After that, they are attributed to the packer. Contamination of the carcass, loss of identity and recent parturition are attributed to the packer.

One of the major factors in the design of a slaughter line is the minimization of the spread of Salmonella (Anon., 1981a). There are well over a thousand species of this bacterium, and they are frequently found in the feces of meat animals and poultry. When the bacteria are transmitted to human food, they may infect the human digestive system and cause a food-borne illness. Although Salmonellae on meat are killed by the heat of thorough cooking, they can cause illness by contaminating other foods which are eaten raw. For example, they may contaminate a salad that has been prepared on a cutting board previously used for contaminated poultry. Another microorganism that causes gastroenteritis, Campylobacter jejuni, also may be transmitted on contaminated meat, particularly poultry. This bacterium is killed by cooking procedures that reach 60oC or more (Kotula and Stern, 1984).

Most of the hygienic precautions in the abattoir are quite straightforward. For example, the knives used to remove animal hides often become severely contaminated. Thus, they must not be used for later operations when the carcass meat has been exposed, and they must be decontaminated by a method such as dipping in hot water at 82oC for 10 seconds (Peel and Simmons, 1978). Contamination is not limited to knives, and relatively large numbers of Salmonellae can be found on steel-mesh safety gloves, cutting boards and stainless steel tables (Smeltzer et al., 1979). Salmonellae also may contaminate the mixtures of ice and water used to chill poultry carcasses after evisceration.

Meat inspection at present is a labor-intensive procedure with a high degree of subjectivity and is poorly suited to industries where volume and speed are essential for economic survival. Many disease conditions are subtle and difficult to detect by inspection alone, while there are other factors such as drug residues that can only be detected by laboratory tests. Thus, it seems likely that meat inspection will move progressively towards blood testing. Acute phase reactants are a group of plasma proteins produced during the acute phase of tissue inflamation and injury, and may provide a useful indicator for meat inspection purposes (Saini and Webert, 1991; Eckersall, 1992).

Lymphatic system

Blood is brought to the body tissues in arteries and is removed by veins. However, interstitial fluid from between the cells of a tissue also is removed by the lymphatic system. Lymph vessels have extremely thin walls, and the lymph fluid they contain is wafted along by body movements that massage the lymph vessels. A system of flap-like valves prevents backward movement of the lymph. As well as fluids, the lymphatic system also recycles proteins that leak from the vascular system. This is an important factor in the determination of the osmotic balance between the interstitial fluid and the blood (Mayerson, 1963). In starved animals, the scarcity of blood proteins unbalances the system and leads to the accumulation of interstitial fluid (edema).

If body tissues are invaded by disease-forming bacteria, some of the bacteria drift into the lymphatic system. The lymphatic system is arranged like a system of rivers leading to an estuary. The final opening of the system is called the right thoracic duct, and this returns the lymph to the vascular system at a point where the main veins of the body enter the heart. Lymph nodes with a gland-like appearance are located at regular intervals throughout the lymphatic system (Figure 1-27). Their function is to filter and destroy invading bacteria. When lymph nodes are successful, they prevent the spread of disease from the region of tissue that has been invaded. The activated lymphocytes of the lymphatic system may play a major role in attacking and destroying invading bacteria.

The lymph nodes that guard healthy tissues are compact in structure and pale brown in color.They become swollen and discolored when they are activated by invading bacteria. The meat inspector systematically examines the lymph nodes of the viscera and the dressed carcass. Lymph nodes that appear to be abnormal are sliced open for inspection. Knives must be re-sterilized once they have been used to open an infected lymph node. Once alerted to the presence of diseased tissue, the inspector determines the type and severity of the disease. The whole of the carcass or just the diseased parts may be condemned. It is essential, therefore, that any offals that have already been removed from the carcass can all be traced back to the carcass from which they originated. This also includes any blood which may have been collected as an ingredient for processed meat products. Blood for human consumption is usually collected with a hollow knife in order to minimize contamination from the surface of the carcass.

Tuberculosis is a bacterial disease transmitted from cattle to humans by the ingestion of milk or meat. Diseases also may be transmitted on by-products such as hides or fleeces. The bacteria that cause anthrax require free oxygen in order to form spores. Workers who handle infected hides or wool may be infected by skin contact or by inhalation. Fortunately, these two serious diseases are rare in the industrialized countries, and the every-day work of the meat inspector is really part of the overall system for the quality control of meat products. Most industrialized nations have a complex system of legislation relating to the disposal of condemned meat. In many cases, condemned meat can be rendered safe for consumption by cooking or prolonged freezing prior to sale.

There are many parasites that attack farm animals and retard their growth. In temperate climates, however, only a few types of parasite occur in the muscles of a dressed carcass. Trichinella spiralis is a small nematode worm that sometimes appears within bundles of muscle fibers in pork carcasses, particularly in wild boar meat produced as a specialty. A recent study (Lee and Shivers, 1987) shows that the nematode larva may in fact be located intracellularly within a nurse cell derived from a parasitized muscle fiber. If the worms are not destroyed as the meat is cooked (at about 60oC), they will reproduce in the human intestine. The larvae burrow through the wall of the intestine and through the body tissues. This causes a disease known as trichinosis. Although mild cases are not serious, heavy infections may be fatal. Pigs may become infected when they eat uncooked garbage or the flesh of rodents that carry encysted worms in their own muscles. Once a pork carcass is infected, the encysted worms are most likely to be found in the muscles of the tongue, diaphragm, larynx, abdomen or under the vertebral column (Kotula et al., 1984). The great problem for the meat inspector is that the encysted worms in pork are too small to be seen without a microscope. In Germany, pork carcasses are examined by a simplified microscope technique, but the number of infected carcasses that are detected is very small (Ten Horn, 1973). Thus, under typical commercial conditions, although no pork carcass can be guaranteed to be free from Trichinella, it is not a serious problem provided that the incidence of the parasite is kept low. For this reason, pork producers have the responsibility to cook any waste food or garbage that is fed to pigs, and consumers have the responsibility to make sure that all pork products are thoroughly cooked.

Echinococcus granulosus is a tapeworm, a cestode parasite. The adult tapeworm is quite small (8 mm long) and lives in the intestine of a dog or fox. The eggs of the parasite leave the body of the host in its feces. If a sheep eats grass contaminated by these eggs, the eggs hatch in the intestine of the sheep and the larvae migrate into the blood stream. The parasite then becomes lodged in the body tissues and grows to form a large (10 cm) hydatid cyst that contains inactive worms. The life cycle of the parasite is completed if hydatid cysts from the flesh of a dead sheep or from abattoir waste are consumed by another carnivore. Any parts of a lamb or mutton carcass that contain hydatid cysts are condemned by the meat inspector following post mortem examination. The hazzard to human health is in the possible contamination of human food by fecal material from dogs. A hydatid cyst may then develop in the human body. In order to prevent the completion of the parasite's life cycle through sheep, it is essential that dead sheep are properly disposed of, and that dogs are prevented from gaining access to abattoirs or to abattoir waste. In areas where there is a high incidence of this parasite, dogs should regularly be dewormed. There are various subspecies of E. granulosus that involve other herbivores and carnivores (Thompson, 1979).

Taenai saginata and T. solium are tapeworms that live in the human intestine. Contamination of feed for cattle and pigs by eggs from human feces completes a life cycle that leads to the presence of tapeworm larvae in meat. Cysticercus bovis is the larval form of T. saginata in beef, and Cysticercus cellulosae is the larval form of T. solium in pork. Cysticercae in meat appear as oval vesicles, almost a centimetre in length and with a white, gray or translucent appearance. Cysticercae are most commonly found in the heart and masticatory muscles. Cysticercae are detected during post mortem examination, after cuts have been made through these muscles. Once a cyst has been found, carcasses should be cooked or made safe by prolonged freezing (Juranek et al., 1976).

Carcass Refrigeration

Carcasses are chilled to reduce the microbial spoilage of their meat. The rate of chilling is determined by the temperature, the relative humidity and the velocity of the air in the meat cooler. In addition to direct heat losses by conduction, convection and radiation, heat is lost from a carcass when water evaporates on its surface. Carcasses cool rapidly if they have a large surface area relative to their mass, and if they have only a thin covering of subcutaneous fat for insulation.

The heat exchange units in meat coolers resemble automobile radiators filled with a refrigerant. The refrigerant is a gas that has been compressed to a liquid by a powerful compressor. Compression liberates heat, and the hot liquid is now pumped to another unit, usually outside on the roof of the meat cooler. The hot liquid, still under pressure in a pipe, is cooled as it passes its heat to the atmosphere or to a water fountain. The cold liquid then is pumped through a small orifice that resembles, in principle, the carburator of an automobile. The conversion of a liquid to gas absorbs heat, so that the resulting gas is very cold. This cold refrigerant passes through the heat exchange units inside the meat cooler where it cools the air inside the meat cooler.

The air inside the meat cooler is kept moving by powerful fans. The arrangement of heat exchangers and their fans in the meat cooler is carefully planned to produce an even distribution of cold air. However, there is always a problem in overcrowded coolers. When hot carcasses are first placed in a cooler, a high air speed is maintained to accelerate initial cooling. Later on, the air speed is reduced so that the surface of the carcass does not become desiccated. To minimize surface dehydration, the air should not blow directly on the carcasses.

Evaporation losses from pork carcasses may be reduced by rapid cooling after slaughter. Pork carcasses can be briefly pre-chilled by very cold air or by immersion in liquid nitrogen for about thirty seconds, sufficient to precool the skin without cracking it (Anon., 1981b). In bad conditions, evaporation losses from pork carcasses within 24 hours of slaughter may exceed 3% of the initial hot carcass weight. With rapid chilling, evaporation losses may be held below 1% in the first 24 hours after slaughter. In laboratory conditions, rapid chilling may enhance the quality of the pork by reducing the incidence of paleness and softness (Borchert, 1972), but there is also some risk causing a deterioration in meat quality under commercial conditions. Evaporation losses from pork carcasses may also be minimized by the use of a spray-on cellulose film (Anon., 1982). Crenwelge et al. (1984) found that color scores for ham muscles were increased favorably by more rapid chilling.

Large carcasses take a long time to cool to 0oC in an ordinary meat cooler, and it is not usually until the day after slaughter that the deep parts of a beef carcass reach the temperature of the surrounding air. Shrinkage due to water loss by evaporation causes economic losses in beef carcasses. The acceleration of carcass cooling by cold water sprays reduces shrink losses, but there may be problems with microbial spoilage even if the water is chlorinated. Intermittent spraying with dilute (1%) acetic or lactic acid solutions, however, largely prevents surface microbial spoilage (Hamby et al., 1987). Once carcasses have been chilled after slaughter, they may be stored just above 0oC at a relative humidity of 90% with some slight air movement (0.3 m/sec). Higher relative humidities reduce evaporation losses but also encourage surface spoilage by micro-organisms.

Meat must never be frozen before rigor mortis has occurred and the meat has become stiff and inextensible. When meat that has been frozen before the onset of rigor mortis is thawed prior to consumption, it undergoes thaw shortening and becomes very tough. Even if the meat is not frozen, cooling that is too rapid may also make the meat very tough. This is called cold shortening. Beef carcasses should not be subjected to air less than about 5oC with a velocity over 1 meter per second within 24 hours after slaughter (Cutting, 1974). The temperature of lamb carcasses should not be forced below 10oC within 10 hours of slaughter. These topics are considered in more detail in Chapter 9.

There is usually no reason why meat cannot be removed from the carcass while it is still warm (hot-boning or hot processing). It is more difficult to handle floppy warm meat, but requirements for refrigerated storage space are reduced and there are favorable changes in the water holding capacity of meat so that drip losses are reduced. The temperature, the hygiene and the shape of isolated pieces of hot meat must be carefully controlled (Cuthbertson, 1980). Electrical stimulation may be used to accelerate the conversion of muscles to meat so that hot-boning or accelerated processing can be undertaken. Satisfactory results have been obtained, even with pork carcasses (Neel et al., 1987). Presumably, in this situation, the tendency for electrical stimulation to cause PSE (pale, soft, exudative) pork is offset by the accelerated rate of meat cooling.

At present, the least expensive method of chilling poultry is by immersion in a mixture of water and ice. Carcasses have a water uptake of 8 to 12% and microbial contamination is controlled by chlorination. Spray cooling wastes water while air cooling may cause dehydration of the carcass (Lilliard, 1982). However, old-fashioned air cooling helps to preserve the flavor of poultry meat and may command a premium price for the product.


The overall objective of rendering is to produce clarified homogeneous substances such as lard and tallow from a heterogeneous mixture of animal trimmings and scraps. Because of the vast volume of such items produced by a large abattoir, rendering and waste product utilization are major factors in the economics of meat processing. Visceral adipose tissue that has been maintained in a wholesome condition is rendered to produce lard and tallow for cooking or for further processing in the food industry. Intrinsically dirty parts of the carcass such as the head and feet are subjected to inedible rendering as the first step in the manufacture of soap and grease.

In the now out-dated method of wet rendering, steam was injected into a pressurized tank full of trimmings with a high fat content. Eventually, the molten fat floated freely on top of an aqueous solution with a high protein content. At the bottom was a slurry of solids. The partial recovery of proteins from the aqueous layer was achieved by a subsequent evaporation process. In dry rendering, the steam is confined to a jacket around the tank, and the contents of the tank are held at a negative pressure. This enables a far greater recovery of proteins which would otherwise greatly elevate the BOD of the abattoir effluent. Thus, a general principle of modern abattoir operations is to minimize the amount of water that is added to animal wastes as they are cleaned from the premises. Not only is water expensive, but much of it has to be removed later by evaporation, and this uses a considerable amount of energy. Many inedible waste products such as clotted blood, bone dust and manure from the rumen and from stockpens are best maintained for recovery operations in as dry a condition as is possible. The engineering techniques that may be used to achieve this goal are described by Jones (1974), but further progress towards reducing water and waste is still required. Anaerobic processing and methane production might provide the best treatment for abattoir effluent with a relatively high content of plant material from rumen and stomach contents (Tritt and Schuchardt, 1992).


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