STRUCTURE OF THE SKELETON

Skull

Sheep

• The brain is situated within the cranium - a box-like posterior part of the skull.

• The brain is connected to the spinal cord through a large hole, the foramen magnum.

• The foramen magnum is flanked by two large knobs or occipital condyles that form a joint with the first cervical vertebra of the neck.

• The skulls of meat animals are damaged in the frontal bone region if animals have been stunned by concussion. Sinuses or spaces are present between the inner and outer cranial walls. Young pig

• There are considerable differences in the size and shape of the skull in different breeds of farm mammals, particularly in pigs where a long narrow skull is a feature that is often associated with a relatively large amount of fat in the carcass.
• The major muscles used in chewing are attached to the coronoid process which is a large expansion of the lower jaw or mandible.

  Mature pig skull showing: C, condyle of mandible; ca, canine teeth; F, frontal bone; i, incisor teeth; M, mandible; mo, molar teeth; MX, maxilla, N, nasal bone; O, occipital bone; P, parietal bone; PM, premaxilla; pmo, premolar teeth; PP, paramastoid process; and Z, zygomatic arch.

• The coronoid process is located medially to the zygomatic arch, between the eye and the ear. The coronoid process allows muscle leverage to be exerted onto the mandible. The joint between the skull and the lower jaw is formed by a mandibular condyle. In cattle and sheep, the mandibular condyle is relatively flat and allows considerable movement in a horizontal plane. Lateral movement is important in animals whose teeth work with a grinding action.
• The jig-saw pattern of suture joints on the skull surface indicates that the whole skull is formed by the fusion of a number of individual bones.
• A saw cut made transversely through the facial region of the skull reveals delicate rolls of the turbinate bones in the nasal cavity. The turbinate bones support a large area of nasal epithelium to warm and moisten the air travelling to the lungs, and to provide a large area for the sense of smell.

Skeleton of the neck

Atlas (on left) and axis of sheep.

• The vertebral column or backbone is the main axis of the skeleton and it protects the spinal cord. The spinal cord is located in a neural canal formed by a long series of neural arches, each contributed by a different vertebra.

  

• The neural arch of each vertebra is supported on the body or centrum of the vertebra. In some types of vertebrae, the neural arch extends dorsally as a prominent spine that may be called a dorsal spine, a neural spine or a spinous process.

• Where movement between vertebrae is possible, the centra are separated by cartilaginous intervertebral discs. In mammals, the anterior and posterior faces of the centra are almost flat.

• The names and numbers of the different types of vertebrae in meat animals are variable.

  NAME--------REGION---------BEEF----------PORK------------LAMB

  Cervical------Neck-------------7--------------7----------------7

  Thoracic-----Ribcage-----------13------------13 to 17-----------13 to 14

  Lumbar-------Loin-------------6-------------5 to 7------------6 to 7

  Sacral ------Sirloin-------------5-------------4-----------------4

  Caudal-------Tail------------18 to 20----------20 to 23----------16 to 18

  

• Meat animals, like most other living mammals, usually have seven cervical vertebrae in the neck region. However, sheep sometimes have only six cervical vertebrae, and as few as five cervical vertebrae have been reported in pigs.

• In cattle and sheep, but to a lesser extent in pigs, the neck is very mobile, and the cervical vertebrae have a series of interlocking articular and tranverse processes that limit excessive bending of the neck to protect the spinal cord.
• The first cervical vertebra, the atlas, articulates with the skull and is greatly modified in shape to form a joint that enables the animal to nod its head up and down.

• Rotation or twisting of the head occurs from the joint between the atlas and the next cervical vertebra, the axis.

• The ligamentum nuchae is a very strong elastic ligament in the dorsal midline of the neck, and it relieves the animal of the weight of its head. Were it not for the ligamentum nuchae, the head of the standing animal would droop between its forelimbs.

  Plan of neck in beef, showing:1, ligamentum nuch; 2, atlas; and 3, axis. The ligamentum nuchae is pale yellow with a thick cord-like or funicular part and a flat sheet-like or lamellar part. Once the head is removed at slaughter, the elasticity of the ligamentum nuchae causes the neck of the carcass to curve dorsally.

• Beef and pork carcasses usually are split into right and left sides soon after slaughter and the series of vertebral centra that are now exposed is called the chine bone.

Ribcage

Sheep

• The number of vertebrae in pork carcasses is rather variable, particularly in certain breeds. The heritability of the number of vertebrae is about 0.74. Each extra vertebra adds about 15 mm to the length of the carcasses at slaughter weight.
• The number of thoracic vertebrae, each bearing left and right ribs, ranges from 13 to 17. Breeds with a large size when mature and with heavy bone development tend to have more thoracic vertebrae than lighter breeds. Sometimes the ribs on extra thoracic vertebrae are only partially formed, but usually they are complete.

• The minimum number of lumbar vertebrae is generally found in cacasses with the maximum number of thoracic vertebrae. However, the variability of vertebral numbers frequently leads to an increase in the total number of vertebrae, so that the phenomenon is not due simply to the substitution of one type of vertebra for another.

• Lamb carcasses usually have either 13 or 14 thoracic vertebrae and a corresponding number of pairs of ribs. Experimental studies suggest that the number of vertebrae in an animal is determined by the number of somites that develop along the length of the spinal cord. By definition, in mammals, the vertebrae that bear ribs are identified as thoracic vertebrae. In the embryo there are ossification centers on each side of the developing vertebrae. In vertebrae that do not normally develop ribs, these lateral ossification centers contribute their bone tissue to the centra of adjacent vertebrae. In the thoracic vertebrae, however, this laterally derived bone tissue remains separate from the centra and forms the ribs. Thus, the numbers of pairs of ribs and the numbers of thoracic vertebrae are determined by the developmental mechanism that controls the fate of the tissue which is derived from the lateral ossification centers.

• The cage formed by thoracic vertebrae, ribs and sternum is an essential component of the respiratory system. Thoracic vertebrae are distinguished by their tall dorsal spines, many of which point towards the hindquarter and are known as the feather bones.

• The ribs are joined to the vertebral column dorsally so that the head of each rib articulates with the bodies of two adjacent vertebrae. Rib showing : 1, head; 2, neck; 3, tubercle; and 4, costal cartilage.Each rib has a tubercle that articulates with the transverse process of the more posterior of its two vertebrae. Ventrally, the anterior ribs articulate with the sternum and are termed sternal ribs. The more posterior ribs are called asternal ribs and they only connect to the sternum indirectly via costal cartilages. The most posterior ribs have only small costal cartilages that do not reach all the way to the sternum. Some of the costal cartilages are very hard and may appear more like bones than typical cartilage.

• The sternum is formed by a number of closely joined bones, the sternebrae. When split through the midline, the interior structure of the sternebrae resembles that found in the centra of the vertebrae. But in an isolated cut of meat, the distinction between sternebrae and vertebral centra may be made by the presence or absence of a neural canal. A neural canal is seen in the vertebrae of carcasses that have been symmetrically separated into right and left sides.

• The structure of the ribcage is rather variable in lamb carcasses. Carcasses have been found with as few as 12 ribs on one side, and left and right sides of the ribcage may differ in their number of ribs. In lambs, rib length is determined mainly by age while the plane of nutrition determines rib thickness.

-----------------------BEEF---------PORK------------LAMB

Total pairs of ribs-------------13----------13 to 17-----------13 to 14

Pairs of sternal ribs-------------8-----------7-----------------8

Pairs of asternal ribs------------5-----------7 to 8-------------5 to 6

Number of sternebrae-----------7-------------6--------------6 to 7

The skeleton of the loin, sirloin and rump

Sheep pelvis

• In a live animal, the lumbar vertebrae act like a suspension bridge to support the weight of the abdomen. The lumbar vertebrae have flat, wing-like transverse processes that broaden the abdominal cavity dorsally to provide a strong attachment for the muscles of the abdominal wall that carry the weight of the viscera.

  Plan of kumbar region in hanging beef carcass showing: 1, ilium; 2, sacrum;3, last lumbar vertebra;4, first lumbar vertebra; and 5, last rib.

• The propulsive thrust generated by the hindlimb during locomotion is transmitted to the sacral vertebrae by the pelvis. To strengthen the sacral vertebrae, they are fused together to form the sacrum.

  Plan of the sacral region in a hanging beef carcass showing:1, ischium; 2, femur;3, sacrum; and 4, ilium.

  Fusion of the sacral vertebrae to form the sacrum is incomplete in young animals and provides an important clue to animal age in the dressed carcass.

• The pelvis is formed by three bones on each side. The most anterior bone on each side is the ilium. The shaft of the ilium expands anteriorly to form a flat wing attached to the sacrum. This joint is called the slip joint. When seen in a sirloin steak, the ilium may appear either as a small round bone or a large flat bone. The anterior edges of the ilia form the hooks of the live animal. The most posterior bone of the pelvis on each side is the ischium.

• The pelvis and the sacrum form a ring of bone completed ventrally by the pubes. The left pubis is separated from the right pubis by fibrocartilage which, at parturition, may soften to allow movement between the bones of the pelvis. The pubes are separated when carcasses are split into left and right sides in the abattoir.

  Plan of the pelvis in a hanging beef carcass showing:1, lesser sciatic notch; 2, ischiatic spine; 3, greater sciatic notch; 4, psoas tubercle; 5, obturator foramen; 6, symphysis pubis;7, ischium; and 8, ilium.

• The pubic bone exposed on a carcass is called the aitch bone. The aitch bone is curved in steer and bull carcasses, is moderately curved in heifers, but is straight in cow carcasses.

  

  Another plan of the both sides of the pelvis in a hanging carcass showing: 1, tiber coxae; 2, acetabulum; 3, acetabular ramus of ischium; 4, tuber ischii; 5, symphysis pubis; 6, ilium; 7, pibis; and 8, ischium.

• Only two caudal or coccygeal tail vertebrae are left on a commercial beef carcass.

The shape of the symphysis pubis seen on a side of beef is a useful guide to the sex of the carcass: 1, steer; 2, heifer; and 3, cow, where x shows the position of the pizzle eye.

Forelimb skeleton

Scapula

• The most proximal bone of the forelimb is the blade bone or scapula.This is the shape of the blade bone in beef. The scapula is not fused to the vertebral column (like the pelvis in the hindlimb), and this allows muscles that hold the scapula to the ribcage to function as shock absorbers during locomotion.

  The scapula has a distal socket joint for the next bone in the forelimb, the humerus. This socket joint is called the glenoid cavity . The glenoid cavity is wide and shallow, unlike the ball and socket joint in the hindlimb which is narrow and deep. Along the dorsal edge of the scapula, the bone merges with flexible hyaline cartilage. On the lateral face of the scapula is a prominent ridge of bone called the spine of the scapula. In beef carcasses, the scapular spine is extended distally as a prominent acromion process.

  Humerus

• Proceeding distally down the forelimb, the bone that articulates with the scapula is the humerus. Proximally, the humerus has a relatively flat knob or head to fit into the glenoid cavity of the scapula. Two well defined condyles on the distal end of the humerus contribute to the hinge joint at the elbow.

  Radius & Ulna

  Beef shankbones showing: 1, distal end of humerus; 2, olecranon fossa; 3, olecranon process;, 4,radius; 5, ulna; and 6, carpal bones.

  The radius is joined to the ulna and is the shorter and more anterior bone of the pair.

• Beef and lamb carcasses have a set of six compact carpal bones remaining on the carcass after slaughter. Before slaughter, the forefeet of cattle and sheep have a large cannon bone located distally to the carpal bones.

  A large plan of the pork foot compared to a small plan of beef, showing:

  AC, accessory carpal;

  C, carpal;

  IC, intermediate carpal;

  MC, metacarpal;

  P, phalanges;

  RC, radial carpal;

  and UC, ulnar carpal.
  

  Beef cannon bones are removed with the feet at slaughter since there is virtually no meat on them. Cannon bones sometimes are left on lamb carcasses in the abattoir to prevent the meat contracting proximally up the limb. Each forelimb cannon bone in ruminants is derived by the enlargement and fusion of the third and fourth metacarpal bones.

• In the human hand, the metacarpal bones lie in the flat part of the hand, between the wrist and the knuckles. If the human hand is placed flat on a desk and is slowly lifted from the wrist, the thumb (digit 1) is the first digit to leave the desk, followed by the index finger and the little finger (digits 2 and 5, respectively). The third and fourth fingers remain on the desk. This demonstrates how the feet of meat animals may have evolved from an original basic plan with five digits.

• In pigs, digits 3 and 4 on each foot bear most of the body weight and are larger than the lightly loaded digits 2 and 5. The first digit is absent in pigs. The evolutionary trend towards lifting of the foot and reduction of digits is even more extensive in cattle and sheep.

• Cattle and sheep have cursorial limbs, long limbs adapted for running. Digits 2 and 5 are reduced to dew claws behind the fetlock. Weight-bearing digits 3 and 4 are enlarged, and their metacarpals are fused to form a long cannon bone.

• The small bones in the toes of both fore and hind feet are called phalanges.

• The feet remain on pork carcasses when they are shipped from the abattoir in shipper's style (unsplit carcasses with head plus perirenal or leaf fat) or in packer's style (split sides with leaf fat and head but not jowls removed). However, in a Wiltshire side for bacon production, the feet are removed, together with the head, pelvis, vertebral column and psoas muscles.

Hindlimb skeleton

Femur

• The proximal bone of the hindlimb is the femur or round bone.

  

• exitThe articular head of the femur is deeply rounded and it bears a round ligament that holds it into the acetabulum. Another distinctive feature of the femur is the broad groove between the two trochlear ridges located distally. The patella or knee cap slides in this groove. The tension generated by muscles above the knee is transmitted over the knee or stifle joint by the patella to avoid having an important tendon in a vulnerable position over the anterior edge of a joint.

• In beef and lamb carcasses there is a single major bone, the tibia or shank bone, located distally to the femur. In the corresponding position in a pork carcass there are two parallel bones, a large tibia and a more slender fibula.

  Tibia and fibula of the pork carcass drawn to the same height and showing:

  1, medial condyle,

  2, lateral condyle;

  3, tibia, and

  4, fibula.
  

  The presence of parallel bones suggests that, at some point in an animal's evolutionary past, rotation of the limb about its axis was possible. For example, rotation of the human wrist involves a partial crossing of the widely spaced ulna and radius but limb rotation is reduced as animals develop cursorial limbs. In cattle and sheep, one of the parallel bones, the fibula, has lost its shaft. Only a remnant of the head of the fibula may be found. In pigs, the fibula retains its shaft and the bone is mobile at birth. After a few years, however, the fibula becomes fused to the tibia.

• Distal to the tibia are the tarsal bones of the hock. The structure of the tarsals, metatarsals and phalanges of the hindlimb is similar to that of the carpals, metacarpals and phalanges in the forelimb. Pork carcasses normally are suspended by a gambrel or hooked bar placed under the tendons of the hind feet. Beef carcasses normally are suspended by a hook under the fibular tarsal bone. This bone projects posteriorly and has a rough knob, the tuber calcis, for the insertion of the Achilles tendon at the hock.

Poultry skeleton

• The skeletons of poultry are radically different from those of the farm mammals. Not only is the avian skeleton adapted for flight, but birds and mammals are only distantly related zoologically.
• The skull has very large eye orbits and a small cranial cavity.

• The long double curved neck contains 14 cervical vertebrae, and the ring-like atlas articulates to the skull with only a single occipital condyle. The axis has a large odontoid process that projects anteriorly.
• There are 7 thoracic vertebrae, but numbers 2 to 5 are fused. Thoracic vertebrae 6 can move freely, but the last thoracic vertebra is fused to the synsacrum. The synsacrum is a fused length of the vertebral column that contains thoracic vertebra 7, 14 lumbo-sacral vertebrae, and the first coccygeal or caudal vertebra, but skeletal fusion in the vertebral column does not occur for many weeks after hatching.
• There are six caudal vertebrae that, apart from the first, are free and mobile. However, only numbers 2 to 5 are normal vertebrae, since the last one is formed into a three sided pyramidal bone called the pygostyle.
• There are seven ribs: the first two are free while the last five are attached to the sternum. There are no costal cartilages. Ribs 2 to 6 each have an uncinate process which overlaps the next posterior rib.

• The sternum is extremely large. It has a conspicuous ventral ridge in the midline, the CARINA, which increases the area available for attachment of the flight muscles. The dorsal surface of the expanded sternum is concave and forms the floor of a continuous thoracic and abdominal cavity.

• The bones of the fore limb are greatly modified to form the wing.

  

  Plan of wing bones showing: 1,2 and 3, digits; 4, carpometacarpus; 5, radius; 6, ulna; and 7, humerus. (The image at the top corresponds to the distal part of the plan view)

  Distal to the humerus are the widely spaced radius and ulna. The carpals, metacarpals and digits are reduced to form a stiff skeletal unit for the anchorage of the primary flight feathers. The three digits of the wing are equivalent to digits 2, 3, and 4 in other animals.

• The wing articulates with the body at the glenoid cavity which is strengthed by the convergence of three bones, the scapula, the coracoid and the clavicle.

  Plan view showing:

  1, glenoid;

  2, scapula;

  3, coracoid,

  4, clavicle; and

  7, carina of sternum.

  
  

  In birds the coracoid is a separate bone, whereas in mammals it has been reduced to a small integral part of the scapula. The clavicles of right and left sides are fused ventrally to form the furcula or wishbone. Although many mammals have a pair of clavicles, they are absent in cattle, sheep and pigs. The clavicle functions as a strut to support the shoulder joint in animals which have complete mobility of the shoulder joint. Since cattle, sheep and pigs have cursorial limbs with a restricted fore and aft movement, they do not need clavicles. In poultry, the distal end of the coracoid is braced against the sternum. In flight, the body of a bird hangs from its wings at the shoulder joint, hence the more elaborate support for the glenoid cavity.

• In poultry, the legs show many cursorial adaptations. Distal to the femur, the fibula is reduced to leave the tibia as the major bone.

  Plan of poultry leg showing:

  1,2,3, and 4, digits,

  5, tarsometatarsus;

  6, tibiotarsus;

  7, fibula; and

  8, femur.
  

  In the embryo this occurs as a result of the differential growth and translocation of the distal part of the fibula to become part of the tibia. The proximal tarsal bones are fused to the distal end of the tibia to increase its length, and the whole skeletal unit may be called the tibiotarsus. The distal tarsal bones are incorporated into the proximal end of a single bone, the tarsometatarsus, which also includes the fused metatarsals 2, 3 and 4. Of the four digits which form the bird's claw, digit 1 is directed posteriorly while digits 2, 3 and 4 are anterior.

  

  This adpatation enables the bird to perch. The ilium is fused to the synsacrum. Instead of being fused in the midline, the pubic bones are separate, and they project backwards as thin rods.

  

  Plan of poultr pelvis (anterior to the left) showing:

  1, obturator foramen;

  2, acetabulum;

  3, sciatic foramen;

  4, ilium;

  5, ischium; and

  6. pubis.
  

  The open structure of the pelvis in the ventral region facilitates the passage of eggs from the body cavity. The ilium, ischium and pubis all contribute to the acetabulum, but the ilium forms more than half of the socket and the floor is membranous.