LAB 8.2 Forequarter muscles



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Neck muscles
 
   The neck muscles produce tough meat because of their high connective tissue content. Like other tough muscles in the carcass, they are usually removed from their bones (cervical vertebrae) and are used as stewing meat or ground beef. Since the anatomy of the neck muscles is rather complex, and since their individual identity is of no consequence once the carcass is subdivided, they need not be considered in detail here. The only muscle worthy of passing note, however, is a strap‑like muscle called the sternomandibularis. This muscle forms the superficial part of a compound muscle in the ventral throat region running from the head to the sternum. Because this muscle is severed when the head of a beef carcass is removed at slaughter, samples may be obtained immediately after slaughter without damaging a commercial carcass.

SHOULDER MUSCLES .
   Shoulder muscles are intermediate in their level of toughness and are usually completely cooked in order to make them tender. An easily recognized group of muscles and bones enables meat from the shoulder or chuck region to be readily identified. The supraspinatus is dorsal to the spine or ridge on the scapula, while the infraspinatus is ventral to the scapular spine. The trapezius is located superficially between the left and right scapular blades. The rhomboideus is ventral to the trapezius. Figuratively speaking, if one were to stab a standing animal between its shoulder blades, the knife would pass through the trapezius first and then through the rhomboideus. The subscapularis is located on the flat medial face of the scapula, towards the ribs. The biceps brachii is anterior to the humerus in an equivalent position to the biceps muscle in the human arm. The adjective brachii is needed to indicate the biceps muscle of the arm, since there is another biceps muscle, the biceps femoris, located in the hindlimb. The triceps brachii is a large muscle located in the triangular area bounded by the ventral edge of the scapula and the posterior edge of the humerus. Its name, tri‑ceps or three‑heads, indicates that this large triangular muscle is subdivided. Thus, when seen in a cut of meat from the shoulder, the triceps brachii may look like more than one muscle.
 



In the view of the superficial muscles (left diagram above) A is the pectoralis, B is latissimus dorsi, C is the position of the longissimus dorsi, D is triceps brachii, E is infraspinatus and G is trapezius. The deep dissection (right diagram above) shows serratus ventralis (A) and rhomboideus (B).


The above diagram is a section through the shoulder to show trapezius (1), rhomboideus (2), supraspinatus (3), the spine of the scapula (4), longissimus dorsi (5), serratus ventralis (6), suscapularis (7), infraspinatus (8), an edge of latissimus dorsi  (9), triceps brachii (10), a thoracic vertebra (11) and a rib (12).

Distal muscles of the limbs
   The distal muscles of the limbs produce tough meat because of their high content of connective tissue. In the distal regions of the forelimb (shank) and hindlimb (leg) are groups of fusiform (cigar‑shaped) muscles with long tendons which extend distally towards the toes or phalanges. If these muscles are cut open longitudinally, it may be seen that most of them contain internal tendons that fan outwards for the attachment of short bundles of muscle fibers. This feather‑like arrangement of muscle fiber bundles is called a pennate structure. In meat animals, the meat derived from pennate muscles is quite tough, because of their high connective tissue content.
   Most of the distal muscles located anteriorly in the limb are extensors and, when they contract in the living animal, they cause the toes to move forwards, as in the start of a new stride. Most of the muscles located posteriorly in the distal part of the limb are flexors which bend the limb during locomotion. In beef and lamb carcasses, tendons from distal muscles pass down the length of the cannon bone and are kept in place by ligamentous rings. Thus, the extremities of a limb are moved by remote control. Beef cannon bones are discarded in the abattoir because they have virtually no meat on them. Flexors and extensors from the distal parts of the limbs are difficult to identify individually once they have been removed from the skeleton.
 
Muscles of the ribcage
    Located between the scapula and the ribcage are several muscles that hold the forelimb onto the body. Like other shoulder muscles around the scapula, most of these muscles are intermediate in their level of toughness. The serratus ventralis is a large fan‑like muscle that radiates from the medial face of the scapula and attaches to the lateral surfaces of the ribs. The serratus ventralis is the major component of a muscular sling that suspends the thorax of an animal from between its forelimbs. This muscular suspension system has no counterpart in the hindlimb since the pelvis is fused to the vertebral column. The muscular sling that holds the forelimb onto the body serves as a shock absorber during locomotion.
 
   The longissimus dorsi is an extremely important muscle. It forms the eye of meat seen when chops and steaks are cut from the posterior rib region and loin. The naming of this muscle is rather a problem since it is really a compound muscle composed of many subunits.  Each subunit acts over the length of several vertebrae and helps to flex the vertebral column. The longissimus dorsi also is involved in respiratory movements, as well as helping to move the neck. Because of its compound structure, the longissimus dorsi may be called by an alternative name, longissimi thoracis et lumborum. In agricultural journals the longissimus dorsi is often simply called the eye‑muscle or longissimus muscle. The muscle fiber bundles of the longissimus dorsi are arranged at an acute angle to the vertebral column. The cross sectional area of the longissimus dorsi increases towards the posterior part of the ribcage, but it has an approximately constant cross sectional area through the loin. Beef carcasses are usually split into forequarters and hindquarters between ribs 12 and 13. The area of the longissimus dorsi seen at this point is often measured or examined in order to assess the amount of meat in a carcass. This may be a useful guide to muscularity when comparing animals with a similar carcass length. However, in the comparison of long carcasses with short carcasses, a smaller cross sectional area does not necessarily indicate a smaller muscle mass, since the mass is spread over a greater length.
 
   The pectoralis muscle is located over the sternum in the brisket, and it extends posteriorly into the plate. The pectoralis is composed of deep and superficial layers. The intercostal muscles are located between adjacent ribs in the wall of the chest, and there are two layers in the depth of the muscle. Intercostal muscles make an important contribution to the meat content of North American pork spare ribs.
 
Loin muscles .
   The loin muscles give rise to tender meat with a desireable taste, and they command a high price when presented for sale as steaks or chops. The longissimus dorsi extends posteriorly from the rib region, it runs through the loin, and most of the muscle terminates on the anterior face of the ilium. Thus, the longissimus dorsi is seen in cuts of meat taken through the ribs and loin, but not in cuts of meat such as sirloin steaks that are posterior to the anterior face of the ilium. The longissimus dorsi is dorsal to the transverse processes of the lumbar vertebrae, and it is dorsal to the ribs in the thoracic region. For most of the length of the ribcage, there are no major muscles immediately ventral to the heads of the ribs. Thus, in a rib steak, there is only one main eye of meat, and that is the longissimus dorsi dorsal to the ribs. However, in the loin, there are muscles both above and below the level of the transverse processes of the lumbar vertebrae. The dorsal muscle above the transverse processes is the longissimus dorsi. The ventral muscle below the transverse processes is the psoas major or filet muscle. The psoas major originates ventrally to the last couple of ribs in the ribcage. The cross sectional area of the psoas major increases towards the sirloin. Medial to the psoas major, almost under the centra of the vertebrae, is a smaller psoas muscle called the psoas minor. The letter P in the word psoas is silent when the word is spoken.



   Immediately lateral to the dorsal spines of the vertebrae (medial to the longissimus dorsi) are some small multifidus dorsi muscles. The longissimus costarum is a relatively small rope‑like muscle, dorsal to the ribs. It appears as a small eye of meat at the separation between the forequarter and the hindquarter. The multifidus dorsi and the longissimus costarum have little commercial significance, since they are such small muscles, but they may create a problem in the measurement of rib‑eye areas since they might be accidently included with the longissimus dorsi. IThese are shown in the bottom part of the diagram below (we will leave the top two parts of the diagram - from the hindquarter  - for  LAB 9).