LAB 8.1 Forequarter skeleton

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

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.