THE SHOULDER AND ARM
The scapula and humerus, and the associated muscles, are enclosed within the skin of the trunk and held closely against the thoracic wall. Some cows, especially Jerseys, stand with their shoulder and elbow slightly abducted, which causes the humerus to angle away from the ribs.
This defect, which seems to arise from inherited weakness of certain girdle muscles, looks awkward but is of little consequence (Figure 30-1). It is not to be confused with “flying scapula,” which is a serious myopathy observed in cattle turned out to pasture in the spring. In this condition, muscle tissue actually degenerates, which causes the dorsal border of the scapula to rise above the withers.The position and slope of the bones can be determined by palpation of certain features: the cranial and caudal angles and the spine of the scapula, and the greater tubercle and deltoid tuberosity of the humerus. The capacious shoulder joint may be punctured at the cranial border of the infraspinatus muscle, just proximal to its insertion on the greater tubercle.
Only those muscles that claim practical attention will be noticed (Figure 30-2). The brachiocephalicus forms the dorsal border of the jugular groove and is joined along its upper margin by the omotransversarius extending between the acromion and the wing of the atlas; the latter muscle covers but does not prevent palpation of the large superficial cervical lymph node. The pectoral group is distinguished by the very rudimentary development of the subclavius; this explains the very abrupt transition from the narrow neck to the much greater breadth at the level of the shoulder joint—a striking difference in conformation between cattle and horses. The rhomboideus rarely attracts attention in cattle of European origin but makes the major contribution to the hump in Zebu stock. The hump varies in position (cervicothoracic or thoracic) and structure in animals of different breeds and strains; in some it is essentially a thickening of muscle, and in others it is a replacement of flesh by fat.
The serratus ventralis, the principal supporter of the trunk, is adapted to this role by the inclusion of many tendinous strands and a stout aponeurotic covering. Its occasional rupture, a disaster of the first magnitude, is made very evident by the projection of the scapular cartilage above the dorsal contour of the thorax (see Figure 26-1).The superficial branch of the infraspinatus tendon is protected by a synovial bursa where it passes over the lateral face of the greater tubercle. The bursa is sometimes the seat of a painful inflammation made obvious by abduction of the arm. The tendon of the biceps brachii is also protected by a synovial (intertubercular) bursa on its deep face; it is a role assumed by a pouch of the shoulder joint capsule in sheep and goats. In the distal part of the arm the biceps detaches a lacertus fibrosus that is palpable despite being much weaker than that of the horse; the lacertus descends in front of the elbow to blend with the covering of the extensor carpi radialis. Two other bursae are associated with the insertion of the triceps: one is interposed between the tendon and the olecranon, and the other, inconstant, is between the tendon and the skin over the point of the elbow.