THE EXTENSOR MUSCLES
The extensor muscles constitute a large mass that fills the triangle between the scapula and humerus. The group comprises the triceps, tensor fasciae antebrachii, and anconeus.
The triceps is by far the most important extensor of the elbow.
It presents three heads (Figure 23-6, B/10,10). The long head arises from the caudal border of the scapula by a short aponeurosis, and the lateral and medial heads arise from the shaft of the humerus. Together they insert on the olecranon where a small bursa is inserted between the tendon and the bone. The division between the long and lateral heads is sometimes visible in thin-skinned animals. A second, acquired (adventitious) bursa is commonly found subcutaneously, over the triceps insertion and expanded part of the olecranon tuber (“capped elbow”; Figure 23-11/5).
Figure 23-10 Medial view of left elbow joint to show the eccentrically placed collateral ligament and the insertions of biceps and brachialis. The internal tendon (5) of the biceps splits off the lacertus fibrosus (5") from the surface of the muscle. 1, Olecranon; 2, humerus; 2’, medial epicondyle; 3, axis of rotation; 4, 4', long superficial and short deep parts of medial collateral ligament; 5, biceps; 5, internal tendon of biceps; 5", lacertus fibrosus; 6, brachialis; 7, radius.
The triceps is extensor to the elbow. Since the long head spans the shoulder joint, it is theoretically available to flex this joint; however, it is probably little used for that purpose.
The tensor fasciae antebrachii (Figure 23-8/6) is a broad, thin sheet covering the medial aspect of the triceps. Its origin is from the caudal border of the scapula and the tendon of the latissimus, while its insertion is spread between the olecranon and forearm fascia. Since it crosses both shoulder and elbow joints, it must be considered as having a potential action at each; neither is likely to be of great importance.
The much smaller anconeus lies within the olecranon fossa, embedded within the deep face of the lateral head of the triceps and directly related to the capsule of the elbow joint. It may be supposed that its principal action is to tense the capsule, thus preventing it from being pinched between the humerus and ulna (Figure 23-12/4).
The radial nerve supplies all muscles of the extensor group.
Figure 23-11 Synovial structures of the left shoulder and elbow regions; lateral view. 1, Shoulder joint capsule; 2, infraspinatus bursa; 3, intertubercular bursa (between biceps tendon and humerus); 4, elbow joint capsule; 5, subcutaneous olecranon bursa; 6, subtendinous olecranon bursa. (For identification of the muscles see Figure 23-6, B.)