16 THE ANATOMY OF THE HORSE. 



the first into the inner surface of the shaft of the humerus above the 

 internal tubercle, the second into a line which begins on a level with 

 the internal tubercle, and runs down the anterior surface of the shaft 

 near its inner border. Between these two insertions the pre-humeral 

 artery and the nerve to the biceps pass. The tendon of origin of the 

 muscle comes out between the supraspinatus'and subscapularis muscles, 

 and the posterior border of the muscle is related to the brachial artery. 



Action. — To adduct and extend the shoulder. 



The Biceps (Plates 5 and 6). This muscle receives its name in the 

 human subject from its having two heads of origin. It is also known 

 as the Jiexor brachii or coraco-radialis. It arises from the whole of the 

 coracoid process of the scapula with the exception of the tubercle on its 

 inner side, which is for the coraco-humeralis. Its strong tendon of 

 origin emerges from between the outer and inner tendons of the 

 supraspinatus, and passes over the shoulder-joint, a pad of fat separat- 

 ing its deep face from the 'capsular ligament of the joint. The tendon, 

 which is of fibro-cartilaginous consistency, then plays over the bicipital 

 groove of the humerus, on which its deep face is moulded, and a 

 synovial bursa facilitates the movements of the tendon in the groove. 

 The central portion of the muscle, which is thick and fusiform, has 

 numerous tendinous intersections, and is traversed throughout by a 

 fibrous cord. It rests on the anterior face of the humerus, and at its 

 low r er end terminates by a tendon which, passing over the anterior 

 ligament of the elbow-joint (to which it is adherent), is inserted into the 

 bicipital tuberosity of the radius. The tendon is partly covered by the 

 internal lateral ligament of the elbow. The muscle has a second in- 

 sertion, in the shape of a strong fibrous band, detached from the main 

 tendon to blend with the sheath of the extensor metacarpi magnus and 

 the deep fascia on the front of the fore-arm. 



Action. — To flex the elbow joint and make tense the fascia of the 

 fore-arm. In the first of these actions it is a good example of a lever 

 of the third order, where the power is applied between the fulcrum — 

 represented by the elbow-joint, and the weight — represented by the 

 distal portion of the limb. The fibrous cord which traverses the 

 muscle is a mechanical extensor of the shoulder-joint, as long as the 

 elbow is kept extended by the triceps extensor cubiti. 



Directions. — The teres major from the shoulder upwards should now 

 be removed, in order to follow more thoroughly the course of the sub- 

 scapular artery with its branches, and to expose the small scapulo- 

 humeral muscle, which lies on the capsular ligament behind the joint ; 

 but care should be taken in dissecting the tendons in the neighbour- 

 hood of the joint, to preserve the capsular ligament intact. 



The Subscapular Artery (Plate 6) springs from the axillary trunk 

 at the interstice between the subscapularis and teres major muscles, 



