MUSCLES OF THE ARM. 381 



of the forearm. Its proximal part is thickened and can be felt subcutaneously as 

 a crescentic border. 



In the arm the biceps conceals the brachialis muscle and the musculo-cutaneous 

 nerve. Its medial border is the guide to the position of the brachial artery and 

 median nerve. 



The biceps is an extremely variable muscle. Its chief anomalies are due to an increase 

 or diminution in the number of origins. A third head of origin is common (10 per cent), and 

 usually arises from the humerus, between the insertions of the deltoid and coracobrachialis. 

 Two or even three additional heads may be present at the same time. The long head of the 

 muscle may be absent, or may take origin from the intertubercular groove. The muscle may 

 have an additional insertion into the medial epicondyle of the humerus, or into the fascia of 

 the forearm. 



Nerve-Supply. The biceps is supplied by the musculo-cutaneous nerve (C. 5. 6.). 



Actions. The actions of the biceps are complex, in that they affect three articulations 

 the shoulder, humero-radial, and radio-ulnar joint. The muscle raises and draws forward the 

 humerus at the shoulder-joint, it flexes the elbow-joint, and it supinates the forearm. The 

 combination of these actions results in a simple movement like that of raising the hand to 

 the mouth. 



M. Brachialis. The brachialis (O.T. brachialis anticus) is a large muscle 

 arising from the distal two-thirds of the anterior aspect of the body of the 

 humerus and from the intermuscular septum on each side (Figs. 340 and 341, 

 p. 380). 



Clasping the insertion of the deltoid proximally, it ends dis tally in a strong 

 tendon, which is inserted, deep in the hollow of the elbow, into the anterior ligament 

 of the elbow-joint, the distal surface of the coronoid process, and slightly into the 

 immediately adjacent part of the volar surface of the body of the ulna (Fig. 348, 

 p. 389). The lateral part of the muscle arising from the lateral epicondylic ridge 

 and lateral intermuscular septum forms a slip more or less separate, which may 

 be partially fused with the brachioradialis muscle. 



It is concealed for the most part by the biceps muscle in the arm. It forms 

 the floor of the cubital fossa, and covers the anterior aspect of the elbow-joint. 



Nerve-Supply. It is supplied by the musculo-cutaneous nerve (C. 5. 6.) ; and also (in most 

 instances) at its lateral border by a fine branch of the radial nerve (C. (5.) 6.). 

 Action. This muscle is a flexor of the elbow-joint. 



M. Triceps Brachii. The triceps brachii is the only muscle on the posterior 

 aspect of the arm. It arises by three heads : a lateral and a medial head, from 

 the hurnerus, and a long or middle head, from, the scapula. (1) The long head 

 (caput longum) begins as a strong tendon attached to a rough triangular surface 

 on the axillary border of the scapula just below the glenoid cavity (infra-glenoidal 

 tuberosity) (Figs. 333, p. 372, and 337, p. 376). This gives rise to a fleshy belly 

 which, after passing between the teres major and teres minor muscles, occupies the 

 middle of the back of the arm. (2) The lateral head is attached by fibres, partly 

 tendinous and partly fleshy, to the curved lateral border of the humerus from the 

 insertion of the teres minor proximally to the radial groove distally, and receives 

 additional fibres from the posterior surface of the lateral intermuscular septum 

 (Fig. 341, p. 380). Its fibres are directed distally and medially over the radial 

 groove, concealing the radial (musculo-spiral) nerve, the profunda brachii artery, 

 and the medial head of the muscle, to the tendon of insertion. (3) The medial 

 head arises by fleshy fibres from an elongated triangular area on the posterior 

 surface of the humerus, extending proximally to the level of the insertion of the 

 teres major, and distally nearly to the margin of the olecranon fossa (Fig. 341, 

 p. 380). It also arises, on each side, from the intermuscular septum, from the 

 whole length of the medial septum, and from the part of the lateral septum which 

 is below the passage of the radial nerve. 



The three heads of origin are inserted, by a broad and membranous common 

 tendon, into an impression occupying the posterior part of the proximal end 

 of the olecranon of the ulna (Fig. 355, p. 397), and into the deep fascia of the 

 forearm on each side of it. The long and lateral heads join the borders of the 

 tendon of insertion, and the medial head is attached to its deep surface. A small 



