586 HUMAN ANATOMY. 



bursa acromialis over the acromion process of the scapula, a bursa olecrani over the 

 olecranon process of the ulna, and a bursa may occur over each condyle of the 

 humerus. 



(a) THE PRE-AXIAL MUSCLES. 

 I. Biceps (Figs. 560, 570). 



Attachments. — The biceps (m. biceps brachii), as its name indicates, takes 

 origin by two heads. The long head arises from the upper border of the glenoid 

 cavity of the scapula by a slender round tendon, which traverses the cavity of the 

 shoulder-joint invested by the synovium and then bends downward into the bicipital 

 groove (intertubercular sulcus ) of the humerus, accompanied by a prolongation of the 

 joint capsule (vagina mucosa intertubercularis ) , and then, becoming muscular, unites 

 with the short head, which arises from the tip of the coracoid process of the scapula 

 in common with the coraco-brachialis. By the union of the two heads a strong 

 muscle is formed which descends in front of the humerus and a short distance above 

 the elbow-joint passes over into a fiat tendon, which is continued downward to be 

 inserted into the tuberosity of the radius, a mucous bursa (bursa bicipitoradialis) 

 being interposed between the anterior surface of the tuberosity and the tendon. 

 Some of the fibres of the muscle, instead of passing into the tendon, are continued 

 into a flat tendinous expansion, the semilunar or bicipital fascia { lacertus fibrosus), 

 which passes downward and medially to become lost in the fascia of the forearm. 



Nerve-Supply. — By the musculo-cutaneous nerve from the fifth and sixth 

 cervical nerves. 



Action. — To flex the forearm on the brachium, and when the forearm is in pro- 

 nation to supinate it. It will also act to a slight extent in movements of the arm at 

 the shoulder-joint, assisting the coraco-brachialis in drawing the arm forward. 



Relations. — The biceps is crossed on its ventral surface by the tendon of the 

 pectoralis major and is covered above by the lateral portion of the deltoid. Deeply 

 it is in relation with the humerus, the brachialis anticus, and the supinator. Upon 

 its inner side lie the coraco-brachialis above and below, in the groove between it and 

 the triceps (sulcus bicipitalis medialis), the brachial vessels, and the median nerve. 



Variations. — The biceps presents numerous variations. Its long head is occasionally want- 

 ing, but more frequently additional heads occur. Of these the most frequent, occurring in some- 

 thing over 10 per cent, of cases, is a head which arises from the medial surface of the humerus, 

 between the insertions of the deltoid and coraco-brachialis. Other heads may arise from the 

 external tuberosity of the humerus or from the outer border of that bone, between the deltoid 

 and brachio-radial muscles. 



2. Brachialis Anticus (Fig. 571). 



Attachments. — The brachialis anticus (m. brachialis) occupies the anterior sur- 

 face of the lower part of the humerus and is for the most part covered by the biceps. 

 It arises from the intermuscular septa and the anterior surface of the humerus imme- 

 diately below the insertion of the deltoid, which it partly surrounds. It passes 

 downward, and the fibres converge to a short tendon which is inserted into the 

 anterior surface of the coronoid process of the ulna. 



Nerve-Supply. — The main mass of the muscle is supplied by branches from 

 the musculo-cutaneous nerve. The fibres which arise from the lateral intermuscular 

 septum and are covered by the brachio-radialis are supplied by a branch from the 

 musculo-spiral nerve. The nerve-fibres come in both cases from the fifth and sixth 

 cervical nerves. 



Action.— To flex the forearm. 



Variations. — The nerve-supply shows the brachialis anticus to be a composite muscle the 

 major portion of which is derived from the pre-axial muscle-sheet, while the lateral portion of 

 it comes from the post-axial sheet. In correspondence with this derivation of the muscle, its 

 lateral portion is occasionally separate from the rest and may terminate below on the fascia of 

 the forearm or on the radius. A longitudinal separation of the pre-axial portion of the muscle 

 may also occur, and it seems probable that the most frequently occurring third head of the 

 biceps (see above) is a derivative of this portion of the brachialis. 



The epitrochleo-ajtconens is a small, usually quadrangular muscle which is present in about 

 25 per cent, of cases. It arises from the posterior surface of the inner condyle of the humerus 



