382 THE MUSCULATURE 



between the pectoralis minor and the subscapularis muscles. More distally it hes medial to the 

 humerus and in front of the chief brachial vessels and nerves. The musculo-cutaneous nerve 

 usually runs through it. 



Variations. — The humeral insertion of the muscle varies considerably. According to Wood, 

 the coraco-brachialis consists primitively of three parts, which arise from the coracoid process 

 and are inserted respectively into the upper, the middle, and the distal part of the humerus 

 along the medial side. The superior division is most deeply, the inferior the most superficially, 

 placed. In man the muscle is composed of parts of the middle and inferior divisions. The 

 inferior division may be completely developed as far as the medial epicondyle. The superior 

 division of the muscle is occasionally found. Slips from the coraco-brachialis to the brachialis 

 have been seen. Complete absence of the muscle has been recorded. 



The biceps brachii (figs. 364, 370) . — The short head arises by a flat tendon closely asso- 

 ciated with that of the coraco-brachialis from the coracoid process. From the dorso-medial 

 surface of this tendon the fibre-bundles descend nearly vertically, though increasing in num- 

 ber, toward their attachment to the tendon of insertion. The fibre-bundles which arise highest 

 on the tendon of origin are inserted highest on the tendon of insertion, while those which have 

 the lowest origin have the lowest insertion. 



The long head arises from the supraglenoid tuberosity and from the glenoid ligament 

 by a long tendon (9 cm.) bifurcated at its origin. The tendon at first passes over the head 

 of the humerus within the capsule of the joint, and then passes into the intertubercular (bicipital) 

 groove, which is covered by the capsule of the joint and an expansion from the tendon of the 

 pectoralis major. To this point the tendon is surrounded by the synovial membrane of the 

 joint. After emerging from this the tendon slowly expands and from its dorsal concave surface 

 arise fibre-bundles which, increasing in number, extend, somewhat obliquely, toward the ten- 

 don of insertion. As in case of the short head, here also the fibre-bundles which arise highest on 

 the tendon of origin have the highest insertion. 



Insertion. — The tendon of insertion begins usually in the distal quarter of the arm as a 

 vertical septum between the two heads of the muscle. More distally this broadens out on each 

 side into a flattened aponeurosis. The fibre-bundles are inserted into the sides of the septum 

 and on each surface of the aponeurosis — those of the long head chiefly on the deep surface, those 

 of the short head chiefly on the superficial surface. The aponeurosis is continued into a strong, 

 flattened tendon which descends between the brachio-radialis and pronator teres muscles to be 

 inserted on the dorsal half of the bicipital tuberosity of the radius. From the medial border 

 of the tendon an aponeurotic expansion, the lacertus fibrosus (semilunar fascia), is continued 

 into the fascia of the ulnar side of the forearm. 



Nerve-supply. — By a branch from the musculo-cutaneous nerve for each head. These 

 branches may be bound in a common trunk for some distance. They enter the deep surface of 

 the muscle in the proximal part of the middle third of each belly often by several rami. Usually 

 there is a distinct intramuscular fissure for the reception of the branches to each head and the 

 blood-ves?els which accompany them. The nerve fibres come from the fifth and sixth cervical 

 nerves. 



Actio7i. — It is a chief flexor of the arm at the elbow and is also a supinator of the forearm. 

 This last action is most marked when the forearm is flexed and pronated. Both heads are 

 flexors and medial rotators of the arm at the shoulder. The long head is an abductor and 

 so also is the short head when the arm is greatly abducted, otherwise the short head is an 

 adductor. 



Relations. — The tendons of origin are concealed by the pectoralis major and deltoid muscles. 

 Beyond this the muscle is covered by the fascia brachii. In the lower part of the arm it lies 

 upon the brachialis muscle. Upon the medial margin lie the coraco-brachialis muscle, the 

 brachial vessels, and the median nerve. 



Variations. — Variations are frequent. The whole muscle or either head may be missing, 

 but such cases are rare. The long head may extend only to the bicipital groove. Frequently 

 the muscle is partially divided into the four primitive portions mentioned above. The two 

 heads may be separate from origin to insertion. There may be an accessory head (1 in 10 

 subjects — Le Double) which arises from the coracoid process, the capsule of the joint, the tendon 

 of the pectoralis major, or the shaft of the humerus near the insertion of the coraco-brachialis. 

 In most instances the origin takes place above the origin of the brachialis from the humerus. 

 Sometimes several accessory heads are seen. Marked variation of insertion is less frequent, 

 but occasionally a supernumerary sHp may go to the medial intermuscular septum or the medial 

 epicondyle. The fusion of the bice])s with neighbouring muscles (pectoralis major and minor, 

 coraco-brachialis, brachialis, palmaris longus, pronator teres, brachio-radialis) by means of 

 tendinous or muscular slips has l)een frequently reported. 



The brachialis (fig. 365). — Origin. — (1) From the distal three-fifths of the front of the 

 humerus, (2) from the medial intermuscular septum, and (3) from (he lateral intcrtiuisculur 

 .septum |)roxiiiial to the heads of the brachio-radialis and extensor carpi radialis longus. Proxi- 

 mally it sends up a [jointed process on the lateral side of the insertion of the deltoid and another 

 between the insertions of the deltoid and the coraco-brachialis. Distally the area of origin 

 .stops a little above the capitulum and the trochlea. 



Structure and Insertion. — The fibre-bundles arise directly from this area of origin, except 

 near the insertion of the deltoid and on the medial margin, where tendinous bands are developed. 

 The fibre-bundles descend, the middle vertically, the medial obliquely lateralvvard, the lateral 

 still more obliquely niedialvvard. The tendon of insertion appears on the dorsal side of the 

 lateral cdgf; of the muscle in its lower fourth, ('ontinuous with this stronger lateral i)ortion of 

 the tendon more distally a thiiuicr band ap))cars upon the ventral surface of the muscle above 

 the joint. The tendon l)ecom(!s thick as it passes distally, is closely united to the capsule of the 

 elbow-joint, and is attached to the ulnar tuberosity. In addition to the nuiin tendon, some of 

 the deeper fibre-bundles of the muscle and .some of tho.se coming from the lateral internuiscular 

 septum are attached by short tendinous bands to the coronoid process. 



