374 THE MUSCULATURE 



the shoulder. When the clavicle is fixed, the muscle aids in forced inspiration. It also serves 

 to keep the clavicle against the sternum. 



Relations. — It is concealed by the clavicle and pectoralis major muscle. Behind it lie the 

 subclavian vessels and the brachial plexus. 



Variations. — It may be replaced by a ligament or by a pectoralis minimus muscle (see below). 

 It may be doubled or may be inserted into the coracoid process, coraco-acromial ligament, the 

 acromion, or the humerus. The subclavius posticus arises near the subclavius, passes backward 

 over the subclavian vessels and brachial plexus and is inserted into the cranial margin of the 

 scapula near the base of the coracoid process. 



Abnormal Muscles of the Pectoral Group 



The following muscles are usually innervated by the anterior thoracic nerves and are 

 probably generally abnormal derivatives of the pectoral mass. Frequently they represent 

 muscles normally found in lovA'er mammals. 



The sternalis. — A flat muscle somewhat frequently seen on the surface of the pectoralis 

 major, usually nearly parallel to the sternum. It arises from the sheath of the rectus and from 

 some of the costal cartilages (third to seventh) and terminates on the sterno-cleido-mastoid, 

 on the sternum, or on the fascia covering the pectoralis major. When present on both sides, 

 the two muscles maj' be fused across the sternum. This muscle is found in 4 per cent, of 

 normal individuals and 48 per Cent, of anencephalic monsters. (Eisler.) Rarely, corresponding 

 muscle slips have been found innervated by the intercostal nerves. These probably represent 

 remains of a thoracic 'rectus' muscle. 



The pectoro-dorsalis (axillary arch). — This muscle in its most complete form extends from 

 the tendon of the pectoralis major over the axillary fossa to the tendon of the latissimus dorsi, 

 to the fascia covering the latissimus dorsi, to the teres major or even more distally. It may, 

 however, be more or less fused with either of the last two muscles mentioned, and it presents a 

 great variety of forms. It may extend from the latissimus dorsi to the brachial fascia over the 

 coraco-brachialis or biceps, to the long tendon of the biceps, to the axillary fascia, to the axillary 

 margin of the pectoralis minor, or to the coracoid process, etc. It is found in about 7 per cent, 

 of bodies. (Le Double.) When supplied from the anterior thoracic nerves, it probably rep- 

 resents a portion of the thoraco-humeral subcutaneous (pannicular) muscle of the lower 

 primates. It is also sometimes supplied by the medial brachial cutaneous or the intercosto- 

 brachial (humeral) nerve and frequently is partly or wholly supplied by the dorsal thoracic (long 

 subscapular) nerve. The part of the muscle supplied by the dorsal thoracic nerve is probably 

 derived from the latissimus dorsi musculature. 



The costo-coracoideus. — A muscular slip which arises from one or more ribs or from the 

 aponeurosis of the external oblique between the pectoralis major and latissimus dorsi muscles, 

 and is inserted in the coracoid process. 



The chondro-humeralis (epitrochlearis). — This is a slip which springs from one or two rib 

 cartilages or from the thoraco-abdominal fascia beneath the pectoralis major, or from its lower 

 border or tendon, and extends on the medial side of the arm to the intertubercular (bicipital) 

 groove, the brachial fascia, the intermuscular septum, or the medial epicondyle. It is found in 

 12 to 20 per cent, of bodies (Le Double), and occurs normally in many of the lower mammals. 



The pectoralis minimus (sterno-chondro-scapularis). — From the cartilage of the first rib 

 and stenuun to the coracoid process. 



The sterno-clavicularis. — From the manubrium of the sternum to the clavicle between the 

 pectoralis major and tlie coraco-clavicular (costo-coracoid) fascia. In 2 per cent, to 3 per cent 

 of bodies, ((irul)cr.) 



The scapulo-clavicularis. — From the coracoid process of the scapula to the outer third of 

 the clavicle. 



The infra-clavicularis. — From above the clavicular part of the pectoralis major to the 

 fascia over the deltoid. 



BURS^ 



B. m. pectoralis majoris. — Between the tendon of insertion of the pectoralis major and 

 the long head of the biceps. Frequent. 



C. MUSCULATURE OF THE ARM 



(Fifrs. 355, 356, 362, 363, 364, 365, 367, 370, 372) 



The muscles in('lu(l('(l in this section are the triceps and anconeus, coraco- 

 })rachi;ins, biceps, iind hrachiaHs. The triceps and anconeus (fig. 363) constitute 

 a mass of musculature extendinfi; along the back of the arm from the scapula and 

 humerus to the olecranon of the ulna. The coraco-brachialis, biceps, and brachialis 

 (figs. 364, 365) constitute a similar mass of musculature extending along the front 

 of the arm from the scapula anrl the humerus to the humerus, and to the radius 

 and ulna near the elbow. In the upper half of the arm the two groups arc 

 separated on the lateral side of the arm l)y the deltoid, pectoralis major, teres 

 minor, supra- and infraspinatus muscles, and by the greater tubercle of the 

 humerus. On tlu; mediul side they are separated the by chief nerves and blood- 



