SUBCLAVIUS 



373 



tendon lying ventral to this. The two tendons are continuous at their distal margins. (W. H. 

 Lewis.) 



Nerve-supply. — From the external and internal anterior thoracic nerves, branches of which 

 enter the sterno-costal portion of the muscle about midway between the tendons of origin 

 and insertion, and the clavicular portion in the proximal third. The nerve fibres are derived 

 from the (fifth), sixth, seventh and eighth cervical and first thoracic nerves. 



Action. — With the thorax fixed, the muscle adducts and flexes the arm and rotates it medial- 

 ward. The clavicular portion draws the arm forward, upward, and medialward; the sterno- 

 costal portion draws the arm downward, medialward, and forward. When the arm is pendent, 

 the upper portion elevates, the lower depresses, the shoulder, ^yith the arm fixed, the muscle 

 draws the chest upward toward it. It is of value in forced inspu-ation. 



Relations. — It hes over the coracoid process, the subclavius, pectoralis minor, intercostal, 

 and serratus anterior (magnus) muscles, the coraco-clavicular (costo-coracoid) fascia, and the 

 thoraco-acromial vessels. It forms the main part of the ventral wall of the axillary fossa, and 

 laterally it enters into relation with the deltoid, biceps, and coraco-brachiahs muscles. 



Variations. — In considering variations the muscle may be looked upon as composed of 

 four portions — a clavicular, a sternal, a costal, and an abdominal, the last being that portion 

 which arises from the aponeiu-osis of the external obhque. These portions vary in the extent 

 of their attachments and in the degree of separation which they present. The abdominal por- 

 tion may extend to the umbihcus. Huntington considers this portion a derivative of the pan- 

 nicular muscle of the lower mammals. On the sternum the muscles of the two sides may de- 

 cussate across the middle line. The sterno-costal portions of the muscle are more frequently 

 deficient or missing than the clavicular, but in rare cases the entu-e muscle is absent. The 

 clavicular portion of the muscle may be fused with the deltoid. The sterno-costal may extend 

 laterally to the latissimus dorsi. There may be an intimate fusion of the abdominal portion 

 with the rectus abdominis or the external obhque. Sometimes a slip may run from the pec- 

 toralis major to the biceps, the pectorahs minor, coracoid process, capsule of the joint, or 

 brachial fascia. 



The pectoralis minor (fig. 388). — Origin. — By aponeurotic slips from the second, third, 

 fourth, and fifth ribs near the costal cartilages. 



Fig. 361. — The Subclavius and the Upper Portiox of the Serratus Anterior. 



Subclavius 



Serratus anterior 



Structure and insertion. — The fibre-bundles converge upward and outward to a flattened 

 tendon which is attached to the medial border and upper surface of the coracoid process of 

 the scapula. 



Nerve-supply. — From the internal anterior thoracic nerve which enters the upper part of 

 the middle third of the deep surface by several branches. Some of the branches extend through 

 to the pectoralis major. The nerve" fibres arise from the seventh and eighth cervical nerves. 



Action. — When the thorax is fixed, the pectorahs minor pulls the scapula forward, the 

 lateral angle of the bone downward, and the inferior angle dorsalward and upward. When 

 the scapula is fixed, the muscle aids in forced inspiration. 



Relations. — It is covered by the pectorahs major. Near its insertion the fibrous investment 

 of the chief nerves and vessels of the arm is adherent to its enveloping fascia. 



Variations. — The origin may extend to the sixth rib or may be reduced to one or two ribs. 

 In the primates below man the insertion of the muscle takes place normally into the humerus. 

 In man its insertion may be continued (in more than 15 per cent, of bodies — Wood) over the 

 coracoid process to the coraco-acromial or coraco-humeral ligaments, to the tendon of the sub- 

 scapularis muscle, or to the great tubercle of the humerus. It may be divided into two super- 

 imposed fasciculi. Fascicuh may extend from the muscle to the subclavius or the pectoralis 

 major. 



The subclavius (fig. 361).— Or ij^in. — From a flat tendon attached to the first rib and its 

 cartilage near their junction. 



Structure and insertion. — The fibre-bundles arise in a penniform manner from the tendon 

 of origin which extends for some distance along the lower border of the muscle. They are 

 inserted in a groove which lies on the lower surface of the clavicle between the costal tuberosity 

 and the coracoid tuberosity. The medial fibre-bundles are inserted directly, the lateral by a 

 strong tendon. 



Nerve-supply.— By a branch which arises usually from the fifth or fifth and sixth cervical 

 nerves and enters the middle of the back part of the muscle. . _ 



Action. — When the first rib is fixed, the subclavius depresses the clavicle and the point of 



