376 MUSCLES AND FASCIA. 



ing the scapula downwards and inwards to the thorax. The Subclavius de- 

 presses the shoulder, drawing the clavicle downwards and forwards. When 

 the arms are fixed, all three muscles act upon the ribs, drawing them upwards 

 and expanding the chest, and thus becoming very important agents in forced 

 inspiration. Asthmatic patients always assume this attitude, fixing the shoul- 

 ders, so that all these muscles may be brought into action to assist in dilating 

 the cavity of the chest. 



LATERAL THORACIC EEGION. 

 Serratus Magnus. 



The Serratus Magnus (Fig. 237) is a broad, thin, and irregularly quadrilateral 

 muscle, situated at the upper part and side of the chest. It arises by nine 

 fleshy digitations from the outer surface and upper border of the eight upper 

 ribs (the second rib having two), and from the aponeurosis covering the upper 

 intercostal spaces, and is inserted into the whole length of the inner margin of 

 the posterior border of the scapula. This muscle has been divided into three 

 portions, a superior, middle, and inferior, on account of the difference in the 

 direction, and in the extent of attachment of each part. The upper portion, 

 separated from the rest by a cellular interval, is a narrow, but thick fasciculus, 

 which arises by two digitations from the first and second ribs, and from the 

 aponeurotic arch between them; its fibres proceed upwards, outwards, and 

 backwards, to be inserted into the triangular smooth surface on the inner side 

 of the superior angle of the scapula. The middle portion of the muscle arises 

 by three digitations from the second, third, and fourth ribs; it forms a thin and 

 broad muscular layer, which proceeds horizontally backwards to be inserted 

 into the posterior "border of the scapula, between the superior and inferior 

 angles. The lower portion arises from the fifth, sixth, seventh, and eighth ribs, 

 by four digitations, in the intervals between which are received corresponding 

 processes of the External Oblique; the fibres pass upwards, outwards, and 

 backwards, to be inserted into the inner surface of the inferior angle of the 

 scapula, by an attachment partly muscular, partly tendinous. 



Relations. This muscle is covered, in front, by the Pectoral muscles ; behind, 

 by the Subscapularis ; above, by the axillary vessels and nerves. Its deep 

 surface rests upon the ribs and Intercostal muscles. 



Nerves. The Serratus Magnus is supplied by the posterior thoracic nerve. 



Actions. The Serratus Magnus is the most important external inspiratory 

 muscle. "When the shoulders are fixed, it elevates the ribs, and so dilates the 

 cavity of the chest, assisting the Pectoral and Subclavius muscles. This mus- 

 cle, especially its middle and lower segments, draws the base and inferior angle 

 of the scapula forwards, and so raises the point of the shoulder by causing a 

 rotation of the bone on the side of the chest; assisting the Trapezius muscle in 

 supporting weights upon the shoulder, the thorax being at the same time fixed 

 by preventing the escape of the included air. 



Dissection. After completing the dissection of the axilla, if the muscles of the back have 

 been dissected, the upper extremity should be separated from the trunk. Saw through the 

 i luvicle at its centre, and then cut through the muscles which connect the scapula and arm with 

 the trunk, viz., the Pectoralis Minor in front, Serratus Magnus at the side, and the Levator 

 Anguli Scapula, the Rhomboids, Trapezius, and Latissimus Dorsi behind. These muscles 

 should be cleaned and traced to their respective insertions. Then make an incision through the 

 integument, commencing at the outer third of the clavicle, and extending along the margin of 

 that bone, the acromion process, and spine of the scapula; the integument should be dissected 

 from above downwards and outwards, when the fascia covering the Deltoid is exposed (Fig. 235, 

 No. 3). 



The sujjerficial fascia, of the upper extremity is a thin cellulo-fibrous lamina, 

 containing between its layers the superficial veins and lymphatics, and the 

 cutaneous nerves. It is most distinct in front of the elbow, and contains very 

 large superficial veins and nerves; in the hand it is hardly demonstrable, the 



