298 MUSCLES AND FASCIJE. 



LATERAL THORACIC EEGIOX. 



Serratus Magnus. 



The Serratus Magnus is a broad, thin, and irregularly quadrilateral muscle, 

 situated at the upper part and side of the chest. It arises by nine fleshy digita- 

 tions 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 ex- 

 tent 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, and forms a thin and broad muscular layer, which proceeds horizontally back- 

 wards, 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 cor- 

 responding 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 muscle, 

 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 

 clavicle 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, behind, the 

 Levator anguli scapula?, the Ehomboidei, Trapezius, and Latissimus dorsi. These muscles should 

 be cleaned and traced to their respective insertions. An incision should then be made through 

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

 of this 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. 



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

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

 neous nerves. It is most distinct in front of the elbow, and contains between 

 its laminge in this situation the large superficial cutaneous veins and nerves ; in 

 the hand it is hardly demonstrable, the integument being closely adherent to the 

 deep fascia by dense fibrous bands. Small subcutaneous bursaa are found in this 

 fascia, over the acromion, the olecranon, and the knuckles. The deep fascia of 

 the upper extremity comprises the aponeurosis of the shoulder, arm, and fore- 

 arm, the anterior and posterior annular ligaments of the carpus, and the palmar 

 fascia. These will be considered in the description of the muscles of these several 

 regions. 



