462 THE MUSCLES AND FASCIAE 



2. The Lateral Thoracic Region. 



Serratus magnus. 



The Serratus magHUS (m. serratus anterior) (Fig. 349) is a thin, irregularly 

 quadrilateral muscle, situated between the ribs and the scapula at the upper and 

 lateral part of the thorax. It arises by fleshy digitations or slips from the outer 

 surfaces and upper borders of the upper ^ight or nine ribs, and from the jiponeu- 

 rosesjgovering the intervening intercostal muscles. Each digitation (except tEe" 

 first) arises from the corresponding rib; the first digitation arises from the first 

 and second ribs and from the fascia covering the first intercostal space. From 

 this extensive attachment the fibres pass backward, closely applied to the thoracic 

 wall, and reach the vertebral border of the scapula, and are inserted into its 

 ventral aspect in the following manner. The first digitation, arising from the 

 first and second ribs, is inserted in to a t riaiigula r & rea on the ventral aspect of the 

 sjiperiQr_aiigle. The next two digitations (from the secon3~and tfilrd riBs) 

 spread out to form a thin triangular sheet, the base of which is directed backward 

 and is inserted into nearly the whole length of the ventral aspect of the vertebral 

 border. The lower five or six digitations converge to form a fan-shaped mass, 

 the apex of which is inserted, by muscular and tendinous fibres, into a triangular 

 impression on the ventral aspect of the inferior angle. The lower four slips 

 interdigitate at their origin with the upper five slips of the External oblique muscle 

 of the abdomen. 



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

 Subscapularis. The axillary vessels and nerves lie upon its upper part, while its deep surf act' 

 rests upon the ribs and intercostal muscles. 



Nerves. The Serratus magnus is supplied by the posterior thoracic nerve, which is derived 

 from the fifth, sixth, and seventh cervical nerves. 



Actions. The Serratus magnus, as a whole, carries the scapula forward, and at the same 

 time raises the vertebral border of the bone. It is therefore concerned in the action of pushing. 

 Its lower and stronger fibres move forward the lower angle and assist the Trapezius in rotating 

 the bone around an axis through its centre, and thus assist this muscle in raising the acromion 

 and supporting weights upon the shoulder. It also assists the Deltoid in raising the arm, 

 inasmuch as during the action of this latter muscle it fixes the scapula and so steadies the 

 glenoid cavity in which the head of the humerus rotates. After the Deltoid has raised the arm 

 to a right angle with the trunk, the Serratus magnus and the Trapezius, by rotating the scapula, 

 raise the arm into an almost vertical position. It is possible that when the shoulders are fixed 

 the lower fibres of the Serratus magnus may assist in raising and everting the ribs; but it is not 

 the important inspiratqry muscle which it was formerly believed to be. 



Applied Anatomy. When the muscle is paralyzed, the vertebral border, and especially the 

 lower angle of the scapula, leaves the ribs and stands out prominently on the surface, giving 

 a peculiar "winged" appearance to the back. The patient is unable to raise the arm, and an 

 attempt to do so is followed by a further projection of the lower angle of the scapula from the 

 back of the thorax. 



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 the Levator 

 anguli scapulae, the Rhomboids, Trapezius, and Latissimus dorsi behind. These muscles should 

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

 ment, 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 

 downward and outward, when the fascia covering the Deltoid will be exposed (Fig. 346, No. 3). 



II. MUSCLES AND FASCIA OF THE SHOULDER AND ARM. 



The superficial fascia of the upper extremity is a thin cellulofibrous layer, con- 

 taining the superficial veins and lymphatics, and the cutaneous nerves. It is 



