THE LATERAL THORACIC REGION. 



381 



membrane, which, with the clavicle, forms an osseo-fibrous sheath in which the 

 muscle is enclosed. 



If the costal attachment of the Pectoralis minor is divided across, and the muscle reflected 

 outward, the axillary vessels and nerves are brought fully into view, and should be examined. 



Nerves. The Pectoral muscles are supplied by the anterior thoracic nerves ; 



the Pectoralis major through these nerves re- 

 ceives filaments from all the spinal nerves 

 entering into the formation of the brachial 

 plexus ; the Pectoralis minor receives its 

 fibres from the eighth cervical and first dorsal 

 nerves. The Subclavius is supplied by a 

 filament from the fifth cervical nerve. 



Actions. If the arm has been raised by 

 the Deltoid, the Pectoralis major will, con- 

 jointly with the Latissimus dorsi and Teres 

 major, depress it to the side of the chest. 

 If acting alone, it adducts and draws for- 

 ward the arm, bringing it across the front 

 of the chest, and at the same time rotates 

 it inward. The Pectoralis minor depresses 

 the point of the shoulder, drawing the scap- 

 ula downward and inward to the thorax, and 

 throwing the inferior angle backward. The 

 Subclavius depresses the shoulder, drawing 

 the clavicle downward and forward. When 

 the arms are fixed, all three muscles act upon 

 the ribs, drawing them upward and expand- 

 ing the chest, and thus becoming very im- 

 portant agents in forced inspiration. Asth- 

 matic patients always assume an attitude 

 which fixes the shoulders, so that all these 

 muscles may be brought into action to assist 

 in dilating the cavity of the chest. 



Spine of 



EXTERNAL 

 OBLIQUE. 



Eighth rib. 



FIG. 231. Serratus magnus. (From a prepnra- 

 f the Royal College of Sur- 



2. Lateral Thoracic Region. 



Serratus magnus. 



The Serratus magnus (Fig. 231) is a thin, 

 irregularly quadrilateral muscle, situated be- 

 tween the ribs and the scapula at the upper and lateral part of the chest. It 

 arises by nine digitations or slips from the outer surface and upper border of the 

 eight upper ribs (the second rib giving origin to two slips), and from the aponeurosis 

 covering the corresponding intercostal muscles. From this extensive attachment 

 the fibres pass backward, closely applied to the chest-wall, and reach the verte- 

 bral border of the scapula, and are inserted into its ventral aspect in the following 

 manner. The upper two digitations /. e.\ the one from the first rib and the higher, 

 of the two from the second rib converge to be inserted into a triangular area on 

 the ventral aspect of the superior angle. The next two digitations 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 digitations converge, as they pass backward from the ribs, to form a fan- 

 shaped structure, the apex of which is inserted, partly by muscular and partly by 

 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. 



