ANTERIOR THORACIC REGION. 



297 



surface is separated from the Pectoralis major by a strong aponeurosis, which, 

 with the clavicle, forms an osteo-fibrous sheath in which the muscle is inclosed. 



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

 outwards, 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 Subclavius, by a filament from the cord formed by the union of the fifth and 

 sixth cervical nerves. 



Actions. If the arm has been raised by the Deltoid, the Pectoralis major will, 

 conjointly with the Latissimus dorsi and Teres major, depress it to the side of the 

 chest ; and, if acting singly, it will draw the arm across the front of the chest. 



Fig. 170. Muscles of the Chest and Front of the Arm, with the boundaries 



of the Axilla. 



The Pectoralis minor depresses the point of the shoulder, drawing the scapula 

 downwards and inwards to the thorax. The Subclavius depresses 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, thus 

 becoming very important agents in forced inspiration. Asthmatic patients always 

 assume this attitude, fixing the shoulders, so that all these muscles may be brought 

 into action to assist in dilating the. cavity of the chest. 



