4 TOPOGRAPHICAL ANATOMY OF 



M. PECTORALis PKOFU^^DUS. — The deep pectoral muscle is readily 

 separated into two distinct parts, the prescapular ^ and humeral (pars 

 praescapularis : pars humeralis). 



The 'prescapular part takes origin from the sternal end of the 

 first four rib-cartilages and the immediately adjacent part of the 

 sternum. Its insertion cannot be determined at the present time, 

 because the muscle disappears by sweeping in a dorsal direction along 

 the cranial border of the scapula under cover of the brachiocephalic 

 muscle. It will ultimately be found on the surface of the supra- 

 spinous muscle. 



The humeral part has a much more extensive origin, part of it 

 being visible before the superficial pectoral muscle was reflected. Some 

 of the bundles of the deep pectoral spring from the elastic covering 

 of the oblique and straight abdominal muscles. Others arise from 

 the sternal ends of the rib-cartilages from the eighth (sometimes the 

 ninth) to the fifth inclusive, and from the adjacent part of the sternum. 

 From this widespread origin the bundles converge with a varying 

 degree of obliquity, and the muscle is inserted mainly into the lesser 

 tubercle of the humerus. In addition, some of tha fibres are attached 

 to the tendon of origin of the coracobrachial muscle and the fascia 

 overlying the tendon of the biceps. 



Dissection. — Cut across both portions of the deep pectoral muscle 

 about midway between the limb and the sternum. When the cut ends 

 have been turned aside, the axilla and its contents are exposed to view. 

 The contents are numerous, and must be cleaned by the careful removal 

 of the loose tissue that surrounds them. 



Axilla. — The axilla ^ is largely an artificial space produced by dis- 

 section. In the living animal it is merely a narrow interval between 

 the limb and the side of the thorax filled with very loose tissue that 

 allows of freedom of movement. When the dissection is completed, 

 the space has considerable dimensions, and may be described as possess- 

 ing fairly definite boundaries. The lateral wall is formed by the 

 prescapular part of the deep pectoral, and the subscapular, teres major, 

 and latissimus dorsi muscles. The medial wall is formed by the 

 wall of the chest, with the associated intercostal, ventral serratus, 

 and transverse thoracic muscles. The lateral and medial walls meet 

 at a very acute angle dorsally, but separate somewhat as they are 

 followed in a ventral direction. Ventrally the axilla is closed by the 



1 Sc(qmlce [L.], shoulder-blades, back of shoulders ; a-Kairdvi^ (scapane) [Gr.], 

 a digging tool. 



2 Axilla [L.] (dim. of ala, wing), arm-pit. 



