4 DISSECTION OF THE DOG 



Within the space will be found the external jugular vein, the communicating 

 branch from the cephalic vein, the omo-cervical arterial and venous trunks, 

 and the nerves to the superficial pectoral and brachio-cephalic muscles. In 

 the depths of the triangle is part of the brachial plexus of nerves. 



All these structures \vill be more fully exposed during the dissection of the 

 axUlary space. 



Dissection. — Reflect the deep pectoral muscle by cutting through it a 

 short distance from its origin. This exposes the axillary space, the 

 structures in which should be cleaned with great care. In reflecting 

 the muscle, a lymph-gland (the axillary gland) will be found lying over 

 the distal part of the teres major and between this and the pectoral 

 muscle. Branches from the brachial plexus of nerves should also be 

 noted bending round the axillary vessels to sink into and supply the deep 

 pectoral muscle. 



Axilla. — The axillary space is bounded laterally by the scapula and the 

 subscapular muscle, and medially by the chest-wall and the muscles clothing 

 it. In the natural condition the space can scarcely be said to exist except as 

 a narrow chink filled by areolar and fatty tissue ; but on dissection its medial 

 and lateral walls are separated from each other and a definite space is produced. 

 DorsaUy the two walls meet at a very acute angle ; ventrally, however, they 

 diverge slightly, and the floor of the space is defined by the pectoral muscles. 

 Cranially the axillary space communicates with the neck ; whereas caudally it 

 is closed in by the latissimus dorsi and cutaneous muscles. The principal 

 contents of the space are the axillary vessels and the brachial plexus of 

 nerves. 



V. BT A. AXILLARIS. — The axillary vessels are among the structures first 

 exposed on reflecting the deep pectoral muscle. The vein drains the blood 

 from the limb, and joins the external jugular vein at the entrance to the 

 chest, thus constituting one of the roots of the innominate vein. Its collateral 

 tributaries are the lateral thoracic, subscapular and omo-cervical veins, and 

 small vessels from the pectoral muscles. Occasionally the omo-cervical venous 

 trunk joins the jugular direct. Of the tributaries the subscapular vein is 

 by far the largest, its volume depending mainly upon the circumstance that 

 it continues the cephaUc vein. 



The axillary artery is a continuation of the subclavian artery. Com- 

 mencing on a level with the first rib, it runs down the hmb as far as the tendon 

 of insertion of the teres major muscle. Most of its branches will be examined 

 in connection with the scapular and arm regions, but two should now be 

 noted as supplying the pectoral muscles and their neighbourhood. These are 

 the thoraco-acromial and lateral thoracic arteries. 



A. THOEACO-ACROMiALis. — The thoraco-acromial artery soon divides into 

 two branches, a ramus deltoideus and a ramus pectoralis. Both of these have 



