560 THE AETERIES. 



situated beneath the limb. In their thoracic portion, the orachial trunks, 

 at first lying beside each other, separate slightly in front to reach the internaJ 

 face of each of the two first ribs. They are accompanied by the cardiac, 

 pneumogastric, inferior laryngeal, and diaphragmatic nerves, and are in- 

 cluded, as already noticed, between the two layers of the anterior medias- 

 tinum. The right occupies nearly the median line beneath the inferior face 

 of the trachea, to the left and above the anterior vena cava. The left 

 slightly rises on the side of the trachea, and generally corresponds inwardly 

 to the thoracic duct. 



In their axillary portion, these vessels accompany the corresponding 

 venous trunks, cross the terminal tendon of the subscapularis muscle in 

 passing below the humeral insertion of the pectoralis magnus, and among 

 the branches of the brachial plexus, but embraced more particularly by 

 the median, anterior humeral, and ulnar nerves. 



Distribution. The axillary arteries give off, on their course, eight col- 

 lateral branches. Four arise from the thoracic portion : three upper, the 

 dorsal, superior cervical, and vertebral arteries ; and an inferior, the internal 

 thoracic. Two are detached at the first rib : one downwards, the other 

 forwards ; these are the external thoracic and superior cervical arteries. Two 

 originate from the axillary portion of the trunk and pass upwards : they are 

 the super- and subscapular arteries. After furnishing the latter vessel, the 

 brachial trunk is continued by the humeral artery. 



Independently of all these branches, the right axillary artery gives off, 

 near its origin, the common trunk of the two carotid arteries, which will be 

 studied in a separate article. 



Preparation. The subject being placed on the right side, remove the skin and the left 

 anterior limb, in order to make the dissection at two periods. 



First period. Dissect all the intra-thoraeic portion of the left axillary artery and its 

 collateral branches, as in figure 282, taking care to leave the inferior cervical artery (which 

 has been cut in the figure to render the drawing more distinct; attached by its superior 

 extremity to the middle portion of the ma stoido-humeralis, which has not been disturbed. 



Second period. Prepare, on the separated limb, the extra-thoracic portion of the 

 vessel and all the arteries it furnishes, in taking us guides figures 283, 290, and 291. 



COLLATERAL BRANCHES OF THE AXILLARY ARTERIES. 



1. Dorsal, Dorso-muscular or Transverse Cervical Artery. (Fig. 282, 4.) 



Chiefly destined to the muscles of the withers, this artery, the first given 

 off by the brachial trunk, crosses outwardly the trachea, thoracic duct, 

 oesophagus, great sympathetic nerve, and the long muscle of the neck, in 

 proceeding beneath the mediastinal layer; it reaches and passes over the 

 second intercostal space, bends slightly backwards, and places itself in the 

 interstice which separates the angular muscle of the scapula and great 

 serratus muscle from the inferior branch of the ilio-spinal muscle (longis- 

 simus dorsi), where it separates into several divergent branches. The majority 

 of these ascend towards the superior border of the withers, neck, and 

 shoulders, by gliding between the latter muscles, the splenius, and the 

 small anterior serratus on the one part, and the great serratus, rhomboideus, 

 and proper elevator of the shoulder on the other, to be distributed to those 

 muscles and the integuments covering them. The most anterior of these 

 branches passes between the splenius and the great complexus muscles, 

 parallel with the superior cervical artery, which is in front of it. and com- 

 municates by its ramuscules with the latter vessel, as well as with the 

 vertebral and occipito-muscular arteries. The last-named branch is some- 



