644 THE ARTERIES. 



plantar interosseous arteries by the anterior and posterior per/orating arteries ; finally, at the 

 base of the toes they bifurcate to form the collateral arteries of the toes. 



The dorsal collateral artery of the fourth space represents the vessel described in tiie Horse 

 by the name of metatarso-pedal artery, or collateral of the cannon; it forms the internal 

 collateral dorsal of the fourtli toe, and external collateral of the great toe. 



The dorsalis pedis, after giving off the last-named vessel, dips into the fourth space and 

 reaches the lower surface of the foot, where it anastomoses with the internal plantar artery. 

 In this last portion of its course it resembles the vessel we have named the perforating pedal 

 in Solipeds. 



Article V. — Anterior Aorta (Fig. 375, 1). 



This vessel, the smallest of the two trunks succeeding the common aorta, is 

 no more than 2 or 2^ inches in length at the most. It leaves the pericardium 

 to pass between the two layers of the mediastinum in an obhque direction from 

 below upwards and behind forwards, above the right auricle, below the trachea, 

 and to the left of the anterior vena cava. After furnishing some insignificant 

 twigs to the pericardium and mediastinum, it divides into two branches which 

 constitute the brachial trunks or axillary arteries. 



In Pachyderms, Carnivora, and Rodents, the anterior aorta does not exist, 

 and the axillary arteries arise directly from the common aorta, towards the point 

 from which the anterior aoita arises in other animals. 



Article YI. — Brachial Trunks, or Axillary Arteries (Fig. 375, 2, 3). 



' The brachial trunks — terminal branches of the anterior aorta — are distinguished 

 into left and right. The latter is much larger than the former, because it fur- 

 nishes arteries to the head. It is also named the arteria innominata. 



Origin. — They separate from one another at an acute angle, the left being 

 a little more elevated than the right. 



Course and direction. — Both branches are directed forwards, between the layers 

 of the anterior mediastinum and beneath the trachea ; gaining the entrance to 

 the chest, and leaving it by turning round the anterior border of the first rib, 

 under the insertion of the scalenus, they become inflected backwards and down- 

 wards, to be placed, one to the right, the other to the left, at the internal face of 

 the anterior limb, in the middle of the nerves of the brachial plexus, and continue 

 within the arm, assuming the name of humeral artery on leaving the interstice 

 which separates the subscapularis muscle from the adductor of the arm. 



In its thoracic course, the left trunk describes a curve — the convexity being 

 upwards — the right taking a rectilinear direction. 



Relations. — In studying the relations of the brachial trunks, we recognize two 

 principal portions — one thoracic, placed in the chest ; the other axillary, situated 

 within the limb. In their thoracic portion, the brachial trunks, at first lying 

 beside each other, separate slightly in front to i-each the internal face of each of 

 the two first ribs. They are accompanied by the cardiac, pneumogastric, inferior 

 laryngeal, and diaphragmatic nerves, and are included, as already noticed, between 

 the two layers of the anterior mediastinum. The right occupies nearly the 

 middle 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 coiTesponding venous 

 trunks, cross the terminal tendon of the subscapularis muscle in passing below 

 the humeral insertion of the deep jKictoral, and among the branches of the 



