THE BRACHIAL OR AXILLARY ARTERIES. 559 



metatarsal spaces, with the plantar interosseous arteries by the anterior and posterior 

 perforating 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 the Horse by the name of metatarso- Fig. 281. 



pedal artery, or collateral of the cannon ; it forms the in- 

 ternal collateral dorsal of the fourth 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. 



Aeticlb V. — Anterior Aorta. (Pig. 282, 1.) 



This vessel, the smallest of the two trunks suc- 

 ceeding the common aorta, is no more than 2 or 2^ 

 inches in length at the most. It leaves the pericar- 

 dium to pass between the two layers of the medias- 

 tinum in au oblique 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 trunJcs or 

 ancillary arteries. 



In the Pachyderms and Carnivora, the anterior 

 aorta does not exist, and the axillary arteries arise 

 directly from the aortic trunk, towards the point 

 from which the anterior aorta springs in other 

 animals. 



ARTERIES OF SOLE OP 

 HUMAN FOOT. 



1, Under surface of os cal- 

 cis ; 2, Musculus acces- 

 sorius ; 3, Long flexor 

 tendons ; 4, Tendjin of 

 peroneas longus ; 5, Ter- 

 mination of posterior 

 tibial artery ; 6, Internal 

 plantar ; 7, External 

 plantar ; 8, Plantar arch 

 giving off four digital 

 branches, three of which 

 are seen dividing into 

 collaterals for adjoining 

 toes. 



Article VI. — Brachial Trunks or Axillary 

 Arteries. (Fig. 282, 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 furnishes the arteries of the head. It is also 

 named the IracMo- cephalic trunk (or arteria innomi- 

 nata). 



Origin. — They separate from one another at an 

 acute angle, the left being a little more elevated 

 than the right. 



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

 laminsB 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 downwards, 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 nervous 

 branches of the brachial plexus, and continue within the arm, assuming the 

 name of humeral artery on leaving the interstice which separates the sub- 

 scapularis muscle from the adductor of the arm. 



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

 is upwards, the right taking a rectilinear direction. 



Belations. — In studying the relations of the brachial trunks, we recognise 

 two principal portions : one thoracic, placed in the chest ; the other axillary. 



