288 COMPARATIVE ANATOMY. 



ARTERIAL SYSTEM. 



It has already been mentioned that in all Vertebrates there 

 is a large sub-vertebral vessel running in the longitudinal axis 

 of the body, called the aorta (Figs. 215 and 223, A), and that 

 this is formed by the union of the branchial vessels. From the 

 latter are also formed the carotids, which go to the head and 

 neck: of these, the internal carotid passes mainly into the 

 cranial cavity, and supplies the brain with blood, while the ex- 

 ternal carotid goes to the external parts of the head (face, 

 tongue, and muscles of mastication). 



The origin of the subclavian artery, which supplies the 

 anterior extremity, is very inconstant, being sometimes symme- 

 trical, sometimes asymmetrical. It arises either in the region 

 of the branchial vessels, or from the roots or main trunk of the 

 aorta (Fig. 215, Sb, and Figs. 232 and 233, Sc). Extending 

 outwards towards the free extremity, the subclavian passes into 

 the axillary artery, and, on reaching the upper arm, becomes 

 the brachial artery. This finally divides into two branches 

 for the fore-arm the radial and ulnar arteries; in the hand 

 these give rise respectively to the deep and superficial palmar 

 arches, as well as to the digital arteries. 



In the dorsal aorta a thoracic and an abdominal portion 

 can be distinguished, and from them arise intercostal, lumbar, 

 and intestinal arteries, supplying the body- walls as well as 

 the thoracic and abdominal viscera. The intestinal arteries may 

 again be divided into two principal groups, namely, those which 

 supply the intestinal tract with its appendages (liver and pancreas) 

 and the spleen, and those which go to the urinogenital organs. 

 The branches of all these vary greatly both in number and 

 relative size ; thus, for instance, there is sometimes a single 

 cceliaco-mesenteric artery (Fig. 232, Cni), sometimes a 

 separate coeliac, and one or more mesenteric arteries. The renal 

 and genital arteries also vary in number and arrangement. 



The abdominal aorta is continued posteriorly into the caudal 

 aorta, which usually lies within a canal formed by the ven- 

 tral arches of the vertebrse (Fig. 232, Aoc, and Fig. 233, 0) ; 

 the degree of its development naturally corresponds to the size of 

 the tail. In cases where the latter is rudimentary, as in Anthro- 

 poids and Man for instance, the caudal aorta is spoken of as the 

 median sacral artery, and the aorta here appears to be directly 

 continued, not by it, but by the common iliac arteries, which 

 pass outwards into the pelvic region (Fig. 232, lie). 



Each common iliac artery becomes divided into an internal 

 iliac, or hypog^astric, supplying the viscera of the pelvis, which 

 is derived from the embryonic allantoic artery, and an exter- 

 nal iliac which is continued into the crural, and supplies the 



