ARTERIAL SYSTEM 319 



finally, a common trunk of origin for the carotids, the subclavians 

 arising independently on either side of it (D). 



ARTERIAL SYSTEM. 



The essential relations of the carotid arteries, dorsal aorta, and 

 pulmonary arteries, as well as the embryonic vitelline arteries, have 

 already been dealt with (pp. 301-305, Figs. 242, 264, &c.). An external 

 carotid and an internal carotid arise on either side independently from 

 the anterior efferent branchial arteries in Fishes and Dipnoans, but 

 from the Amphibia onwards these vessels are formed by the bifurca- 

 tion of each common carotid. In these higher types, the internal 

 carotid passes entirely into the cranial cavity, and supplies the brain 

 with blood, while the external 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 symmetrical, some- 

 times asymmetrical. It arises either in connection with the 

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

 the aorta (Figs. 262-264, &c.). Extending outwards towards the free 

 extremity, the subclavian passes into the brachial artery, from 

 which a dorsal and a ventral branch arise, and these subdivide again in 

 the lirnb. 



From the dorsal aorta, in which a thoracic and an abdominal 

 portion can be distinguished in Mammals in addition to the caudal 

 portion, arise parietal (intercostal, lumbar}, and cceliac, mesenteric, 

 and urinogenital arteries, supplying the body-walls and viscera re- 

 spectively. These all vary greatly both in number and relative 

 size ; thus, for instance, there is sometimes a single cceliaco-mesen- 

 teric artery (Fig. 262), sometimes a separate cceliac, and one or more 

 mesenteric arteries (Fig. 264) ; x the renal and genital arteries also 

 vary in number and arrangement. All the branches of the dorsal 

 aorta, however, present primarily an approximately metameric 

 character, their number becoming more or less reduced owing to a 

 concentration of the vessels, which is more marked in short-bodied 

 than in long-bodied Vertebrates. 



The aorta is continued posteriorly into the caudal artery, which 

 usually lies within a ccelomic canal enclosed by the ventral arches of 

 the vertebra? (Figs. 262-264) ; the degree of its development is 

 naturally in correspondence with the size of the tail. In cases 

 where the latter is rudimentary, as in Anthropoids 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 



1 The cceliac typically supplies the stomach, liver, and spleen ; one or more 

 anterior mesenteries the whole intestine with the exception of the rectum, as well 

 as the pancreas ; and a posterior mesenteric the rectum. 



