416 



COMPARATIVE ANATOMY 



From the dorsal aorta arise parietal (intercostal, lumbar\ and 

 cceliac, mesenteric, and urinogenital arteries, supplying the body- 

 walls and viscera respectively. These all vary greatly both in number 

 and relative size : thus, for instance, there is sometimes a single 

 coeliaco-mesenteric artery, sometimes a separate coeliac, and one 

 or more mesenteric arteries (Figs. 322 and 323). 1 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 



sb.a. 



d-d. 



in.c 



H 



afb.a. 



Hy.b.a. 



FIG. 321. BRANCHIAL ARTERIAL SYSTEM OF COD (Gadus morrhua). Lateral 

 view. (From Bridge, altered from T. J. Parker.) 



af.b.a, 1st and 2nd afferent branchial ; cl.a, coeliac ; d.a, median dorsal aorta ; 

 ef.b.a, first afferent branchial; ex.c, " external " carotid ; H, heart; hy.a, 

 hyoidean; ffy.b.a, hypobranchial (to heart and pelvic fins); hy.ps, 

 spiracular pseudobranch ; in.c, "internal" carotid; I. d.a, left supra- 

 branchial; m.a, mesenteric; on, orbitonasal ; oph.a, ophthalmic; r.d.a, 

 right suprabranchial ; sb.a, subclavian ; v.a, ventral aorta; 1 5, hyo- 

 branchial and succeeding gill-clefts. 



more marked in short-bodied than in long-bodied Vertebrates. 

 In other cases a reduction in the number of vessels takes place by 

 the formation of anastomoses, one branch taking to itself the 

 peripheral twigs of another branch, so that the main stem of the 

 latter disappears. 2 



1 The ccaliac 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. In Hatteria a very 

 primitive arrangement of the branches of the aorta is retained. 



2 Special mention must be made of the collateral vertebral artery, which in 

 Urodeles arises on either side from the root of the aorta and extends backwards 

 along the vertebral column to the end of the tail (Fig. 322). At the base of the 

 transverse processes numerous branches extend from it into the vertebral canal, 

 while others pass along the ribs and reach as far as the skin ; it also communicates 

 with the aorta along its entire length. 



