IX 



ARTERIES 



323 



of the paired portions of the ventral aorta are known as the ventral or 

 external carotids : those of the paired portions of the dorsal aorta or 

 aortic roots are known as the dorsal or internal carotids. 



The arterial system of Scyllium is illustrated by Fig. 735. The 

 ventral aorta supplies five pairs of afferent branchial vessels (II-VI) and 

 of these V and VI have become approximated together at their origin 

 from the ventral aorta while II and III have gone a step further and 

 become completely fused throughout a 

 great part of their length. Such fusions 

 and alterations of position are common 

 in the vascular system of vertebrates 

 and are potent factors in bringing about 

 differences between the vascular arrange- 

 ments in closely allied creatures. The 

 carotid and efferent systems of vessels 

 in Scyllium undergo complicated changes 

 during the course of development, lead- 

 ing to an arrangement in the adult which 

 is of interest to specialists rather than 

 to general students of vertebrate mor- 

 phology and which therefore need not be 

 further described here. 



The efferent vessels pour their blood 

 into the dorsal aorta which retains the 

 unsplit condition and this is continued 

 back, immediately ventral to the verte- 

 bral column, to the tip of the tail in 

 which region of the body it is known as 

 the caudal artery (Fig. 132, p. 316, A). 

 The dorsal aorta by its branches supplies 

 with oxygenated blood the whole of the 

 trunk and tail regions. Numerous paired 

 branches (parietal arteries) pass to the 

 wall of the body, sending branches to the kidneys (renal arteries) : a 

 pair of subclavian arteries supply the pectoral fin, a pair of iliac arteries 

 the pelvic fins. 



Important unpaired branches pass out ventrally in the sub- 

 stance of the mesentery to supply the alimentary canal. Of these 

 in the Dogfish there are four, the coeliac artery (liver, part of stomach, 

 part of pancreas, anterior end of intestine), the anterior mesenteric 

 (intestine, branches to genital organs), the lieno-gastrie (spleen, part 



coe: 



Fig. 135. 



Illustrating the arterial system of the 

 Dogfish {Scyllium). A, Dorsal aorta; 

 c, conus arteriosus ; coe, coeliac ; e.b, 

 efferent branchials ; sc, subclavian ; 

 v.a, ventral aorta. The roman numerals 

 indicate the ventral portions of the 

 several aortic arches (afferent branchials). 



