290 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



Cll 



m 



vOa 



the pharynx and anterior to the junction of the two tubes. It marks 

 the line of division of the transverse tubes into 

 ascending and descending, those in front of the 

 heart carrying the blood upward while those 

 behind return it to the ventral vessels which 

 carry it forward. The transverse vessels are not 

 continuous but capillaries intervene between their 

 dorsal and ventral moieties. 



With this introduction, an outline of the cir- 

 V^j[|^ culation in the lower vertebrates (ichthyopsida) 



and in the embryos of the amniotes may make 

 the following clearer. 



In all vertebrates the heart lies on the ventral 

 side of the alimentary canal in a pericardial sac 

 which, is a part of the coelom (p, 18). In the 

 heart as soon as it is functional, several parts are 

 differentiated. At the posterior end is a thin- 

 wailed sac, the sinus venosus, which is followed, 

 going forward, by a larger division, the atrium, 

 with slightly thicker walls. The next division, 

 the ventricle, is much thicker walled on account 

 of the development of the muscles necessary to 

 force the blood through the whole system. In 

 front of the ventricle, but still inside of the per- 

 icardial cavity, is a tube, the arterial trunk, which 

 is continued farther forward, in front of the peri- 

 cardial cavity, by the ventral aorta. 



The ventral aorta gives off a series of pairs 



Fig. 311. — Diagram . , , ,. ■, /r \ 1 • 1 • 



of early circulation of of vessels, the aortic arches (fig. 311) which rise 



r bulbus^*arteriosus-°i' °^ ^^^ ^^^^^ ^^ ^^^ pharynx, in the gill septa, to 

 caudal artery; ce, ci, ex- the dorsal side of the alimentary canal where 



ternal and internal ^, , ., ... , •*. j- 1 1 xi. 



carotid arteries; d, those of a Side Unite in a longitudinal canal, the 

 Cuvierian duct; da, radix aortas, the radices of the two sides uniting 



dorsal aorta; h, hypo- ^ 



gastric artery: i, iliac farther back to form the dorsal aorta, which con-. 

 rT4hl£ienreri^ tinues to the posterior end of the body. From 

 vein; oa, omphalomes- the arches and from the dorsal aorta (all of these 

 are arterial) numbers of arteries arise, some of 

 which are mentioned here, others being considered 

 later. 



The most anterior aortic arch gives rise to two 



enteric artery; pc, post- 

 cardinal vein; ra, 

 radices aortae; sa, sv, 

 subclavian artery and 

 vein; v, ventricle; va, 

 ventral aorta, 1-6, 

 aortic arches. 



