STRUCTURE OF THE VERTEBRATES 



223 



ing of only two chambers, one auricle and one ventricle. The 

 ventricle is thick-walled and muscular, and is the pumping region 

 of the heart. Anterior to the ventricle is the muscular conus 

 arteriosus which continues anteriorly as the ventral aorta. The 

 most posterior chamber is the sinus venosus which receives the 

 blood from the veins and empties it into the thin-walled auricle, 

 from which it passes to the ventricle. 



yeniral' aorta 



Coronary artery 



Fig. 130. Branchial Vessels of the Dogfish. Only the gross relationships of 

 the left side are shown. The vessels break into capillaries in the gills. 



The six pairs of afferent branchial arteries of the embryo (see 

 page 235) are reduced to three in the adult. Beginning pos- 

 teriorly, the fifth and sixth rise close together and in Squalus are 

 fused for a short distance from their point of origin. The fourth 

 embryonic arch passes into the gill arch as the second afferent 

 artery of the adult. The second and third unite to form the first 

 afferent artery, and then separate to supply the two anterior gill 

 arches. The first afferent arch is not complete in the elasmo- 

 branchs, and forms the internal and external carotids. In Squalus 

 these vessels appear to rise as branches from the first efferent 

 arter}^ 



