324 HISTORY OF THE HUMAN BODY 



into two branches. These pass along the outer sides of the 

 kidneys and are resolved entirely into a set of small branches, 

 the vena renales advehentes, which enter these organs and 

 break up into capillaries. From these the blood is re- 

 collected by veins which emerge from their inner edges, the 

 vence renales revehentes, which unite to form the posterior 

 cardinals. 



There is thus formed a portal system similar to that of 

 the liver, and called the renal portal, in distinction from the 

 latter, the hepatic portal. This relationship is a permanent 

 one in fishes and amphibians, but in the Sauropsida and Mam- 

 malia the kidneys in which this portal system is developed 

 function as such only in the embryo, and become eventually 

 replaced as kidneys by a new organ in connection with which 

 no such portal system becomes developed. 



Thus far in the development of the circulatory system all 

 Classes of vertebrates agree, allowing for slight differences 

 in the relations of the extra-embryonal parts, such as the rela- 

 tive development of the allantois, or the amount of yolk ; one 

 Class, the lowest, that of fishes, remains permanently at this 

 stage, while the others progressively modify the fundamental 

 plan. It may be well, then, to consider the condition in the 

 adult selachian, which is practically that of the foregoing 

 sketch, after which the later development of the various parts 

 of the system may be taken up one after another. 



The circulatory system of the selachians is represented in 

 the accompanying diagram (Fig. 90), in considering which 

 the reader may begin at the heart and trace the vessels an- 

 teriorly. The heart is situated very far forward, immedi- 

 ately behind the gills, its embryonic position in higher animals, 

 and consists of four chambers arranged in a single longi- 

 tudinal row along the median line. The most posterior 

 of these, the sinus venosus, is the receptacle into which is 

 brought the impure blood from all parts of the body. Next 

 in order, into which the blood passes in succession, are the 

 atrium, the ventricle, and the conus arteriosus. This last 

 and most anterior compartment is prolonged into an 

 arterial trunk (truncus arteriosus), which breaks up into 



