COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 269 



arch, the right side having disappeared; the fifth aortic arches are absent; the bases of the 

 sixth arches are the right and left pulmonary arteries (Fig. s8G). The embryonic connection 

 of the aorta and the pulmonary (Fig. 58) persists in the adult as the arterial ligament. The 

 branches of the dorsal aorta are similar to those of the other animals studied. 



The venous system is not markedly different from that of reptiles. There are two precaval 

 veins as in the turtle, or in some mammals, the left precaval joins the right precaval by a cross- 

 connection in front of the heart forming a single precaval (Fig. 59). The proximal end of the 

 left precaval then becomes the coronary sinus of the heart. The coronary sinus and bases of 

 the precavals are the common cardinal veins of the dogfish and of the embryo. The internal 

 jugular branch is the original anterior cardinal vein, but in the adult mammal is often exceeded 

 in importance by its branch, the external jugular. The proximal portions of the posterior 

 cardinal veins are absent as in the turtle, except that the azygos vein, at least in part, is the 

 remnant of the right posterior cardinal (Fig. 59) . Usually, however, other veins also contribute 

 to the formation of the azygos. The postcaval vein has the same relations as in the turtle, but 

 has now extended itself posteriorly. Whereas in the turtle it extends no farther caudad than 

 the posterior ends of the kidneys, it is now seen to collect from the entire posterior part of the 

 body. This change has been accomplished very simply; the postcaval vein unites with that 

 part of the renal portal system posterior to the kidneys. The renal portal system then passes 

 out of existence and there is no longer in the adult any trace of a portal circulation through 

 the kidneys. It will be remembered that it was explained in connection with the turtle that 



renal collar by vertical hatching. A, early stage, showing the anterior and posterior cardinal veins 

 a, b, c, the common cardinal vein d, the subcardinal veins/, and the outgrowth e from the hepatic veins 

 of the liver. B, next stage, showing the union of the hepatic outgrowth e with the subcardinal veins/, 

 to form the proximal part of the postcaval vein; the two subcardinals have united with each other at h. 

 C, the anterior part of the posterior cardinal vein b has separated from the posterior part c, c now being 

 the renal portal vein; the postcaval vein is seen to be formed of the hepatic vein e, the right subcardinal/, 

 and to be united by means of the two subcardinals below h with the renal portals c. D, the supracardinal 

 system of veins *', represented by open stippling, has appeared and has united anteriorly with the 

 anterior parts of the posterior cardinals b, medially with the subcardinals by an anastomosis k, named 

 the renal collar, and posteriorly with the renal portals c. E, union of the two anterior cardinals by a cross- 

 connection p, and development of the renal veins from the renal collar k; the supracardinal veins have 

 separated into anterior parts connected with the posterior cardinals b and posterior parts connected 

 with the subcardinals and renal portals c. F, continuation of E. G, adult stage; the left anterior 

 cardinal joins the right by means of the cross-vein p, which is the left innominate vein; the common 

 stem a, which is the right anterior cardinal, enters the heart by way of , which is the right common 

 cardinal vein; the left common cardinal vein persists as the coronary sinus o; the right anterior parts 

 of the posterior cardinal vein and supracardinal form the azygos vein, b and i, while on the left side 

 these are obliterated at v; the postcaval vein is now complete and is seen to be composed of the hepatic 

 vein e, the right subcardinal, the anastomosis between the two subcardinals at h, the right renal collar k, 

 the posterior part of the supracardinal vein i, and the posterior parts of the renal portals (posterioi 

 cardinals) c: the left subcardinal and posterior cardinal contribute to the vein of the left gonad, hence 

 the asymmetrical arrangement of the genital veins of mammals, a, anterior cardinal; b, anterior part 

 of the posterior cardinal; c, posterior part of posterior cardinal or renal portal; d, common cardinal; 

 e, hepatic portion of the postcaval (this is partly removed in Figs. D-G); f, subcardinal; g, gonad; 

 h, union between the two subcardinals; i, supracardinal; j, kidney (metanephros); k, renal collar or 

 union between subcardinals and supracardinals; /, adrenal gland; m, vein to adrenal gland; n, base 

 of the precaval vein or right common cardinal; o, coronary sinus or left common cardinal; p, left 

 innominate or connection between the two anterior cardinals; q, internal jugular; r, subclavian; s, 

 external jugular; /, external iliac; u, internal iliac. (After Huntington and McClure in the Anatomical 

 Record, Vol. XX.) 



