CIRCULATORY SYSTEM 795 



tends to the end of the tail. This fused portion is known as the caudal 

 vein. In the cyclostomes this connection persists. It disappears in other 

 vertebrates. 



The left Omphalomesenteric vein, which passes along the left side 

 of the liver, continues to carry blood from the caudal or posterior part 

 of the body to the heart, while the right disappears with the exception of 

 the small portion between the sinus venosus and the liver. 

 The Portal System: 



It will be remembered that the liver develops from a simple sac into 

 a compound tubular glandular structure. The left omphalomesenteric 

 breaks up into a great mass of capillary-like tubules or sinusoids, which 

 pass among the tubules of the liver and end by reconnecting at .the 

 cephalic end of the liver. As the liver increases in complexity so do 

 these sinusoids. The left omphalomesenteric is consequently quite im- 

 portant during this period and it is known as the ductus venosus 

 (Arantii), (Fig. 451). A little later, however, this importance is lost 

 by a part of the omphalomesenteric becoming the portal vein, which 

 brings all the blood from the posterior regions of the body to the liver, 

 sending it through the tiny sinusoids. The ends between the heart and 

 the liver, formerly called the ductus venosus, now become the hepatic 

 veins. It is in and through the hepatic veins that the collected blood 

 from the liver sinusoids is sent to the heart. 



When a vein breaks up into capillaries of this kind, as in the liver 

 and kidneys, and its contents are again gathered in a vein, it is called a 

 portal system. That of the liver is the Hepatic portal, while that of the 

 kidney the Renal portal system. 



In elasmobranchs and sauropsida, which produce eggs with large 



D, the supracardinal system of veins i, 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 fc, named the renal 

 collar, and posteriorly with the renal portals c. 



E, union of the two anterior cardinals by a cross-connection p, and develop- 

 ment of the renal veins from the renal collar fc; 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 n, 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 (posterior cardinals) c: the left subcardinal 

 and posterior cardinal contribute to the vein of the left gonad, hence the 

 asymmetrical arrangement of the genital veins in mammals. 



H, composite diagram of the veins of a cat. 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 re- 

 moved in Figs. D-G ;) /, subcardinal; g, gonad; h, union between the two sub- 

 cardinals; i, supracardinal; j, kidney (metanephros) ; fc, renal collar or union 

 between subcardinals and supracardinals ; I, adrenal gland ; m, vein to adrenal 

 gland ; n, base of the precaval vein or right common cardinal ; o, coronary sinus 

 or left common cardinal ; omph.mes., omphalomesenteric artery ; p, left innominate 

 or connection between the two anterior cardinals ; q. internal jugular ; r, sub- 

 clavian ; 8, external jugular; t, external iliac; u, internal iliac. (Partly from 

 Hyman after Huntington and McClure in Anatomical Record, Vol. XX.) 



