gradual condensation of the coeliac and superior mesenteric 

 stems from segmental vessels can be observed. 



Fishes The arterial pattern of the fishes, whether dipnoan, 

 actinopterygian, or shark does not suggest anything beyond 

 that which is seen in the more advanced forms. 



In the cyclostomes there is marked contrast between the 

 lamprey and the hagfish. In the lamprey there is a large 

 coeliacomesenteric artery which comes off the dorsal aorta 

 near the duct of Cuvier and, passing down to the left of the 

 gut along with the bile duct, enters the typlosole. It extends 

 most of the length of the intestine. Far posteriorly are two or 

 three mesenteric arteries, each enclosed by a venous chan- 

 nel, which also serve the gut wall. 



In the hagfish the coeliacomesenteric passes down to the 

 left of the gut, to the right of the bile duct, and posteriorly 

 a short distance next to the hepatogastric mesentery. Most 

 of the length of the intestine is served by numerous mesen- 

 teric arteries — the primitive style as suggested in the Am- 

 phibia. 



Veins 



Mammals In the human, blood is returned to the heart 

 through a large postcaval vein or posterior vena cava 

 which parallels the course of the dorsal aorta but lies be- 

 neath that vessel. The posterior vena cava receives blood 

 from the various hepatic vessels and, more posteriorly, a 

 pair of renal veins and several vertebral branches; it is 

 formed posteriorly by the union of two common iliac veins 



and a middle sacral vein. Each iliac has femoral and hypo- 

 gastric branches. The pattern of the postcava and its branches 

 is much the same as the pattern of the main arteries. 



The difference between these two systems lies in the fact 

 that the veins draining the digestive tract form an hepatic 

 portal vein. This enters the liver and branches to all parts 

 of that structure in order to supply the small sinusoids. 

 Blood is collected from these sinusoids by the hepatic veins 

 and drained into the postcava. 



EMBRYOLOGiCAL DEVELOPMENT The earliest vessels to ap- 

 pear are the vitelline veins. These are soon augmented by 

 and functionally replaced by the umbilical veins which ex- 

 tend back to the allantois and out along the allantois to the 

 placenta. The umbilical and vitelline veins join the ducts of 

 Cuvier (common cardinals) to form the sinus venosus. The 

 development of the circulation within the liver involves for- 

 mation of hepatic veins and the hepatic portal stem from 

 the vitelline veins; the right umbilical disappears, while the 

 left connects the embryo with the placenta and continues 

 through the liver as the ductus venosus. Behind the liver, 

 the vitelline veins unite above and below the gut. As a re- 

 sult of these anastomoses, an hepatic portal channel extends 

 up on the left side of the gut, over the gut, and down on 

 the right side where it comes to drain the channels in the 

 supporting mesenteries of the digestive tract. 



The development of the postcava is a complex story (Fig- 

 ure 11-36). The posterior cardinals (or postcardinals) at 

 first are the main drainage of the body wall. With develop- 

 ment of the mesonephric kidney, there is a rearrangement 



subcardinal (anterior portion 



posterior cardinal 



nephric duct 

 umbilical artery 



ordinal 

 n vessels 



cardinal 

 orsal aorta 



jcardinal (anterior portion) 



anterior part of postcardinal 



coeliac artery 



adrenal vein 



nterior mesenteric artery 

 bcardinal anastomosis 

 erior part of postcardinal 



metanephros 

 posterior mesenteric artery 

 rtery 



caudal artery , , . 



caudal artery 



Figure 1 1-36. Two stages in the development of the renal circulation of the cov/, (After Grau, 

 1933) 



368 • THE CIRCULATORY SYSTEM 



