596 



COMPAEATIVE ANATOMY. 



in some, even of tlie larger, trunks. The inferior vena cava (Fig. 

 339, ci), which collects the blood from the kidneys and generative 

 glands, is developed very early ; it accompanies the united umbilical 

 veins, and, when the right one disappears, it receives the left. 

 After the cardinal veins (c) disappear, the veins of the pelvis {hy) 

 are connected with the end of the trunk of the vena cava, as are 

 also the veins of the hinder extremities (iZ), and the caudals. At the 

 time when the umbilical is the largest of the veins, the inferior cava 

 appears to be merely a branch of it. Where the umbilical vein 



enters the liver, the hepatic vessels are f oi'med, 

 while at the same time similar branches from 

 the liver pass to the point where the umbilical 

 unites with the vena cava inferior ; these form 

 the hepatic veins. As the blood which is 

 returned to the heart from the umbilical veins 

 passes through the liver, that portion of them 

 which lies between the afferent and efferent 

 veins is atrophied, and forms the ductus 

 venosus Arantii. That portion of the omphalo- 

 mesenteric vein which receives the mesenteric 

 veins is then converted into the trunk of the 

 portal vein, while the hepatic branches of the 

 umbilical form the branches of the portal vein 

 after the obliteration of the ductus Arantii. 

 The inferior vena cava is thus converted into 

 the chief hinder trunk, into which open the 

 veins of the pelvis, of the hinder extremities, 

 of the renal and generative organs, while the 

 veins of the digestive canal and spleen form 

 the portal vein. 



Fig. 339. Diagram of 

 tho chief trunks of 

 the venous system 

 of Man. cs Vena cava 

 superior, s Vena sub- 

 clavia. je Jugnlaris ex- 

 tema. ji Jugularis in- 

 terna. az Vena azygos. 

 ha Vena hemiazygos. 

 c Indication of the car- 

 dinal veins. ci Vena 

 cava inf. h Venae hepa- 

 ticae. r Vense renales. 

 il Vena iliaca. hy Vena 

 hypogastrica. 



§ 442. 



The blood-vessels are ordinarily distri- 

 buted in the body by the gradual branching 

 of the different trunks, until at last the finest 

 branches of the arteries and veins give rise 

 to the capillary system, which connects the 

 two kinds of blood-vessels with one another. 

 To say nothing of the various special arrangements in certain organs, 

 a somewhat different method of distribution obtains in the blood- 

 vascular apparatus of several regions of the body. A vein or artery 

 suddenly breaks up into a tuft of fine branches, which either do, or 

 do not anastomose, and which lose themselves in the capillary 

 system, or are again soon collected into one trunk. This distribution 

 of the vessels has been long known as a ret e mirabile. Its function 

 is clearly to slacken the blood-current, and to increase the surface of 

 the walls of the vessels, so that there must bo a change in the amount 

 of nutrient fluid diffused by osmosis. If from this rete a vascular 



