218 



HITCHCOCK'S ANATOMY 



most of them empty into the great veins of the neck. Their 

 probable service is to afford a free passage of blood from the 

 brain, even if by excess of arterial action this organ should be 

 overcharged with blood. 



380. Portal System— Use of the Portal System.— An- 

 other apparent exception to the ordinary system of veins is 

 seen in what is called the Portal System. This comprises 

 those vessels which receive their blood from the intestinal 

 canal, the stomach, and the spleen. As these small vessels 

 unite into a larger trunk, instead of passing directly to the 

 heart, they form what is called the Portal Vein, which emp- 

 ties itself into the liver. This vein ramifies into every part 

 of the liver, where the blood is again collected by a series of 

 vessels which unite into several trunks, called the Hepatic 

 Veins, and which convey the blood to the heart. The design 

 of this arrangement is not certainly known as yet, although 

 it is probable that the blood which returns from the alimentary 

 canal is not fitted to enter the general circulation until it has 

 gone through some change in the liver. 



fig. 221. 381. Origin of the Vnhs 



— Valves— Their Discoverer. 

 — The Veins all take their ori- 

 gin in the capillary vessels in 

 every part of the body except 

 those of the stomach, and in 

 number and length of tube 

 greatly exceed the vessels of 

 the arterial system. In the 

 lining membrane of the veins, 

 also, w r e find a peculiarity 

 not presented in the arteries. 

 This is the presence of folds, 

 so that pouches or bags are 

 formed, which readily suffer the flow of blood towards the 



Of what especial service are they ? 380. Describe the Portal System. What is the 

 probable use of so marked an exception to the general circulating System ? 381. Where 

 do the Veins take their origin ? What is said of the Valves of the Veins ? 



Vein laid open to show the Valves, a, 

 Vein. &, Valves. 



