398 FOODS CIRCULATED AND USED IN THE BODY 



and increase in size until they form two large vessels known as 

 the venae cavae. 



Portal circulation. Some of the blood, on its way to the heart, 

 passes to the walls of the food tube and to its glands. From these 

 parts it is sent with its load of absorbed food to the liver. Here 

 the vein which carries the blood (called the portal vein) breaks up 

 into capillaries around the cells of the liver, which takes out the 

 excess sugar and stores it as glycogen. From the liver, the blood 

 passes directly to the right auricle. The portal circulation con- 

 nects the stomach and the small intestine with the liver. It is 

 the only part of the circulatory system where the blood passes 

 through two sets of capillaries on its way from auricle to auricle. 



Demonstration 2. To show circulation in the web of a frog's foot. 



Examine under a compound microscope the web of the foot of a living 

 frog. Note the network of tiny blood vessels, capillaries. Those 

 vessels in which the blood moves in spurts are tiny arterioles; the 

 larger vessels in which the blood moves slowly or steadily are veins. 



Structure of the arteries, veins, and capillaries. A distinct 

 difference in structure exists between the arteries and the veins in 

 the human body. The arteries, because of the greater strain 

 received from the blood which is pumped from the heart, have 

 thicker muscular walls, and in addition are very elastic. Veins 

 are much thinner-walled than arteries and have small valves which 



1. 1... 



2 - 2... 



3 3... 



lepitbelial.-- 

 layer 



.2. smooth ... 

 muscle 

 tissue 



2>.Connectr 

 "tisstxe 



I epitheliia 



Capillary 



Explain the difference between an artery, a vein, and a capillary. 



