The Circulatory System 



171 



foot. These walls are sufficient to retain the cells inside but allow gases, 

 foods, and other substances to diffuse in and out. 



In general, the connections between the arteries and veins are 

 through the capillaries. The arteries gradually break into arterioles and 

 then into capillaries ; the larger veins are formed as the capillaries com- 

 bine first into venuoles and then into veins. At times, there are larger 

 connections between the small arteries and veins. These are known as 

 anastomoses. When a whole network of blood vessels is formed by 

 numerous anastomoses, a plexus results. 



B 



Fig. 51. — Valve of a vein. A, Closed; B, open. 



THE PHYSIOLOGY OF THE CIRCULATORY SYSTEM 



The proper functioning of the circulatory system is ultimately de- 

 pendent upon the heart. By its pumping action it forces the blood 

 through the arteries to the capillaries where the work of the blood 

 takes place. From the capillaries, the blood returns through the veins 

 to the heart where it is once again pumped through the systemic and 

 pulmonary circuits. The heart is a muscular pump which utilizes 

 many familiar mechanical principles to perform its essential work. 



The Heartbeat. — The heartbeat is neither initiated nor main- 

 tained by impulses from the central nervous system. The heart muscle 

 itself has an inherent rhythmicity and automaticity which remains even 

 after all nerves supplying it are severed. Even isolated bits of heart 

 muscle placed in the proper solution will contract for a considerable 

 period. 



The real cause of the rhythmic heartbeat is still unknown, but it 

 can be shown that it originates in special masses of tissue known as 

 neuromuscular tissue which is unique to the heart. One of these masses, 

 the sino-auricular node (the "pacemaker") is located in the wall of the 

 right auricle near the point where the precaval enters. The name sino- 



