86 PHYSIOLOGY 



the blood is forced to return to the heart after its pass- 

 age through the body. The elastic walls of the arteries 

 stretch to receive the blood and by pressing upon it 

 force it to go onward in a steady stream. The tone, or 

 the sustained activity of the muscles in the walls of the 

 small arteries, regulates the capacity of the entire sys- 

 tem, so that the tubes are always full of blood; it con- 

 trols the amount of blood going to the tissues so that 

 they always get the supply of oxygen and food that they 

 need; and it keeps the body temperature constant. 





