66 APPLIED PHYSIOLOGY 



121. The heart beats and pulse. While the heart is 

 squeezing the blood, it can be felt through the sides of the 

 body ; this is called a heart beat. Each heart beat makes 

 a wave of blood in the artery which can be felt when the 

 artery is near the skin. This wave is called a pulse and is 

 best felt in the wrist. The heart beats about seventy-five 

 times each minute. Running or other work of any kind 

 makes it beat faster, so as to send more blood to the cells 

 which are working. 



122. How cells eat. Let us learn exactly how the cells 

 of the body eat. We have seen how albumin and mineral 

 matters soak through the sides of the capillaries along 

 with the liquid parts of the blood, so as to reach the cells 

 of the body. Each cell is thus bathed in food, and eats 

 by soaking it in from all sides. So each cell is like the 

 ameba, but with these differences : 



First. The ameba must seek, catch, and digest its 

 food, while the cells of the body are always soaked in food 

 ready for their instant use. A few cells of the body in 

 the stomach, the liver, the heart, and the blood tubes do all 

 the work of making the food ready for the rest of the cells. 



Secondly. The ameba can move about, while the only 

 cells of the body which can move about are the white 

 blood cells. The rest are held in place by little strings 

 called connective tissue. 



Thirdly. The ameba does just what it wants to, while 

 the cells of the body each have one kind of work to do 

 and must obey the orders of the mind in doing it. 



Besides eating, each cell takes air from the blood and 

 gives off carbonic acid gas and water and other waste 

 matter from every side. Thus each cell breathes. We 

 shall see how it breathes when we study the lungs. 



123. The lymph. The cells cannot eat all the plasma 



