108 HUMAN BIOLOGY 



151. Hygiene of the plasma. — All the nutrition of the 

 tissues is derived from the blood., and all the nutrients of the 

 blood come from the foods we eat. If these foods are in- 

 sufficient or of an improper kind, the blood will, of course, 

 be deprived of necessary ingredients, and the cells must 

 inevitably suffer in consequence. Hunger and thirst are 

 the sensations that tell us that the blood is in need of new 

 material. That this is true is demonstrated by the fact that 

 these s_ensations disappear when water and liquid food, in- 

 stead of being swallowed, are injected directly through the 

 skin into the blood vessels. 



162. Blood clotting. — When blood escapes from the body, 

 it is a liquid of a bright red color. It soon changes to a dark 

 maroon, however, and later this thickens to the consistency 

 of jelly. This dark red mass is called a blood clot, and the 

 process is known as clotting or coagulation. Coagulation is 

 of great practical importance, since it provides a natural 

 means of closing injured blood vessels, and of preventing 

 loss of blood. 



II. ClKCULATION AND ITS OkGANS 



153. Necessity for the circulation. — From our study thus 

 far, we have found that our bodies are composed of complex 

 chemical compounds that are constantly being consimied 

 in the development of heat and other forms of energy. It 

 is evident, then, that every organ of the body, and indeed 

 every living cell, must be supplied with new material to make 

 good these losses and to provide for growth. The source 

 of all this material is the food we eat. 



In the last chapter we considered some of the processes 

 by which foods are converted into liquid form and made ready 

 for use in the cells. We found that after being liquefied these 



