EXCRETORY SYSTEM 363 



appears gradually after the age of forty. Little by 

 little the system fails to supply the coloring matter for 

 the central part of the hair. Its place is taken by air. 

 There is no way of preventing this change. 



The nails. The nails are hardened epidermis. 

 They grow from the root which is underneath the skin 

 and are gradually pushed forward. Their function 

 is to protect the ends of the fingers and toes. They 

 should be kept trimmed even with the tip of the finger. 

 The skin at the base of the nail should be rubbed back 

 to keep it from adhering to the nail. 



Temperature of the body. Our bodies are usually 

 warmer than the things around us, but the interesting 

 fact is that our temperature is always the same, about 

 98.5 F. The body is thus as warm on a cold day as 

 it is on a hot day. If the temperature of the body does 

 not change, the amount of heat produced must be 

 equal to the amount of heat lost. If a greater amount 

 of heat is produced, a greater amount must be lost 

 and vice versa. We will now consider how heat is 

 produced in the body, and how it is lost. 



Source of heat. The source of heat in the body is the 

 result of the process of slow oxidation, w r hich is con- 

 stantly taking place. Since the muscles form so large 

 a portion of the body they contribute the most of the heat. 

 The heat produced in different parts of the body is 

 distributed by the blood. In passing through an organ 

 producing heat the blood becomes warmer, and then as 

 it circulates through different parts of the body, this 



