2/0 COORDINATION AND SENSATION 



gets rid of an excess of heat and, by so doing, avoids 

 overheating. 1 



How the Skin cools the Body. The skin is a means 

 of ridding the body of an excess of heat in at least two 

 ways : 



1. By the conduction and radiation of heat from its sur- 

 face as from a stove. This goes on all the time, but varies 

 with the amount of heat brought to the surface by the 

 blood. 



2. By the evaporation of the perspiration. It is a well- 

 established and easily demonstrated principle that liquids 

 in evaporating use up heat. (See Practical Work.) It is 

 also a matter of everyday experience that the perspiration 

 has a cooling effect upon the body and that its flow in- 

 creases with the amount of heat to be gotten rid of. The 

 quantity of perspiration secreted, and of heat disposed 

 of through its evaporation, also varies with the amount of 

 blood circulating through the skin. 



Temperature Regulation by the Skin. Variations in 

 the quantity of blood circulating through the skin enable 

 this organ to throw off just the right amount of heat for 

 keeping the body at the normal temperature. If it is 

 necessary for the body to rid itself of an excess of heat, 

 the quantity of blood circulating in the skin is increased. 

 This brings the blood near the surface, where more heat 

 can be radiated and where it may cause an increase in the 

 perspiration. On the other hand, if the body is in danger 

 of losing too much heat, the circulation diminishes in the 

 skin and increases in the internal organs. This stops the 

 rapid loss of heat from the surface. The skin in this work 



1 Heat also leaves the body by the lungs, partly by the respired air and partly 

 through the evaporation of moisture from the lung surfaces. Respiration in some 

 nnimals, as the dog, is the chief means of cooling the body. 



