CHAPTER XIX 

 THE HEAT OF THE BODY 



Warm and cold animals. The heat of the body re- 

 sults from the oxidation of food materials in its tissues. 

 The unicellular and lower forms of animals have no 

 means of retaining the heat which they generate within 

 their small bodies, as their skins are so thin and moist 

 as readily to transmit it. Indeed, in many cases, there 

 is but little heat for retention, as their movements are 

 so infrequent or sluggish as to call for but little oxida- 

 tion of food material. As a result, their temperature 

 corresponds to that of the air or water in which they 

 live. They must therefore depend for their warmth 

 upon such external sources as the sun, air and water. 

 In warm-blooded animals, on the contrary, the body 

 is kept at a fairly uniform temperature partly by means 

 of the protective skin and partly by means of the vaso- 

 motor mechanism. 



Heat control. When too much heat is generated dur- 

 ing muscular activity and the temperature of the body 

 tends to rise, the excess heat is taken up by the blood, 

 carried away from the active tissues and glands, and 

 distributed throughout the body. In the course of its 

 circulation, the blood reaches the special nerve centre 

 in the upper end of the spinal cord which has to do 

 with the regulation of heat in the body. If, in spite 

 of its distribution, the blood still contains too much 

 heat, this centre at once actively arranges for the re- 

 moval of the superfluous heat by causing the arterioles 



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