THE FOOD AND DIGESTION 361 



During the period of active digestion the temperature of the 

 blood coming from the liver may be nearly 1 C. higher than 

 that of the blood going to the organ. The liver alone among 

 glandular structures contributes an appreciable amount of 

 heat to the body, since the amount of blood passing through 

 the organ is large, and thus a considerable amount of heat is 

 derived from it. 



C. Brain. Some physiologists have maintained that the 

 fact that the temperature of the brain rises during cerebral 

 activity indicates that the chemical changes going on are 

 sufficient to yield a certain amount of heat. But it is more 

 probable that the rise of temperature is due to the increased 

 flow of blood through the organ, since a study of the blood 

 gases in the brain gives no indication of any marked increase 

 of chemical change during periods of increased cerebral 

 action. 



2. Regulation of Temperature. 



Since heat is constantly being produced, the temperature 

 of the body would tend to rise higher and higher, were 

 there not some arrangement by which just as much heat is 

 eliminated as is produced, and by which the temperature is 

 thus kept constant. 



Elimination of Heat. Heat is got rid of by three 

 channels. A. Skin. Since the body is generally warmer 

 than the surrounding air, heat is constantly lost by conduc- 

 tion, convection, and radiation, and the extent of this loss 

 depends mainly upon the difference between the temperature 

 of the body and that of the air. Radiation plays the most 

 important part when a person is sitting quiet in still air ; 

 conduction and convection when the exchange of air over 

 the surface is rapid. The temperature of the skin is increased 

 when, from dilatation of the cutaneous vessels, more blood is 

 brought to the surface, and conversely it is lowered by con- 

 striction of these vessels. The influence of variations in the 

 temperature of the air is generally minimised in man by the 

 covering of clothes, and in animals by the covering of fur or 

 feathers, which retains a stationary layer of air at about 25 

 to 30 C. over the skin. It has been calculated that over 



