THE FASTING KATABOLISM 263 



as upon an inanimate body. A fall of temperature, for example, 

 tends to increase the rate of outflow of heat and a rise of tem- 

 perature to diminish it. In the so-called physical regulation, 

 these tendencies are offset and the rate of heat emission main- 

 tained constant chiefly by means of changes in the temperature 

 and state of moisture of the skin, brought about on the 'one 

 hand by an adjustment of the blood flow and on the other 

 through the perspiration. 



The tendency of a rise of external temperature to check 

 the outflow of heat is compensated for by a vaso-motor reflex 

 which causes the arterioles leading to the surface of the body to 

 relax (187), so that more blood flows through the skin capillaries, 

 thus tending to raise the temperature of the surface and increase 

 the outflow of heat. This phenomenon is readily observed 

 in the flush which follows exposure to high temperatures. This 

 method of regulation might be compared to opening the win- 

 dows of a heated room to cool it. 



If the external temperature continues to rise, visible per- 

 spiration occurs, or in the case of animals which have no sweat 

 glands, like the dog, a peculiar form of breathing sets in and 

 relatively large amounts of water are evaporated from the skin 

 or from the tongue and the interior of the mouth and throat. 

 In this way, large quantities of heat are carried off as the latent 

 heat of vaporization of water, somewhat as an overheated room 

 may be cooled by sprinkling the floor. 



If the external temperature falls again, the process is reversed. 

 Sensible perspiration decreases, the blood is diverted from the 

 capillaries of the skin to the internal capillaries, and if the change 

 takes place too rapidly, may even lead to congestion of the latter. 

 The .process is analogous to closing the windows of a room as 

 the weather grows colder. 



352. Chemical regulation of body temperature. There 

 are evidently limits to the possibilities of physical regulation. 

 On the one hand, the external temperature may rise so high 

 that it is impossible for the heat to escape from the body as fast 

 as it is produced by the necessary katabolism, and heat apoplexy 

 results. On the other hand, the temperature may fall so low 

 that the utmost restriction of evaporation and the greatest 

 possible diversion of the blood from the superficial capillaries 

 is insufficient to conserve the body temperature. If the windows 



