Chap. XVIII] BODY HEAT 355 



respiratory centres, increases the rate of the respirations, and con- 

 sequently increases the loss of heat. In man respiration plays 

 only a small part in heat regulation, but in animals that do not 

 perspire, respiration is an important means of regulating the 

 temperature. 



Mechanism of heat regulation. — Just how the nervous system 

 controls the amount of heat produced by metabolism is not known. 

 Various theories are advanced, one being that there are special 

 nerves and special heat centres : (1) thermogenetic, which is 

 concerned in the production of heat, (2) thermolytic, which is con- 

 cerned in the dissipation of heat, and (3) thermotactic, which is 

 concerned in regulating the former two. Another theory is that 

 the temperature of the blood influences the motor nerves of the 

 muscles. Stimulation by cold causes increased contraction and 

 increased oxidation. Heat causes relaxation and a decrease in 

 oxidation. Metabolism is also influenced by the action of the 

 vasomotor system, for under the influence of cold, the blood is 

 driven to the interior of the body and metabolism is increased; 

 under the influence of heat, the blood is driven to the surface of 

 the body and metabolism is decreased. 



Heat regulation by muscular exercise and diet. — Muscular 

 contractions give rise to heat, therefore muscular activity is used 

 as a means to counteract the effects of external cold. On the other 

 hand, muscular activity does not increase the temperature in hot 

 weather to any marked extent. This is accounted for by the 

 fact that when muscular exertion causes the blood to circulate 

 more quickly than usual, the blood-vessels in the skin dilate, the 

 sweat-glands at the same time are excited to pour out a more abun- 

 dant secretion, and the heated blood passing in larger quantities 

 through the cutaneous vessels (which are kept well cooled by the 

 evaporation of the perspiration), the general average temperature 

 of the body is maintained. 



During digestion heat is produced partly by the peristaltic ac- 

 tion of the intestines, and partly by the activity of the various 

 digestive glands (particularly the liver). The quantity of food 

 eaten, and the relative amount of heat-producing food, influences 

 the temperature of the body. In cold weather an increase in food 

 (usually accompanied by an increase of fats) serves to replace the 

 greater amount of heat lost. When muscular exercise is impos- 



