ANIMAL HEAT. 369 



several degrees. When it is cooled, its metabolism is diminished, and 

 the temperature of the body may be lowered in spite of the fact that 

 the rest of the body still possesses its regulative mechanism. 



The loss of heat from the body depends upon the amount of blood 

 passing through the vessels of the skin and upon the amount of sweat 

 formed, both of these being under the control of the central nervous 

 system. The maintenance of the body temperature when the surround- 

 ing air becomes colder might be effected either by an increased produc- 

 tion of heat or by a diminished loss of heat. In many animals the 

 adaptation is brought about by changes in the production of heat, more 

 heat being evolved ; in man the adjustment is made in a more 

 economical manner chiefly by variations in the heat loss, and to a much 

 smaller extent by alterations in the production of heat. On a cold day 

 the vessels of the skin are constricted, so as to diminish the loss of heat 

 by radiation and convection, and the formation of sweat is scanty. 

 Conversely, on a hot day the skin is flushed and moist, and the loss of 

 heat is more marked. 



Within moderate limits of external temperature the production of 

 heat varies but little, though it is diminished when the surrounding 

 temperature becomes high. During muscular exercise both heat 

 production and heat loss are increased, the production exceeding the 

 loss, so that for a time the temperature rises above the normal level. 



The intimate relation between heat production and heat loss is also 

 shown in the relationship between production of heat and the size of the 

 animal. The greater loss of heat relative to its weight which occurs in 

 a small animal is met by a correspondingly larger production of heat, 

 with the result that the animal's temperature remains constant. In 

 many animals, including man, the regulative mechanism is not fully 

 developed at birth ; and the temperature of the new-born infant falls 

 unless an excessive loss of heat is prevented by keeping the child in a 

 warm atmosphere. 



The part of the nervous system which regulates the production and 

 loss of heat and keeps the temperature constant is not known, though 

 possibly it lies in the corpus striatum, injuries to which have been 

 found to cause a marked rise of temperature. Wherever its seat may 

 be, the mechanism is so perfect that in man the temperature remains 

 constant, whether he lives in the tropics or in the arctic regions, 

 though the adjustment fails when the heat or cold is extreme. When 

 a man is exposed to excessive cold, the temperature gradually falls till 

 consciousness is lost and finally death supervenes. When the surround- 

 ing temperature is extremely high, and particularly if loss of heat by 

 sweating is interfered with, the temperature of the body rises, pro- 



24 



