368 ESSENTIALS OF PHYSIOLOGY. 



In the process of evaporation much heat becomes latent and is lost to 

 the body ; and the rate at which this loss takes place may be increased 

 either by greater formation of sweat or by hastening the rate of 

 evaporation by exposing the body to a current of air. 



Conversely the loss of heat in this way is checked when an indi- 

 vidual is surrounded by air which is already nearly saturated with 

 moisture. Owing to the heat taken up by water as it evaporates, heat 

 continues to be lost even when the temperature of the surrounding 

 air is higher than that of the body, provided the air is dry ; and in 

 tropical climates the loss of heat from the skin takes place chiefly by 

 evaporation. 



When sweating is very profuse, the amount of heat lost by the skin 

 relatively to that lost through the lungs is increased, whereas, when the 

 skin is cold and perspiration is scanty, the reverse is the case. In dogs, 

 in which, owing to their hairy coat and paucity of sweat glands, loss of 

 heat by evaporation is comparatively slight, an increase in the loss of 

 heat is largely effected by increased respiratory movements. 



The Regulation of Temperature. In cold-blooded animals the 

 amount of carbonic acid given off from the body varies directly with 

 the temperature of their surroundings ; and their metabolic activities, 

 including the production of heat, rise with the temperature, resembling 

 in this respect chemical reactions in the laboratory, which are 

 accelerated at a higher temperature. These animals possess no regula- 

 tive nervous mechanism by which they can counteract the effects 

 of heat or cold. When the surrounding temperature falls, their 

 metabolic activities diminish until they sink into a state of torpor. 

 When the temperature rises, their metabolic activities increase, and 

 their only means of evading the ill effects of an unduly high tempera- 

 ture is to hide in a stream or to burrow into moist earth. 



The maintenance of a constant temperature in warm-blooded 

 animals is effected by an exact adjustment through the central nervous 

 system of the heat production and heat loss. That the production of 

 heat takes place mainly in the muscles and is under the control of the 

 central nervous system is shown by two observations. In the first 

 place, when the motor nerve endings are paralysed by curare so that 

 the muscles are cut off from nervous influences, the animal behaves 

 like a cold-blooded animal. Secondly, when the spinal cord is injured 

 in man or in the lower animals in such a way that the -lower part of 

 the body no longer receives impulses from the brain, this portion 

 becomes poikilothermic. When it is warmed, its metabolism becomes 

 more active, and the heat produced warms the blood passing through 

 it, and may be sufficient to raise the temperature of the whole^body 



