WATER ITS RELATION TO METABOLISM 265 



requires about O550 Cal. heat. Fourthly, on the heat lost in 

 the excreta, an amount which is insignificant in comparison. 



The heat loss, then, depends on two sets of conditions : (1) the 

 external conditions the temperature, relative dryness, and move- 

 ment of the air ; (2) the internal conditions of the body, controlled 

 by the vaso-motor and sweat nerves. 



Since all the energy spent in the internal work of the body 

 finally becomes heat, the determination of the total heat produced 

 by the fasting animal, in the state of rest, either directly in a 

 calorimeter or by computation from the amount and kind of 

 matter metabolised, gives us a measure of the total energy of 

 this internal work. It is found to be relatively constant, for 

 the fasting animal adjusts itself to the minimum metabolism 

 necessary for the maintenance of the action of the heart, respira- 

 tory muscles, &c., and of the body heat. Thus 



Dog Fasting-. 



(Rulmer.) 



Since the animal is constantly producing heat, it must be able 

 to give off heat at the same average rate, for if not, the heat 

 produced in one day would raise the body to a temperature at 

 which bioplasm is destroyed. Moreover, since the external con- 

 ditions are subject to frequent and sudden changes, the balance 

 between heat production and heat loss must be capable of prompt 

 adjustment. This adjustment is, however, possible only within 

 narrow limits. With the rise of external temperature a point is 

 reached at which the animal is unable to lose heat as fast as it is 

 produced, and with the fall of temperature a point is reached at 

 which the greatest amount of metabolism cannot balance the heat 

 loss. The extent of this thermic range varies for different animals, 

 and in any given animal can alter with changes in its surface, such 

 as in the thickness of its summer and winter coats, in its colour, &c. 



