438 ANIMAL HEAT 



It has also been experimentally ascertained that the rate of heat produc- 

 tion varies with the kind of food taken: for example, if sugar be added to the 

 meal of meat given to the dog, the height of maximum production is reached. 

 It is often said that the various nations have found by experience what food 

 is most suitable for the climate in which they live, and that such experience 

 can be trusted to regulate the quantity consumed. Although there have 

 been no very conclusive experiments to prove the view, yet it is a matter of 

 general observation that in northern climates and in colder seasons the quan- 

 tity of food takerris greater than in warmer climates or in warmer seasons. 

 Moreover, the kind of food is different. For example, persons living in the 

 colder climates require much fat in order to produce the requisite amount 

 of heat. 



Influence of the Nervous System on Heat Production. The in- 

 fluence of the nervous system in modifying the production of heat must be 

 very important, as upon the nervous influence depends the amount of the metab- 

 olism of the tissues. The experiments and observations which best illus- 

 trate it are those showing, first, that, when the supply of nerves to a part is 

 cut off, the temperature of that part falls below its ordinary degree after a 

 time; and, second, that when there is severe injury to or removal of the 

 nervous centers the temperature of the body rapidly falls, even though arti- 

 ficial respiration be performed, the circulation maintained, and to all appear- 

 ance the ordinary conditions for chemical changes in the body be com- 

 pletely maintained. 



There is a heat-regulating nervous apparatus closely comparable to that 

 which regulates the secretion of saliva or of sweat, by means of which the pro- 

 duction of heat in the warm-blooded animals is increased or diminished, as 

 occasion requires. This apparatus probably consists of a center or centers 

 in the brain which may be reflexly stimulated, as, for example, by impulses 

 from the skin, and which act through special nerves supplied to the various 

 tissues. The evidence upon which the existence of this regulating appara- 

 tus depends is the marked effect in the increase of the oxygen consumed by 

 a warm-blooded animal when exposed to cold, and the corresponding increase 

 in the output of carbon dioxide, indicating that there is an increase of the 

 metabolism and so an increased production of heat under such circumstances, 

 and not a mere diminution of the amount of heat lost by the skin, etc. A 

 cold-blooded animal reacts very differently to exposure to cold; instead of 

 increasing the metabolism as in the case of the warm-blooded animal, cold 

 diminishes the metabolism of its tissues. It is clear, therefore, that in warm- 

 blooded animals there is some apparatus not possessed by cold-blooded ani- 

 mals, which counteracts the effects of cold. In warm-blooded animals poi- 

 soned by curara, or in which section of the medulla has been done, it has been 

 found that this regulating apparatus is no longer in action, and under such 

 circumstances no difference appears to exist between such animals and those 



