590 A MANUAL OF PHYSIOLOGY 



ance with physiological laws, that a man overpowered by the 

 heat should lie down, as it is that he should walk about and 

 stamp his feet or clap his hands on a cold winter morning. In 

 the one case a diminution, in the other an increase, in the heat- 

 production is aimed at by a corresponding change in the amount 

 of muscular contraction. The quantity and quality of the food 

 also influence the production of heat. The Eskimo, who revels 

 in train-oil and tallow-candles, unconsciously illustrates the 

 experimental fact that the heat of combustion of fat is high ; 

 the rice diet of the ryot of the Carnatic, with its low heat-equiva- 

 lent, seems peculiarly adapted to the dweller in tropical lands. 

 But it would be easy to attach too much weight to considerations 

 such as these. The Arctic hunter eats animal fat, and the 

 Indian peasant vegetable carbo-hydrate, not only because fat 

 has a high and carbo-hydrate a low heat-equivalent, but because 

 in the climate of the Far North animals with a thick coating of 

 badly-conducting fat are plentiful, and vegetable food scarce ; 

 whereas in the river-valleys of India Nature favours the growth 

 of rice, and religion forbids the killing of the sacred cow. 



The production of heat is also controlled by an involuntary 

 nervous mechanism, through which the ' chemical ' regulation 

 of the body-temperature is achieved, as the ' physical ' regulation 

 is accomplished by the nervous mechanisms that control the 

 circulation, the sweat-glands, and the respiratory movements. 

 It is a matter of everyday experience that cold causes involuntary 

 shivering involuntary muscular contractions the object of 

 which seems a direct increase in the heat-production. But 

 besides this visible mechanical effect, the application of cold to 

 a warm-blooded animal, when not carried so far as to greatly 

 reduce the rectal temperature, is accompanied by a marked 

 increase in the metabolism, as shown by an increased produc- 

 tion of carbon dioxide and consumption of oxygen. In cold- 

 blooded animals like the frog the metabolism, on the other 

 hand, rises and falls with the external temperature ; there is 

 no automatic mechanism which answers an increased drain upon 

 the stock of heat in the body by an increased supply. Or, in 

 the light of recent experiments, we ought rather to say that, 

 although the rudiments of a heat-regulating mechanism may exist 

 in such animals as the frog, the newt, and even the earthworm 

 (Vernon), it is only able to modify to a certain extent the effects 

 of changes of external temperature, not to balance or even 

 override them, as in the homoiothermal animal. The warm- 

 blooded animal loses its heat-regulating power when a dose of 

 curara sufficient to paralyze the voluntary muscles is given. A 

 curarized rabbit, kept alive by artificial respiration, reacts to 

 changes of external temperature like the cold-blooded frog. 

 Now, the only action of curara adequate to account for this 



