92 ZOOLOGY. 



To complete this sketch of the phenomena of nutrition, all 

 that remains is to speak of the sources of animal heat. 



OF ANIMAL HEAT. 



174 Animals vary so much in their different heat-pro- 

 ducing powers, that although all produce heat, some are 

 called cold-blooded animals with reference to others. The 

 difference may be shown by comparing the amount of heat 

 produced by a fish and a rabbit, placed in a vessel surrounded 

 with ice ; the quantities of ice dissolved will give the amount 

 of the difference, which is enormous ; for, after three hours, 

 the heat produced by the fish will scarcely have acted on the 

 ice, whilst that originating from the rabbit will have produced 

 more than a quart of water. Now the amount of heat 

 required to convert so much ice into water will be found 

 equal to that necessary to raise 'the temperature of three 

 quarts of water from the freezing to the boiling point. 



Hence the distinction of animals into cold and hot blooded. 

 In man the heat of the skin varies from 97 to 100 Fahr. ; 

 that of the interior of the body is always 100. It is the 

 same in most mammals. In birds the temperature rises to 

 108. The blood in both is hotter. 



Fig. 63. The Marmot (Arctomys marmota). 



175. In general, birds and mammals maintain the 

 same temperature at all seasons of the year and in all climates; 

 but there are some in which the temperature lowers as winter 

 proceeds: these are the hybernating animals, such as the 

 marmot (Fig. 63), the bat, and hedgehog. 



