CHAPTER XIV. 

 ANIMAL HEAT. 



The chemic changes which take place in the tissues and organs 

 of the living body and which underlie all manifestations of life are 

 attended by the evolution of heat. In consequence of this each 

 animal acquires a certain body-temperature. 



In man, as well as in other mammals and in birds, the chemic 

 changes are extremely active and the evolution of heat very great. 

 Through some special heat-regulating mechanism, by which heat- 

 production and heat-dissipation are kept in equilibrium, these animals 

 have acquired and maintain within limits a constant temperature 

 which is independent of and generally above that of the surrounding 

 atmosphere. As the temperature of these animals is high and per- 

 ceptible to the sense of touch, they were originally designated "warm- 

 blooded" animals. As this temperature is constant notwithstanding 

 the great variations in external temperature during the summer and 

 winter seasons, they are more appropriately termed constant-tem- 

 peratured or homoiothermous animals. The intensity of the body- 

 temperature determined by the insertion of a thermometer in the 

 rectum varies in different classes of mammals from^37.2 C. to 40 C. 

 The causes of this variation are doubtless connected with peculiarities 

 of organization. In birds the rectal temperature is usually higher, 

 varying from 40.9 C. in the pigeon to 44 C. in the titmouse and 

 the swift. 



In reptiles, amphibians, and fish chemic changes, as a rule, are 

 not very active and heat-production relatively slight. As they are 

 devoid of a sufficiently active heat-regulating mechanism, the tem- 

 perature of the body is largely dependent on that of the medium in 

 which they live, though it is always one or more degrees above it. In 

 winter the body-temperature of frogs, for example, may decline as 

 low as 8.9 C., the temperature of the surrounding medium being 

 6.7 C. When subjected to temperatures below zero, the temperature 

 of the body may fall below the freezing-point also, when the lymph 

 and fluids of the body become ice. /Though apparently dead, the 

 gradual elevation of the temperature restores their vitality>/In 

 summer-time, on the contrary, the body-temperature may attain to 

 38 C. Similar variations have been observed in other animals. As 

 the temperature of these animals is low and perceptibly below that of 

 our own bodies, they were originally termed "cold-blooded" animals; 

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