CHAPTER XVI. 

 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 main- 

 tain within limits a constant temperature which is independent of and gen- 

 erally above that of the surrounding atmosphere. As the temperature of 

 these animals is high and perceptible to the sense of touch, they were origi- 

 nally designated "warm-blooded" animals. As this temperature is con- 

 stant notwithstanding the great variations in external temperature during 

 the summer and winter seasons, they are more appropriately termed con- 

 stant-temperatured or homoio-ihermous 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 organ- 

 ization. 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 suffi- 

 ciently active heat-regulating mechanism, the temperature 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; as their temperature is 

 inconstant, varying with the temperature of the surrounding medium, they 

 are more appropriately termed " variable-tempera tured" or poikilo-ther- 

 mous animals. 



THE TEMPERATURE OF THE HUMAN BODY. 



The determination of the temperature of the human body under the 

 changing conditions of life is a matter of the greatest physiologic and 

 clinical interest. The temperature of the superficial portions of the body 



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