TEMPERATURE OF THE BODY AND ITS REGULATION 1221 



that their surface is moist, and that with a rise of external temperature the 

 rate of evaporation on the surface increases, so that the increase in the rate 

 of cooling by evaporation more than corresponds to the rate of increase in 

 the heat production, which would tend to raise the body temperature. Most 

 of these animals however escape from any extreme rise of external tempera- 

 ture by burrowing underground or taking to the water, while in plants a rise 

 of external temperature assists transpiration to such an extent that the 

 temperature of the plant is generally several degrees below that of the 

 surrounding atmosphere. The extreme variability in the metabolism of 



10 20 30* 40~ 



-*> External temp. C. 



FIG. 557. Effect of alterations in the temperature of the surrounding medium 

 on output of C0 2 in cold-blooded (poikilothermic) animals. (C. J. MAHTIN.) 



such animals implies a state of dependence of all the activities of the body on 

 the environment, which would prevent the utilisation to the full of the 

 available sources of energy. An animal whose metabolism was more or 

 less independent of the surrounding temperature must have a great ad- 

 vantage over an animal liable to have his activities reduced and paralysed 

 by a sudden spell of cold weather ; this greater independence of the environ- 

 ment, which is characteristic of elevation of type, has been achieved by the 

 warm-blooded animals including man. Such animals are often spoken of as 

 homoiolhermic, i. e. animals possessing a uniform temperature, in contra- 

 distinction to the cold-blooded animals, which are poikilothermic and possess 

 a temperature varying with that of their surroundings. 



Amongst the warm-blooded animals the body temperature may be 

 very different according to the species. In birds it is generally from 39 

 to 40 C. ; in mammals it varies from 35 to 40 C. The temperature of 

 man varies within slight limits about 37 C. (98-4 F.). 



