406 ANIMAL HEAT AND ITS REGULATION 



As a result of these protective measures, the temperature of the air imme- 

 diately surrounding the body is generally somewhat above 30 C. The skin 

 itself in places where it is clothed has a temperature of 33-35 C., on naked 

 places its temperature is lower (cf. page 399). 



That warm-blooded animals can maintain a constant body temperature, 

 when exposed to a very low external temperature, is due to their natural or 

 artificial clothing. This is perfectly evident from the fact that the tempera- 

 ture of an animal declines more or less when it is shorn, as well as by the 

 experience that a naked man at rest can only maintain his temperature at 

 the normal level when the surrounding temperature is at 27 C. or higher 

 (Senator). 



By experiments with the calorimeter Rubner has determined the saving of 

 heat to the body accomplished by clothing in some special cases. A guinea pig 

 lost normally by radiation and conduction on the average 3.37 Cal. per hour; 

 after being shorn the hourly loss was 4.19 Cal. i. e., 33.3 per cent more. In the 

 human subject the loss from the naked arm by radiation and conduction at 

 ordinary room temperature was about thirty per cent more than that from the 

 clothed arm. 



And yet the saving actually accomplished by the clothes is somewhat less 

 than this would indicate; for the output of water vapor from the clothed body 

 is greater than from the naked because of the higher temperature of the air 

 immediately adjacent to the skin. From experiments on the naked forearm 

 and on the hand it has been found that in a dry atmosphere at a temperature 

 of 15-20 C. about twenty per cent of the total heat loss takes place by evapo- 

 ration. From the naked arm the elimination of water amounted to 3.59 g., and 

 from the clothed arm 4.39 g. a difference of twenty-two per cent. Using this 

 value the saving of heat due to clothes may be calculated as follows : 



The total loss of heat through the skin 100 



By radiation and conduction 80 



By evaporation 20 



The loss by radiation and conduction is diminished by the clothes 



twenty per cent, leaving therefore 56 



While the loss by evaporation is increased twenty-two per 



cent making 24 



Total 80 



The saving according to this amounts to about twenty per cent at ordinary 

 room temperature, and of course at lower temperature is much greater. 



Just as man seeks to reduce as much as possible the loss of heat during 

 the winter by wearing heavier clothing, so the animals offset the influence of 

 the lower temperature by a thicker coat of hair or feathers. What this thick 

 coat actually does for its owner may be seen in polar animals living in the 

 air, which maintain a normal body temperature even at an external tempera- 

 ture of 40 C. (Parry). 



5. REGULATION OF THE BODY'S TEMPERATURE 



The facts thus far discussed relate only to the necessary conditions for 

 the maintenance of the body temperature, but by no means suffice to explain 

 this phenomenon theoretically. For, while both animals and men are all the 



