442 THE REGULATION OF TEMPERATURE 



Every one knows that certain substances do not readily conduct heat. 

 In the hot room of a Turkish bath, where all the objects are at the same 

 temperature, metallic objects feel much hotter than those of wood, bone, 

 rubber, etc. A familiar example of the retention of heat by a body sur- 

 rounded by some bad conductor is found in the hay box or Norwegian 

 cooker, wdiich consists of a wooden box having a thick lining of felt. The 

 partially cooked meal while still hot is- placed in the box and the inter- 

 vening space firmly packed with hay or paper. The felt-lined lid is closed 

 and the meal left to cook itself. After several hours the temperature will 

 have fallen only a few degrees. The Dewar or thermos flask depends on 

 a vacuum as non-conductor. 



Animals which have a good layer of subcutaneous fat lose heat 

 much more slowly than lean ones. 



Fat acts as a heat insulator and retards loss by conduction. 

 On a moderately warm day, the obese person becomes uncomfort- 

 ably warm. He is unable to eliminate heat with sufficient rapidity, 

 and, as a consequence, his temperature rises, and may cause an 

 increase in general metabolism amounting to 50 per cent, over 

 that of a thin person. Aquatic mammals rely on their adipose 

 tissue to protect them from a too rapid loss of heat. 



(d) Duration of exposure. The loss of heat varies directly with 

 the duration of exposure of the surface. In actual practice, this 

 can be determined for non-living systems only, because, as we 

 shall see, in the living body secondary reactions take place ; 

 alterations in the state of the surface and also of the deep-lying 

 tissues are produced, rendering difficult the application of this law 

 to living beings. If the student uses his common sense, however, 

 he will readily see that, if all conditions arc kept constant except 

 time, the amount of heat lost by conduction must vary directly with 

 the time of exposure. 



(ii.) Temperature Gradient. 



Newton's law states that the amount of heat lost by a body in 

 unit time is proportional to the difference in its temperature from 

 that of the surrounding medium. The warm body loses heat and 

 becomes colder, while the environment becomes warmer. This 

 loss and gain goes on till the body and its environment are at the 

 same temperature. Inspired air or cold food is warmed to body 

 temperature at the expense of body heat. The drinking of cold 

 water may cause a temporary reduction of rectal temperature. 

 Conversely, hot food and drink tend to increase the temperature 

 of the body. 



The heat lost by excretion depends not so much on this factor 

 as on the amount and nature of the excreta. Compared with 

 the total heat lost, the amount lost by the heating of inspired air, 

 cold ingesta, and by the excreta is trivial. 



