THE HEAT OF THE BODY 489 



Clothing. While the majority of other warm-blooded animals 

 have coats of their own, formed of hairs or feathers, over most of 

 man's Body his hairy coating is merely rudimentary and has lost 

 nearly all physiological importance as a protection from cold; ex- 

 cept in tropical regions he has to protect himself by artificial gar- 

 ments, which his esthetic sense had led him to utilize also for pur- 

 poses of adornment. Here, however, we must confine ourselves to 

 clothes from a physiological point of view. In civilized societies 

 every one is required to cover most of his Body with something, 

 and the question is what is the best covering; the answer will vary, 

 of course, with the climatic conditions of the country dwelt in. 

 In warm countries, clothing, in general terms, should allow free 

 radiation or conduction of heat from the surface; in cold it should 

 do the reverse; and in temperate climates, with varying temper- 

 atures, it should vary with the season. If the surface of the 

 Body be exposed so that currents of air can freely traverse it 

 much more heat will be carried off (under those usual conditions 

 in which the air is cooler than the skin) than if a stationary layer 

 of air be maintained in contact with the surface. As every one 

 knows, a " draught " cools much faster than air of the same temper- 

 ature not in motion. All clothing, therefore, tends to keep up the 

 temperature of the Body by checking the renewal of the layer of 

 air in contact with it. Apart from this, however, clothes fall into 

 two great groups : those which are good, and those which are bad, 

 conductors of heat. The former allow changes in the external 

 temperature to cool or heat rapidly the air stratum in actual con- 

 tact with the Body, while the latter only permit these changes to 

 act more slowly. Of the materials used for clothes, linen is a good 

 conductor; calico not quite so good; and silk, wool, and fur are bad 

 conductors. 



Whenever the surface of the Body is suddenly chilled the skin- 

 vessels are contracted and those of internal parts reflexly dilated; 

 hence internal organs tend to become congested; this within limits 

 is a protective physiological process, but if excessive it is danger- 

 ous since the congested membranes of the nose, throat, and lungs 

 are especially liable to fall victims to the agencies which pro- 

 duce colds, influenza or even pneumonia. When hot, therefore, 

 the most unadvisable thing to do is to sit in a draught, throw off 

 the clothing, or in other ways to strive to get suddenly cooled. 



