486 THE HUMAN BODY. 



creased heat formation in its body. This has been interpreted 

 either as due to the excitation of thermogenic nerve-centres 

 which then stir up increased katabolisms in the tissues or to 

 injury and paralysis of inhibitory centres which normally 

 hold tissue metabolisms in check. The fact that a similar 

 result may be obtained by electrical stimulation of this region 

 of the brain is in favor of the excitation theory, but the possi- 

 bility of the existence also of febrile paralysis of nerve-cells 

 which normally inhibit a heat-production centre should be 

 borne in mind. 



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 capillary coating is merely rudi- 

 mentary and has lost nearly all physiological importance as a 

 protection from cold; except in tropical regions he has to 

 protect himself by artificial garments, which his aesthetic 

 sense has led him to utilize also for purposes 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 temperatures, it should vary with the 

 season. If the surface of the Body be exposed so that cur- 

 rents 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 main- 

 tained in contact with the surface. As every one knows, a 

 " draught " cools much faster than air of the same tempera- 

 ture 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, how- 

 ever, clothes fall into two great groups: those which are 

 good, and those which are bad, conductors of heat. The- 

 iormer allow changes in the external temperature to cool or 

 heat rapidly the air stratum in actual contact 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; 



