THE HEAT OF THE BODY 545 



perature 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. 

 Moreover, while in the American summer it is tolerably safe to 

 wear good-conducting garments, and few people take cold then, 

 this is by no means safe in the spring or autumn, when the tem- 

 perature of the air is apt to vary considerably within the course of 

 a day. A person going out, clad only for a warm morning, may 

 have to return in a very much colder evening; and if his clothes 

 be not such as to prevent a sudden surface chill, will get off lightly 

 if he only "take" one of the colds so prevalent at those seasons. 

 In the great majority of cases, no doubt, he suffers nothing worse, 

 but persons, especially of the female sex, often thus acquire far 

 more serious diseases. When sudden changes of temperature are 

 at all probable, even if the prevailing weather be warm, the trunk 

 of the Body should be always protected by some tolerably close- 

 fitting garment of non-conducting material. Those whose skins 

 are irritated by anything but linen should wear immediately out- 

 side the under-garments a jacket of silken or woolen material. 



