THE SKIN. 213 



CHAPTER VIII. 



Animal Heat permanent. Skin regulates it. Excess of Heat 

 carried off by Evaporation of the perspired Fluids. Cold 

 Sweats. Sensations of Heat and Cold comparative. 



508. The skin is itself a bad conductor of heat ; that is, 

 it does riot allow heat to pass easily, either outwardly or 

 inwardly, and therefore it is a good protector against high or 

 low temperatures. The natural and usual temperature of 

 the body is 98 ; ..but the surrounding air is often at 100 in 

 summer, and afc-0 in winter; and, in some- extreme climates, 

 it is 30 warmer, or 150 colder, than our bodies. In the 

 experiments of Sir Charles Blagden, ( 402, p. 172,) it was 

 more than 160 higher than the standard of 98 ; and yet in 

 neither case is the heat of the body materially changed. In 

 the heated room, a thermometer placed in the mouth was 

 hardly raised, and, beyond the arctic circles, it scarcely tell 

 below our usual temperature. 



509. There is, of course, a constant tendency to radiation 

 of heat from the skin when the air is colder than our bodies, 

 as from any other substance : and, on the other hand, there 

 must be a tendency to receive heat from the air when that 

 is warmer than the body. In the first case, in cold weather, 

 more internal heat is produced, ( 443, p. 189,) to supply 

 loss from increased radiation. In warm seasons, the evapo- 

 ration of the perspiration absorbs the excess of animal heat, 

 and thus the equilibrium of the internal temperature is 

 maintained. 



510. The evaporation of the cutaneous fluids is the outlet 

 of much of the surplus heat. ( 441. p. 188.) Every one is 

 familiar with the fact, that a wet skin is colder than a dry 

 one, because the evaporation carries off more of the heat. 

 The inhabitants of hot climates make use of this princi- 

 ple, and put water into porous jars, the surface of which is 

 constantly wet with the moisture that oozes through; and 



