878 SPECIAL PHYSIOLOGY. 



complished in extreme cases of frost-bite by friction with snow or ice- 

 cold water. By warming the frozen parts too rapidly, the gases of the 

 blood and tissues, set free at the moment of congelation of the water, 

 are expanded in the interior of the capillaries, or amongst the fine 

 structural elements of the tissues, bursting the former or destroying 

 the latter, and so inducing gangrene. In fatal cases of exposure to 

 cold, the body may not, or may, be completely frozen, for the process 

 of congelation of the water in the tissues may penetrate through the 

 whole dead body; but death long precedes such a result, owing, on the 

 one hand, to the fatal benumbing influence of cold on the nervous sys- 

 tem, and, on the other, to the retention of some of the proper heat of 

 the body, after actual death. 



Effects of Heat on the Human Body. 



These are also of great interest. In resisting moderately high tem- 

 peratures, the problem to be solved is that of maintaining the temper- 

 ature of the body, within 2, or at most 5, of its ordinary standard, 

 at a time when it is not only producing heat within itself by its vito- 

 chemical actions, but is also, like any other material mass, receiving 

 heat from a surrounding medium hotter than itself. Thus, the mean 

 daily temperature of the air, in the tropics, during the six summer 

 months, ranges as high as from 80 to 90, in the shade, the highest 

 daily temperature in the shade varying from 104 to 118, and the 

 lowest nocturnal temperature ranging from 60 to 66, the heat thus 

 exhibiting a variation of from 44 to 52 in the course of 12 hours. 

 In exceptional seasons, the highest temperature rises to 130. Again, 

 in the direct rays of the sun, the heat is, of course, still greater. The 

 effect on the system is aggravated by motion of the hot air. The 

 adaptability of Man to extremes of temperature enables him, however, 

 to live in such a climate. 



The chief means of maintaining the normal temperature uf the body, 

 in hot climates, consists in a large increase in the amount of the water 

 exhaled from the surface of the lungs and of the skin, especially, 

 however, from the latter. The skin becomes bathed with fluid, the 

 evaporation of which, at the high temperature of the surface and of 

 the surrounding air, occasions a loss of heat and a reduction in the 

 temperature of the evaporating surface. In this way chiefly, the heat 

 of the body is lowered, and maintained nearly at its normal standard. 

 The effect in reducing the temperature of the body is greater, if the 

 atmosphere be dry as well as warm, and then, also, if it be in motion ; 

 these conditions favor cutaneous exhalation and evaporation. 



It has been shown, by an ingenious experiment, that the evaporation from 

 the surface of the skin of the living frog subjected to high temperature, is suf- 

 ficient to maintain the temperature of the animal at a stationary point, after 

 it has reached a certain height ; whereas moist mushrooms, subjected simul- 

 taneously to the same conditions, soon obey the temperature of the surround- 

 ing air. The frog and the mushrooms were placed in a chamber filled with 

 dry air, heated from 122 to 140 ; at the end of a quarter of an hour, both the 

 frog and the mushrooms were nearly at an equal temperature, viz., from 62 



