5 o6 A MANUAL OF PHYSIOLOGY 



variable, that a fall in the production of heat may be com- 

 pensated by a diminution of heat-loss, and an increase in 

 the loss of heat balanced by a greater heat-production. 



The loss of heat from the surfaces of the body may be 

 regulated both by involuntary and by voluntary means. It 

 is greatly affected by the state of the cutaneous vessels, and 

 these vessels are under the influence of nerves. A cold skin 

 is pale, and its vessels are contracted. In a warm atmo- 

 sphere the skin is flushed with blood, its vessels are dilated, 

 its temperature is increased ; an effort, so to speak, is being 

 made by the organism to maintain the difference of tempera- 

 ture between its surface and its surroundings on which the 

 rate of heat-loss by radiation and conduction depends. A 

 still more important factor in man, and in animals like the 

 horse, which sweat over their whole surface, is the increase 

 and decrease in the quantity of water evaporated and of 

 heat rendered latent. It is owing to the wonderful elasticity 

 of the sweat-secreting mechanism, and to the increase of 

 respiratory activity, and the consequent increase in the 

 amount of watery vapour given off by the lungs, that men 

 are able to endure for days an atmosphere hotter than the 

 blood, and even for a short time a temperature above that 

 of boiling water. The temperature of a Turkish bath may 

 be as high as 65 to 80 C. Blagden and Fordyce exposed 

 themselves for a few minutes to a temperature of nearly 

 127 C. Although meat was being cooked in the same 

 chamber by the heat of the air, they experienced no ill 

 effects, nor was their body temperature even increased. 

 But a far lower temperature than this, if long continued, 

 is dangerous to life. In the summers of 1892 and 1896 

 hundreds of persons died in the United States within a few 

 days from the excessive heat. During the unusually hot 

 summer of 1819 the temperature at Bagdad ranged for a 

 considerable time between 108 and 120 F. (42 to 49 C.), 

 and there was great mortality. A much higher temperature 

 may be borne in dry air than in air saturated with watery 

 vapour. A shade temperature of 100 F. (37*7 C.) in the 

 dry air of the South African plateaux is quite tolerable, 

 while a temperature of 85 F. (29*4 C.) in the moisture- 



