718 METABOLISM 



fan was then started so as to set the air in motion. Immediately all of 

 the men recovered and remained in a perfectly comfortable condition 

 so long as the fan was kept going. The practical application of these 

 facts to the hygienic control of the working conditions in mine shafts, 

 in submarines, in workshops, etc., will be self-evident. 



The stimulus to increased sweating seems to be dependent mainly on 

 changes in the temperature of the blood; for sweating does not im- 

 mediately set in when the body is subjected to heat, as by a warm bath or a 

 hot pack. It usually takes from ten to twenty minutes after the person 

 has been placed in the bath or surrounded by the warm blankets of the 

 pack before sweating becomes pronounced. It can usually be shown that 

 before it sets in the body temperature has been raised from 0.1 to 0.8 

 degrees C. (0.2 to 1.4 degrees F.). In this regard, therefore, the response 

 of the sweat glands does not occur so promptly as does the dilatation of 

 the cutaneous vessels. 



Loss of heat by evaporation of sweat occurs only in certain animals. 

 It is practically absent, for example, in the dog. The degree to which 

 it may occur also varies in different individuals of the same species. The 

 power of withstanding high temperatures is proportional in man to the 

 facility with which he perspires. Where sweating is interfered with by 

 skin diseases, by ichthyosis, for example, exposure to heat or in- 

 creased heat production, as by muscular activity, may raise the body 

 temperature to a dangerous degree. 



.Another factor upon which the efficiency of evaporation of sweat in 

 cooling the body depends is the relative humidity of the air. When this 

 is high, evaporation of water into it can not occur, and it is on this 

 account that an increase in body temperature is much more likely to 

 occur in warm, humid atmospheres than in those that are dry. At the 

 same temperature people can live in perfect comfort in the dry air of the 

 open plains, but suffer immediately from rise of temperature when they 

 go into the humid air of the river valleys. Similarly, work in hot fac- 

 tories or in mines is quite possible at very high temperatures if the air 

 is kept dry and in motion, but becomes impossible when the air is moist. 

 In judging of the adequacy of air from this point of view, it is there- 

 fore important to take not the ordinary dry-bulb thermometer reading 

 but that of the wet-bulb.* 



In animals, like the dog, that do not perspire over the surface of the 

 body, vaporization of the water in the expired air is the most important 

 method of regulation of heat loss. When such an animal is exposed to 



*The wet-bulb thermometer registers a temperature that is lower than that of the dry-bulb in 

 proportion to the relative humidity of the air. When the air is completely saturated with moisture, 

 the temperature recorded by the two instruments will be the same; when it is perfectly dry, the 

 difference will be maximal. 



