746 METABOLISM 



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 

 warmth or when the region of the corpora striata is artificially warmed, 

 the breathing immediately becomes much quicker and deeper, so that 

 pulmonic ventilation, is greatly increased and much more water is carried 

 out as vapor with the expired air. To vaporize the water large quanti- 

 ties of heat are required (seen in the latent heat of steam). In man this 

 method is, ordinarily, not of great importance, but it may become so 

 when sweating is interfered with, as in ichthyosis. The more rapid 

 breathing also facilitates cooling by increasing the conduction of heat 

 from the mucous membranes of the tongue, mouth, throat, etc. The im- 

 portance of this method of cooling has been shown by finding that after 

 the introduction of a tracheal cannula a dog can not withstand an in- 

 crease of external temperature nearly so well as a normal animal. 



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

 inverse 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. 



