DUTIES OF THE SKIN. 17 



absorb water. When air has absorbed all the moisture it can 

 hold, it is said to be saturated, and then evaporation cannot 

 go on. The degree of humidity in the atmosphere varies 

 between the limits of absolute dryness and saturation point, 

 neither of which are ever attained in nature. The degree of 

 saturation varies according to the temperature ; hence, the 

 warmer the air, the more moisture it is capable of absorbing. 

 Also, the drier the air, the more rapidly does evaporation 

 take place, and consequently, the greater is its cooling effect. 

 Dry atmospheric heat can therefore be far more easily borne 

 than moist heat. The difference which the presence of 

 moisture in the air makes on the rate of evaporation, is well 

 exemplified by a comparison drawn between the condition of 

 the skin of a man or horse in a hot moist climate like that of, 

 say, Singapore, and in a hot dry one like that of the Arabian 

 desert. In the former, the skin remains more or less wet from 

 sweat which is unable to escape into the nearly saturated air ; 

 but in the latter, its usual condition is one of extreme dryness. 



It is evident that the cleaner the skin and the shorter the 

 coat, the more freely will evaporation take place from the 

 surface of the body. 



In addition to the evaporation of perspiration, there is an 

 evaporation of moisture which passes through the skin 

 mechanically, in the same way as it would do from a freshly 

 killed dead body. This evaporation, which is far less than 

 that from perspiration, has also of course a cooling influence 

 on the system. 



4. To secrete oil, which serves to lubricate the skin and hair. 

 This oil is necessary to keep the skin in a pliable condition ; for 

 we find that when a large proportion of it has been removed, 

 especially from those parts which are subjected to a good deal 

 of bending (flexion and extension), the skin is liable to crack, 

 as in cases of " cracked heels " and mud fever. This oil gives 

 to the skin a polish, which naturally helps to check loss of 



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