82 HUMAN PHYSIOLOGY. 



takes place so gradually that as fast as it is formed it passes off by evapora- 

 tion as insensible perspiration. Under exposure to great heat and exercise 

 the evaporation is not sufficiently rapid, and it appears as sensible perspi- 

 ration. 



COMPOSITION OF SWEAT. 



Water 995-573 



Urea . 0.043 



Fatty matters ... 0.014 



Alkaline lactates O-3i7 



Alkaline sudorates 1.562 



Inorganic salts .... 2.491 



Urea is a constant ingredient. 



Carbonic acid is also exhaled from the skin, the amount being about 

 zfa of that from the lungs. 



Perspiration regulates the temperature, and removes waste matters from 

 the blood ; it is so important, that if elimination be prevented death occurs 

 in a short time. 



The nervous system influences the secretion of watery vapor by causing a 

 dilatation of the capillary blood vessels around the tubular coil. It is 

 increased by mental emotions ; section of the sympathetic fibres in the 

 neck is followed by a copious perspiration; stimulation of the nerves pro- 

 ducing contraction of the vessels is followed by an arrestation of the elimi 

 nation of the sweat. 



