414 



THE SKIN. 



these glands in the body were straightened out and put end to 

 end, they would extend a distance of 4 kilometers. 



This brief consideration of the perspiratory glands suggests 

 that their function must be very important. They are constantly 

 at work pouring out their secretion upon the surface of the skin. 

 Ordinarily this secretion is not perceptible, and it is then called 

 " insensible perspiration." Upon active exercise or when the 

 temperature of the air is high this secretion becomes visible, and 

 it is then called " sweat " or " perspiration." The average total 

 amount daily formed is 900 grams. This amount is subject to 



FIG. 231. Vertical section of the skin, diagrammatic (after Heitzmann). 



considerable variations, being increased in summer and diminished 

 in winter. During violent exercise the amount may be as much 

 as 380 grams per hour, and during exposure to very high tempera- 

 tures it has been known to reach 1814 grams in the same time. 



Sweat has a salty taste, a specific gravity of 1003 to 1005, and 

 is acid in reaction. It is claimed by some writers that its true 

 reaction is alkaline, and that its acidity is in reality due to the 



