THE SKIN. 



159 



The tubes are like 



up, and flow over on the surface, 

 springs, drawing their supply from 

 the blood-vessels beneath. It has 

 been estimated that there are 2,500,- 

 000 of them altogether, and that, if 

 they were all joined in one tube, it 

 would be ten miles long. 



PERSPIRATION. 



7, We do not see any moisture on 

 the surface when we are cool, but 

 there always is some pouring out 

 of the tubes. Ordinarily it changes 

 into vapor, and is wafted away so 

 fast that it can not gather in drops. 

 But, when we are very warm, it 

 wells up so rapidly that drops 



appear; and these sometimes flow in streams, 

 remain quiet in a cool place, we dry off; that is, the per- 

 spiration does not come so fast, and that which is already 

 on the surface is changed to vapor, and carried away by 

 the air. 



The moisture which is constantly coming from the 

 pores, but which we can not see, is called insensible per- 

 spiration. That which we can see and feel, as water, is 

 called sensible perspiration, or sweat. 



HAIR. 



8. Hairs grow from the skin. On most of the body 

 they are short and fine. On the scalp and the face they 

 grow long. The hair which covers the bodies of the 

 lower animals is very useful in keeping them warm. 



Fig. 62. 



SWEAT-GLAND, WITH CAPIL- 

 LARIES SURROUNDING IT. 



If we 



