236 



ADVANCED PHYSIOLOGY 



of exercise a person takes and the condition of the air about 

 him. One perspires less on a wet day than on a dry one, and 

 less in cold weather than in warm. Ordinarily this secretion 

 is produced so slowly that it evaporates as fast as it is formed 

 and except for a realization that the 

 skin is kept moist and flexible, one is 

 unconscious of it. When the glands 

 secrete very rapidly, their output may 

 not evaporate as fast as it is formed, 

 and collects in small drops; this occurs 

 particularly when the weather is hot, or 

 when one is exercising vigorously. Phy- 

 siologists speak of this "sweat" as sensi- 

 ble perspiration, while the ordinary slow 

 secretion is called insensible perspiration. 

 The amount of insensible perspiration 

 increases with a rise in temperature, 

 from a cold state up to about 92 F; a 

 very sudden increase in the perspiration 

 rate then sets in, and sweat drops collect 

 on the skin; at the same time, CO 2 is 

 eliminated in considerable amount 

 through the skin. In this connection it 

 is interesting to note that in some ani- 

 mals, e. g. the earthworm, frog, etc., the 

 skin is the principal breathing organ. 



Sweat Glands. There are about two 

 and a half million sweat glands scat- 

 tered through the dermis of the human 

 body. They occur in least numbers in the middle of the 

 back, where there are about four hundred per square inch, 

 while on the palm of the hand and sole of the foot there may 

 be two thousand five hundred in the same area. Each sweat 

 gland is a minute tube of uniform diameter, closed at its inner 

 end where much of its length is coiled into a knot; Fig. 119. 



i/oocC 



FIG. 119. DIAGRAM 



Showing a sweat gland, its 

 duct and its blood supply^ 



