212 EXCRETION BY THE KIDNEYS AND SKIN 



The secreting part lies just underneath the true skin and, as 

 a whole, resembles a small nodule ; however, the nodule con- 

 sists of an intricate coiling of the tube itself which is of ap- 

 proximately uniform diameter throughout. It curls upon 

 itself some 6-12 times and ends by a blind extremity. It is 

 lined by epithelial cells. 



The duct passes away from the glandular coil, runs 

 through the cutis vera. in a comparatively straight course and 

 assumes a spiral shape as it traverses the epidermis to open 

 obliquely on the surface. With the ducts of the larger 

 glands are connected a few non-striped muscular fibers 

 which may aid in the discharge of the secretion. (Fig. 61.) 



Properties and Composition of Sweat. The secretion is 

 colorless, has a slight characteristic odor, and a salty taste. 

 Its specific gravity is about 1003-4, and its reaction is usu- 

 ally acid when just discharged. It contains a large propor- 

 tion of water, a little urea and fatty matter, and quite a 

 quantity of inorganic salts of which the chief is sodium 

 chloride. All the constituents in health are of subsidiary im- 

 portance except the water. Under average conditions of 

 temperature and exercise the amount secreted in 24 hours 

 is about 2 pounds. But the quantity is very variable as 

 much so as~the urine, and may be said in a general way to 

 vary inversely as the urinary secretion. 



Mechanism of the Secretion of Sweat. Sweat is pro- 

 duced continuously, though up to a certain point it passes 

 off as vapor or "insensible perspiration." Beyond that point 

 it accumulates on the skin as an evident fluid and becomes 

 "sensible perspiration." Whether it escapes as sensible or 

 insensible perspiration, it is secreted as a fluid. 



The activity of the cells lining the glandular coils in sep- 

 arating sweat from the blood is undoubted. Distinct secre- 

 tory fibers are distributed to them, and through the influence 

 of these fibers the glands will secrete sweat even without an 

 increase in the blood supply. But usually a determination 

 of blood to the surface means an increase of perspiration. 



