212 EXCRETION BY THE KIDNEYS AND SKIN 



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 usually acid 

 when just discharged. It contains a large proportion 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 importance 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 produced 

 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 perspira- 

 tion." Whether it escapes as sensible or insensible perspiration, 

 it is secreted as a, fluid. 



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

 ing sweat from the blood is undoubted. Distinct secretory 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 sur- 

 face means an increase of perspiration. This occurs during 

 violent exercise, e. g. However, that the production of sweat is 

 not altogether dependent on this factor is shown by profound 

 sweating in shock, nausea and like conditions when the skin is 

 pale and cold, and by dryness of the flushed skin in febrile 

 diseases. Furthermore, experiments on inferior animals have 

 revealed fibers which influence the secretion of sweat without 

 affecting the blood flow. 



