1 64 ELEMENTARY PHYSIOLOGY 



dermis. These nerve-endings vary considerably in form, and 

 possibly also in function, and will be described in a future 

 lesson. 



The pores of the skin have already been mentioned as the 

 openings of the ducts of the sweat glands. By making a section 

 of the skin we are enabled to trace these ducts to the deeper 

 portion of the dermis, where the glands are situated. As the ducts 

 pass through the epidermis they are twisted like a corkscrew, but 

 lower down they are straight or only slightly 

 curved. On reaching the deeper layer of the 

 dermis, or, sometimes, on entering the sub- 

 cutaneous tissue, the ducts are coiled up into 

 little balls. These little bodies constitute 

 the sudoriferous, sudoriparous (Lat. sudor, 

 sweat) or sweat glands. Each gland is sur- 

 rounded by a dense network of blood capil- 

 laries, giving it the appearance of a reddish 

 ball. As the blood circulates through these 

 vessels, the fluid which we call the perspira- 

 tion or sweat is separated from it, passing 

 up the duct till it teaches the surface of the 

 skin. 



The perspiration is a watery fluid, con- 

 taining a small amount of dissolved common 

 salt, together with smaller quantities of other 

 salts (see page 81). A little carbonic acid gas 



Fie IM -Magnified is also S iven off b ? the sweat lands ' but this 

 View of a Sweat- 1S insignificant compared with that which 

 gland, with its escapes by the lungs. 

 Duct. From what has been said it is clear that 



a .the gland surrounded we must look upon the skin as an excretory 



by fat-cells ; b. the duct . . L . . . . . / 



passing through the organ that is, one by which waste materials 



d e on m iUough C the tin refe are separated from the blood circulating in 



mucosum, and d, it. Like the lungs, it is a source of loss to 



d h ernl h " s **** epl ~ the blood ; but while the lungs give off a 



great amount of carbonic acid gas and water 



vapour, the skin excretes but little carbonic acid gas. 



The excretion of the perspiration is continuous but exceed- 

 ingly variable, the average quantity discharged being probably 

 about two pounds j>er day in the adult. 



The chief use of the perspiration is undoubtedly the protec- 

 tion of the body from too great heat, or, in other words, the 

 equalisation of the temperature of the system. When the surface 

 of the body is cold, the supply of blood to the skin is decreased 

 by the contraction of involuntary muscular fibres which reduce 



