SWEAT GLANDS 



357 



ply modifications of normal skin 

 layers, and have their origin in the 

 dermis. 



Sweat glands. — If we trace a 

 spiral duct which leads downward 

 from a pore in the epidermis, we 

 shall find that it passes through the 

 dermis also and ends finally in the 

 subcutaneous layer as a coil. This 

 knotted coil is surrounded by a 

 network of capillaries, and the com- 

 bination is called a sweat gland. 

 These coils are lined with a secreting 

 membrane of epithelial cells, and the 

 power of the skin to excrete wastes 

 in the form of sweat or perspiration 

 is due to their activity. Their ac- 

 tion is as follows: From the blood 

 brought to them by the capillaries 

 the cells select the salts and nitrog- 

 enous wastes (urea). These wastes, 

 together with water and some salts, 

 are passed on by the cells as a secre- 

 tion to the inside of the tubes where 

 they collect, and finally leave the 

 bod}'' through the epidermal ends fig. i64— s-weatgiand; a, epi 

 of the ducts or pores. This action 

 may take place so slowly as to be 

 invisible to the eye (insensible per- 

 spiration), or in summer, and in violent exercise, drops 

 may collect on the surface of the body (sensible perspira- 

 tion). By placing the skin against a cool surface, such as 



dermis; b, dermis; c, sub- 

 cutaneous layer; d, sweat 

 gland; e, moutli of sweat 

 duct (spiral duct). 



