Excretion. 133 



. The nails, like the hair, are outgrowths of the 

 epidermis. At the base the nail is supplied with blood, and 

 here it grows. It is alive, hence this part is called the 

 quick ; but at the outer surface and the tip it is dead. 



Examination of the Skin with a Lens. Place a linen tester, or other 

 hand magnifier, over the palm, and note the sweat pores or openings of 

 the ducts of the sweat glands. Count the pores within the square shown. 

 Measure this square, and then estimate the number of sweat glands to a 

 square inch of the palm. 



The Sweat Glands. The sweat glands are minute tubes 

 whose inner ends are closed, and whose outer ends open 

 upon the surface of the skin. The tube going inward pur- 

 sues a corkscrew-like course through the epidermis, then 

 becomes straighter, and is coiled up in a ball in the deeper 

 layer of the dermis, or, more often, in the connective tissue 

 just beneath the skin. (See Fig. 64.) The cells forming 

 the walls of the coiled part are different from those of the 

 duct, or straighter part of the tube. As the blood flows 

 through the capillaries of the skin it gives off lymph. In 

 this lymph are waste matters brought from the muscles 

 and other tissues that have been at work. The sweat 

 glands absorb this waste matter, with considerable water, 

 and pass it out to the surface. 



Composition of Sweat. Sweat is mostly water. About 

 one per cent is solid matter, including salt and certain 

 matter which like the organic waste matter from the lungs, 

 easily putrefies. Sweat varies greatly in its wateriness and 

 hence in the relative amount of solid matter contained. 

 Ordinarily the sweat is evaporated as fast as it is poured 

 out. In distinction from this insensible perspiration, there 

 is the sensible perspiration when it accumulates enough 

 to be seen. These are not two kinds of sweat, but it is con- 

 venient to distinguish between the visible and the invisible. 



