192 ELEMENTARY PHYSIOLOGY less. 



functions of the skin as a sense-organ, they Avill be 

 described later on (see Lesson VIII.). 



The epidermis lies on the dermis and dips down into all 

 its depressions. It is composed entii-ely of cells and has 

 no blood-vessels. 



The cells may be divided into two layers. Of these the 

 innermost or Malpighian layer (Fig. 57, Em.) is made 

 up of nucleated cells which are tall and columnar where 

 they rest on the dermis, become more rounded and wrinkled 

 as they pass outwards, and then flattened and granular. 

 The outer layer of the epidermis or corneous layer 

 (Fig. 57, Ec.) is made up of cells which, losing their 

 nuclei become converted into flattened, thin scales, con- 

 sisting of, horny material. These are the cells which 

 become so strongly developed on parts of the body sub- 

 ject to friction such as the hands and soles of the feet. 

 They are always being shed from the surface of the skin, 

 and their place is take by new cells passed up from the 

 deeper layers of the epidermis (see also Lesson XII.). 



All over the body the skin presents minute apertures, 

 the ends of channels excavated in the epidermis, and each 

 continuing the direction of a minute tube, usually about 

 80/i (j5j^(j of an inch) in diameter, and a quarter of an inch 

 long, which is imbedded in the dermis. Each tube is 

 lined with an epithelium continuous with the epidermis 

 (Fig. 57, d). The tube sometimes divides, but, whether 

 single or branched, its inner end or ends are blind, and 

 coiled up into a sort of knot, interlaced with a mesh work 

 of capillaries (Fig. 57, (il and Fig. 58). 



This coiled-up portion is called a sweat-gland, and the 

 tube leading from it to the surface of the skin is its duct. 

 The cells lining the duct are small and flat, those in the 

 tube of the gland are larger and more colunuiar, and may 

 be readily stained. 



The blood in the capillaries of the gland is separated 

 from the cavity of the sweat-gland only by the thin walls 

 of the capillaries, that of the glandular tube, and its 



