474 THE HUMAN BODY 



Diuretics. Various substances, caffein, digitalis, urea, salts, 

 and even water, stimulate the kidney to increased activity. Sub- 

 stances which have this effect are known as diuretics. It appears 

 that these act for the most part by stimulating the secreting cells 

 of the tubules to greater activity, although some of them, notably 

 the salts, may bring about an increased pressure in the glomeruli 

 and so an increased transudation through the capsule. 



The Skin, which covers the whole exterior of the Body, consists 

 everywhere of two distinct layers; an outer, the cuticle or epider- 

 mis, and a deeper, the dermis, cutis vera, or corium. A blister is 

 due to the accumulation of liquid between these two layers. The 

 hairs and nails are excessively developed parts of the epidermis. 



The Epidermis, Fig. 141, consists of cells, arranged in many 

 layers, and united by a small amount of cementing substance. 

 The deepest layer, d, is composed of elongated or columnar cells, 

 set on with their long axes perpendicular to the corium beneath. 

 To it succeed several layers of roundish cells, b, the deepest of 

 which, prickle-cells, are covered by minute processes (not indicated 

 in the figure) which do not interlock but join end to end so as to 

 leave narrow spaces between the cells; in more external layers the 

 cells become more and more flattened in a plane parallel to the 

 surface. The outermost epidermic stratum is composed of many 

 layers of extremely flattened cells from which the nuclei (conspic- 

 uous in the deeper layers) have disappeared. These superficial 

 cells are dead and are constantly being shed from the surface of 

 the Body, while their place is taken by new cells, formed in the 

 deeper layers, and pushed up to the surface and flattened in their 

 progress. The change in the form of the cells as they travel out- 

 wards is accompanied by chemical changes, and they finally con- 

 stitute a semitransparent dry horny stratum, a, distinct from the 

 deeper, more opaque and softer Malpighian or mucous layer, b and 

 d, of the epidermis. 



The rolls of material which are peeled off the skin in the "sham- 

 pooing" of the Turkish bath, or by rubbing with a rough towel 

 after an ordinary warm bath, are the dead outer scales of the 

 horny stratum of the epidermis. 



In dark races the color of the skin depends mainly on minute 

 pigment-granules lying in the cells of the deeper part of the Mal- 

 pighian layer. 



