412 THE HUMAN BODY. 



timate products of proteid degradation from which urea 

 is made, are carried to the kidneys, and the final formation 

 of urea occurs in these organs. The results of blood analy- 

 sis are conflicting, but on the whole it seems proved that 

 more urea exists in renal-artery blood than in renal -vein 

 blood, which indicates that urea is not made in the kidneys. 

 In death, too, from suppression of the renal secretion, urea 

 is found to accumulate in the blood which would not be the 

 case unless it were normally formed elsewhere and car- 

 ried off by the kidneys. The whole urea question, which 

 is one of great importance, will be more fully considered in 

 Chapter XXVIII. , in connection with the chemistry of 

 nutrition in general. 



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

 consists everywhere of two distinct layers; an outer, the cuticle 

 or epidermis, 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. 117, 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 strata of 

 roundish cells, #, which in the outer layers 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 (conspicu- 

 ous in the deeper layers) have disappeared. These super- 

 ficial 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 outwards is accompanied by 

 chemical changes, and they finally constitute a semitrans- 

 parent dry horny stratum, a, distinct from the deeper, more 

 opaque and softer Malpighian or mucous layer, b and 

 d, of the epidermis. The cells of this latter are soluble in 

 acetic acid; those of the horny stratum, not. 



