2IO EXCRETION BY THE KIDNEYS AND SKIN 



Structure. The skin consists of an external covering, the 

 epidermis, with its modifications, hair and nails, and of the 

 cutis vera. Imbedded in the cutis vera are sebaceous and 

 sweat glands and hair- follicles. (Fig. 61.) 



Epidermis. The epidermis consists of at least four lay- 

 ers of epithelial cells. From above downward these are ( I ) 

 the stratum corneum } (2) the stratum lucidum, (3) the 

 stratum granulosum, (4) the rete mucosum or Malpighii. 

 All these except the stratum corneum have a fairly constant 

 thickness. The stratum corneum is thick or thin according 

 to location and degree of exposure, and its cells are flat and 

 horny. The lowest cells of the rete mucosum are columnar. 

 From this last-named layer the cells pass gradually upward, 

 and as gradually assume the shape of the horny layer. The 

 horny cells are thrown off and their place is taken by others 

 from beneath. (Fig. 60.) 



Hairs are to be found on almost all parts of the cutaneous 

 surface. They consist of a bulb and a shaft. A depression 

 of the skin involving both epidermis and cutis vera consti- 

 tutes the hair-follicle in which the bulb rests. A projection 

 at the bottom of the follicle corresponds to a papilla, and 

 upon it the bulb is placed. The shaft has an oval shape in 

 cross section. It is composed of fibrous tissue, outside 

 which is a layer of imbricated cells. 



Nails consist of a superficial layer of horny cells and a 

 deeper one corresponding to the rete mucosum. The root of 

 the nail is received into the matrix a specialized portion 

 of the cutis vera. 



Cutis Vera. The cutis vera is tough but elastic. It rests 

 upon cellular and adipose tissue. Its structure is areolar 

 with some non-striated muscle fibers. Projecting from the 

 cutis vera into the epidermis are minute conical elevations, 

 the papilla. Many of them contain sensory nerve terminals. 



Sweat Glands. Practically the whole cutaneous surface 

 contains sweat glands. Some two and a half mil- 

 lions are thought to exist in the skin of the av- 



