STRUCTURE OF THE CUTICLE. 



411 



Waldeyer), staining deeply with carmine, and they are thought by some to have an 

 intimate relation to the formation of the horny substance in the more superficial 

 cells. Immediately above the stratum granulosum is a clear-looking layer in which 

 the outlines of the cells are somewhat indistinct, owing to the fact that they have 

 undergone a transformation into horny material. This layer, which is not always 

 sharply marked off from the one superficial to it, has been termed stratum 

 lucidum, and may be looked upon as the commencement of the horny layer of the 

 epidermis. Superficial to it is a stratum which in some parts is of considerable thick- 

 ness, and in which the cells are much enlarged, and the nuclei in many cases no 

 longer visible (a) : still nearer the surface this passes into a stratum of hard 

 flattened scales which are being thrown off by desquamation. As the cells change 

 their form, they undergo chemical and physical changes in the nature of their 

 contents ; for in the rete mucosum they consist of a soft, granular, protoplasmic 

 matter, whilst the superficial ones are transparent, dry, and horny. These dry hard 



Fig. 467. SECTION OF EPIDERMIS FROM THE SKIN OF THE FINGER, COLOURED BY PICROCARMINE. 



(Ranvier.) 



a. stratum corneum ; b, stratum lucidum, some of the cells of which are filled with flakes of eleidin ; 

 c, stratum granulosum, full of eleidin granules or drops ; d, spring and fibrous cells of rete mucosum ; 

 e, dentations of deepest cells, for attachment to cutis vera. 



Fig. 468. SKIN OF THE NEGRO, VERTICAL SECTION, MAGNIFIED 250 DIAMETERS. (Kb'lliker.) 



a, a, cutaneous papillae ; b, undermost and dark-coloured layer of oblong vertical epidermis-cells ; 

 c, mucous or Malpighian layer ; d, horny layer. 



scales may be made to reassume their cellular form, by exposure for a few minutes 

 to a solution of caustic potash or soda, and then to water. Under this treatment 

 they are softened by the alkali, and distended by imbibition of water. 



As Zander has pointed out, there are two types of horny layers in the epidermis. 

 The epidermis which covers the greater part of the surface of the body has a 

 horny layer which is composed exclusively of thin flattened scales, whereas that 

 which covers those parts of the skin which have a thicker epidermis and are not 

 provided with hairs, is mainly composed of large swollen-out cells, with a central 

 cavity. These cells perhaps represent the epitrichial layer which during a certain 

 period of foetal development covers the whole surface of the body, but is thrown off 

 elsewhere as the hairs become developed (see next page). 



Many of the cells of the cuticle contain pigment-granules, and in parts give the 

 membrane more or less of a tawny colour, even in the white races of mankind ; the 

 blackness of the skin in the negro depends entirely on the cuticle. The pigment is 



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