524 STRUCTURE OF THE EPIDERMIS. 



papillary layer. With loss of contact with the dermis, the vital force 

 is progressively diminished ; the cell becomes subject to the influence 

 of physical laws, and evaporation of its fluid slowly ensues. In con- 

 sequence of this evaporation the cell becomes collapsed and flattened, 

 and assumes an elliptical form ; the latter is by degrees converted into 

 the flat cell with parallel and contiguous layers, and an included nu- 

 cleolated nucleus; and lastly, -the flattened cell desiccates into a thin 

 membranous scale, in which the nucleus is no longer apparent, and 

 is thrown off by desquammation. 



From this explanation it is apparent that the epidermis must be 

 composed of a series of strata of nucleolo-nucleated cells, which exhibit 

 a progressive increase of flattening, from the plastic fluid and cyto- 

 blasts of the deepest layer to the thin and horny scales of the outer- 

 most layers. It is this peculiarity that enables us to split the epider- 

 mis into laminae, the deepest of these laminae, composed of the soft 

 and newly formed cyto-blasts and cells, being the rete mucosum. In 

 the deepest layer, the cyto-blasts are connected with each other by the 

 unemployed portion of plastic lymph which constitutes an intercellu- 

 lar substance ; the cells are connected by their parietes, and the flat- 

 tened cells and scales by their surfaces and borders. 



The under surface of the epidermis is accurately modelled on the 

 papillary layer of the dermis, each papilla having its appropriate 

 sheath in the newly-formed epidermis or rete mucosum, and each 

 irregularity of surface of the former having its representative in the 

 soft tissue of the latter. On the external surface, this character is lost; 

 the minute elevations corresponding with the papillae, are, as it were, 

 polished down, and the surface is rendered smooth and uniform. The 

 palmar and plantar surfaces of the hands and feet are, however, an ex- 

 ception to this rule ; for here, in consequence of the large size of the 

 papillae and their peculiar arrangement in rows, ridges corresponding 

 with the papillae are strongly marked on the superficial surface of the 

 epidermis. The epidermis is remarkable for its thickness in situations 

 where the papillae are large, as in the palms and soles. In other 

 situations, it assumes a character which is also due to the nature of the 

 surface of the dermis ; namely, that of being marked by a network of 

 linear furrows, which trace out the surface into small polygonal and 

 lozenge-shaped areae. These lines correspond with the folds of the 

 dermis produced by its movements, and are most numerous where 

 those movements are the greatest, as in the plexuses and on the con- 

 vexities of joints. 



The dark colour of the skin among the natives of the South is due 

 to the presence of granules of pigment, which are disseminated in the 

 interstices of the cyto-blasts and cells of the rete mucosum, or deve- 

 loped in the same situation into proper pigment cells. In the super- 

 ficial layers of the epidermis, as evaporation proceeds, the colour of 

 the pigment is gradually lost. 



The pores of the epidermis are the openings of the perspiratory ducts, 

 hair follicles, and sebaceous follicles. 



