STRUCTURE OF THE EPIDERMA. 471 



nished ; the cell becomes subject to the influence of physical laws, and 

 evaporation of its fluid slowly ensues. In consequence of this evapora- 

 tion, the cell becomes collapsed arid flattened, and assumes an elliptical 

 form ; the latter is by degrees converted into the flat cell with parallel and 

 contiguous layers, and an included nucleolated nucleus ; and lastly, the 

 flattened cell desiccates into a thin membranous scale, in which the nucleus 

 is no longer apparent. 



My own investigations* have shown that, after the original granules of 

 the liquor sanguinis have become aggregated into a granular nucleus, other 

 granules are formed in successive circles, around the circumference of the 

 nucleus, until the entire breadth of the epidermal scale is attained ; that 

 the cell never acquires a greater thickness than that of the original nucleus, 

 and, that the formation of the scale results from the desiccation of the cell, 

 as it is gradually pushed outwards from the derma towards the surface. 

 Consequently, the cell never possesses any other than the flattened form ; 

 all its phases of growth are perfected in the deepest layer of the epiderma ; 

 and, in its internal structure, it is a parent cell containing secondary and 

 tertiary cells and granules, its growth being the result of the growth of 

 these secondary formations. 



The under surface of the epiderma is accurately modelled on the papil- 

 lary layer of the derma, each papilla having its appropriate sheath in the 

 newly-formed epiderma 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 corre- 

 sponding 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 exception to this rule ; for here, in con- 

 sequence 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 epiderma. The epiderma 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 derma ; namely, that of being marked by a 

 network of linear furrows, which trace out the surface into small polygonal 

 and lozenge-shaped areas. These lines correspond with the folds of the 

 derma produced by its movements, and are most numerous where those 

 movements are the greatest, as in the flexures and on the convexities of 

 joints. 



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

 the coloration of the primitive granules of which the cell is composed, es- 

 pecially the nucleus. As the cells desiccate, the colour of the granules is 

 gradually lost; hence the deeper hue of the rete mucosum. 



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

 hair follicles, and sebiparous glands. 



Vessels and Nerves. The Arteries of the derma which enter its struc- 

 ture through the areoke of the under surface of the corium, divide into inr 

 numerable intermediate vessels, which form a rich capillary plexus in the 

 superficial strata of the skin and in its papillary layer. In the papillae of 

 some parts of the derma, as in the longitudinal plications beneath the nail, 

 the capillary vessels form simple loops, but in other papillae they are con 

 voluted to a greater or less degree in proportion to the size and importance 



* Diseases of the Skin, 2d edition, p. 5. 



