EPIDERMIS 



1043 



the pujinentum uti/runi. When examined separately these cells are 

 found to have a polygonal form (fig. 776, ft), and to have a distinct 

 nucleus (b) in their interior. The black colour is due to the accu- 

 mulation, within each cell, of a number of flat rounded or oval 

 granules, of extreme minuteness, which exhibit an active movement 

 when set free from the cell, and even whilst inclosed within it. 

 The pigment-cells are not always, however, of this simply rounded or 

 polygonal form ; they sometimes present remarkable stellate pro- 

 longations, under which form they are well seen in the skin of the 

 frog (fig. 791,cc). The gradual formation of these prolongations 

 may be traced in the pigment-cells of the tadpole during its meta- 

 morphosis (fig. 777). Similar varieties of 

 form are to be met with in the pigmentary 

 cells of fishes and small Crustacea, which 

 also present a great variety of hues ; and 

 these seem to take the colour of the bottom 

 over which the animal may live, so as to 

 serve the better for its concealment. 



The structure of the epidermis may be 

 examined in a variety of ways. If it be 

 removed by maceration from the true skin, 

 the cellular nature of its under surface is at 

 once recognised, when it is subjected to a 

 magnifying power of 200 or 300 diameters, 

 by light transmitted through it, with this 

 surface uppermost ; and if the epidermis be 

 that of a negro or any other dark-skinned 

 race, the pigment cells will be very distinctly 

 seen. This under-surface of the epidermis 

 is not flat but is excavated into pits and 

 channels for the reception of the papillary FIG. 777. Pigment -cells 



elevations of the true skin ; an arrangement from tail of tadpole : a a 

 ,.,. , , i j.1 j_i i simple forms of recent 



which is shown on a large scale in the thick origin; b b, more complex 

 cuticular covering of the dog's foot, the sub- forms subsequently as- 

 jacent papillae being large enough rn lie dis- 

 tinctly seen (when injected) with the naked 



eye. The cellular nature of the newly forming layers is best seen 

 I iy examining a little of the soft film that is found upon the surface 

 of the true skin, after the more consistent layers of the cuticle have 

 been raised by a blister. The alteration which the cells of the 

 external layers have undergone tends to obscure their character ; 

 but if any fragment of epidermis be macerated for a little time in a 

 weak solution of soda or potass, its dry scales become softened, and 

 are filled out by imbibition into rounded or polygonal cells. The 

 same mode of treatment enables us to make out the cellular struc- 

 ture in warts and corns, which are epidermic growths from the 

 surface of papillae enlarged by hypertrophy. 



The epithelium may be designated as a delicate cuticle, covering 

 all the free intfritnl surfaces of the body, and thus lining all its 

 cavities, canals, &c. Save in the mouth and other parts in which 

 it approximates to the ordinary cuticle, both in locality and in 



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