EPIDERMIS : — PIGMENT-CELLS. 



719 



Fig. 379. 



find others which secrete Colouring matter instead of horn ; these 

 which are termed ' Pigment-cells, ' are especially to he noticed in 

 the Epidermis of the Negro and other dark races, and are most 

 distinguishable in the Malpighian layer, their colour appearing to 

 fade as they pass towards the surface. — The most remarkable 

 development of Pigment-cells in the higher animals, however, is on 

 the inner surface of the Choroid coat of the Eye, where they have 

 a very regular arrangement, and 

 form several layers, known as the 

 Pigmentum nigrum. When ex- 

 amined separately, these cells are 

 found to have a polygonal form 

 (Fig. 379, a), and to have a dis- 

 tinct, nucleus (b) in their interior. 

 The black colour is given by the 

 accumulation, within the 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 enclosed 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. 393, c, c). The gi-adual formation of these prolonga- 

 tions may be traced in the pigment-cells of the Tadpole during 

 its metamorphosis (Fig. 380). 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 for its concealment. 



562. 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 recognized, 

 when it is subjected to a magnifying power of 200 or 300 dia- 

 meters, by light transmitted through it, with this surface upper- 

 most ; 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 elevations of 

 the True Skin ; an arrangement which is shown on a large scale in 

 the thick cuticular covei'ing of the Dog's foot, the subjacent papillae 

 being large enough to be distinctly seen (when injected) with the 

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

 seen by examining a little of the soft film that is found upon the 



Cells from Pigmentum Nigrum : 

 — a, pigmentary granules con- 

 cealing the nucleus ; b, the nu- 

 cleus distinct. 



