BPIDEEMIS. 



605 



the internal to the external surface of the Epidermis, the cells 

 undergo a series of well-marked changes. When we examine 

 the innermost layer, we find it soft and granular ; consisting of 

 nuclei, in various stages of development into cells, held together 

 by a tenacious semi-fluid substance. This was formerly con- 

 sidered as a distinct tissue, and was supposed to be the peculiar 

 seat of the color of the skin ; it received the designation of rete 

 mueosum. Passing outwards, we find the cells more completely 

 formed ; at first nearly spherical in shape, but becoming poly- 

 gonal where they are flattened one against another. As we 

 proceed further towards the surface, we perceive that the cells 

 are gradually more and more flattened, until they become mere 

 horny scales, their cavity being obliterated ; their origin is indi- 

 cated, however, by the nucleus in the centre of each. This 

 change in form is accompanied by a change in the chemical 

 composition of the tissue, which seems to be due to the meta- 

 morphosis of the contents of the cells, into a horny substance 

 identical with that of which hair, horn, nails, hoofs, &c., are 

 • composed. Mingled with the epidermic cells, we find others 

 which secrete coloring-matter instead of horn; these are termed 

 "pigment-cells." The most remarkable development of "pig- 

 ment-cells" in the higher animals, is on the inner surface of the 

 Choroid coat of the eye, where they have a very regular arrange- 

 ment, and form several layers, known as the Pigmentum nigrum. 

 When examined separately, these cells are 

 found to have a polygonal form (Fig. 319, a), 

 and to have a distinct nucleus (6) in their 

 interior. The black color is given by the 

 accumulation, within the cell, of a number 

 of flat, rounded, or oval granules, of extreme 

 minuteness, which exhibit an active move- 

 ment 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 prolongations, 

 under which form they are well seen in the 

 skin of the Frog (Fig. 327, c, c). The gra- 

 dual formation of these prolongations may 

 be traced in the pigment-cells of the Tadpole during its meta- 

 morphosis (Fig. 320). 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 

 hav* the power of likening their color to that of the bottom 

 over which the animal may live, so as to serve for its conceal- 

 ment. 



420. The structure of the Epidermis may be examined in a 

 variety of ways. K it be removed by maceration from the true 

 skin, the cellular nature of its under-surface is at once recognized. 



Cells from Pigmentum, 

 Nigrum,: — a, pigmpiit- 

 ary granules concealing 

 the nucleus; &, the nu- 

 cleus distinct. 



