EPIDERMIS. PIGMENT-CELLS. 



249 



Fig. 39. 



they dry up, and become flattened scales, in passing towards the surface, thus 

 constantly remaining dispersed through its substance, and giving it a dark tint 

 when it is separated and held up to the light. The color is more apparent in 

 the cells of the "stratum Malpighii," than it is in those of the horny layer; 

 and it is particularly deep in the stratum of columnar cells that lies in immedi- 

 ate contact with the surface of the Cutis (Fig. 38, b, 6). In all races of men, 

 however, we find the most remarkable development of Pigment-cells on the inner 

 surface of the Choroid coat of the eye ; where they form several layers, known 

 as the Piymentum nigrum. When examined separately, these are found to have 

 a polygonal form, and to have a distinct 

 nucleus in their interior (Fig. 39, A). 

 The black color is given by the accumu- 

 lation, within the cell, of a number .of 

 flat, rounded or oval granules, measuring 

 about 1-20, 000th of an inch in diameter, 

 and a quarter as much in thickness; 

 these, when separately viewed, are ob- 

 served to be transparent, not black and 

 opaque ; and they exhibit an active move- 

 ment when set free from the cell, and 

 even whilst enclosed within it. The 

 Pigment-cells are not always of a simple 

 rounded or polygonal form; they some- 

 times present remarkable stellate pro- 

 longations, such as those seen in the 

 skin of the Frog (Fig. 86); and occa- 

 sionally, the cells being more nearly ap- 

 proximated to each other, these prolonga- 

 tions communicate, so as to form a kind of 



network. The Chemical nature of the Black pigment has not yet been distinctly 

 ascertained ; it has been shown, however, to have a very close relation with that 

 of the Cuttle-fish ink, or Sepia, which derives its color from the pigment-cells 

 of the ink-bag, and to include a larger proportion of carbon than most other 

 organic substances every 100 parts containing 58 J of that element. 



243. It cannot be doubted that the development of the Pigment-cells of the 

 Skin is very much influenced by exposure to light; and in this respect there is 

 a remarkable correspondence between Animals and Plants the coloration of 

 the latter, as is well known, being entirely due to that agent. Thus, it is a 

 matter of familiar experience, that the influence of light upon the skin of many 

 individuals, causes it to become spotted with brown freckles j these freckles being 

 aggregations of brown pigment-cells ; which either owe their development to the 

 stimulus of light, or are enabled by its agency to perform a decided chemical 

 transformation, which they could not otherwise effect. In like manner, the 

 swarthy hue, which many Europeans acquire beneath exposure to the sun in 

 tropical climates, is due to a development of dark pigment-cells ; and to this we 

 usually find the greatest disposition in individuals or races that are already of 

 a somewhat dark complexion. The deep blackness of the Negro skin seems de- 

 pendent upon nothing else than a similar cause, operating through successive 

 generations (CHAP. XX.). It is well known that the new-born infants of the 

 negro and other dark races, do not exhibit nearly the same depth of color in 

 their skins, as that which they present after the lapse of a few days, when light 

 has had time to exert its influence upon their surface ; and further, that in those 

 individuals who keep themselves during life most secluded from its influence, 

 we observe the lightest hue of the epidermis. Thus among the intertropical 

 nations, the families of Chiefs, which are not exposed to the sun in the same 



A. Choroid Epithelium, with the cells filled with 

 pigment, except at a, where the nuclei are visible. 

 The irregularity of the pigment-cells is seen. 6. 

 Grains of pigment. 



B. Pigment-cells from the substance of the Cho- 

 roid. A detached nucleus is seen. Magnified 320 

 diameters. 



