SIMPLE ISOLATED CELLS. EPIDERMIS. 



143 



tint when it is separated and held up to the light. 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 seve- 

 ral layers, known as the Pigmentum nigrum. Here they have a very 

 regular arrangement ; which is best seen where they cover the blood- 

 vessels of the Choroid coat in a single layer, as shown in Fig. 22. 



Fig. 22. 



Fig. 23. 





a, a, a, 



on of the clioroid coat from the eye of the Ox, showing the pig- 

 ment-cells, where they cover, , a, a, the veins, in a single layer : 5, 6, 

 ramifications of the veins near the ciliary ligament, covered with less 

 regular pigment-cells ; c, c, spaces between the vessels, more thickly 

 covered with pigment-cells. 



When examined separately, they are found to have a polygonal form 

 (Fig. 23, a), and to have a distinct 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, measuring about 1-20, 000th of an inch 

 in diameter, and a quarter as much in thickness ; these, when separately 

 viewed, are observed to be transparent, not black and opaque ; and they 

 exhibit an active movement 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 sometimes present remarkable stellate 

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

 Frog. The Chemical nature of the black pigment has not yet been 

 made evident ; it has been shown, however, to have a close relation with 

 that of the Cuttle-fish ink or Sepia, which derives its colour from the 

 pigment-cells lining the ink-bag ; and to include a larger proportion of 

 Carbon than most other organic substances, every 100 parts contain- 

 ing 58J of this element. 



230. That the development of the Pigment- cells, or at least the for- 

 mation of their peculiar secretion, is in some degree due to the influence 

 of Light, seems evident from the facts already mentioned ( 93). To 

 these it may be added, that the new-born infants of the Negro and other 

 dark races do not exhibit nearly the same depth of colour in their skins, 

 as that which they present after the lapse of a few days ; which seems 

 to indicate that exposure to light is necessary for the full development 

 of the characteristic hue. An occasional development of dark pigment- 

 cells takes place during pregnancy in some females of the fair races ; 

 thus it is very common to meet with an extremely dark and broad areola 



