PIGMENT-CELLS. 



49 



coat of the eye, at the back of the iris, in the skin, etc. 

 In all cases the dark colour is due to the presence of so- 

 called pigment-cells. 



These are for the most part polyhedral (fig. 12) or 

 spheroidal, although sometimes they have irregular 

 processes,^. as shown in fig. 13. The cell- wall itself is 

 colourless, the dark tint being produced by small dark 

 granules heaped closely together, and more or less con- 

 cealing the nucleus, itself colourless, which each cell 

 contains. The dark tint of the skin, in those of dark com- 

 plexion and in the coloured races, is seated chiefly in the 



Fig. 12* 



Fig. i3-t 



epidermis, and depends on the presence of pigment-cells, 

 which, except in the presence of the dark granules in their 

 interior, closely resemble the colourless cells with which 

 they are mingled. The pigment- cells are situate chiefly in 



* Fig. 12. Pigment- cells from the choroid; magnified 370 diameters 

 (Henle). A, cells still cohering, seen on their surface; , nucleus 

 indistinctly seen. In the other cells the nucleus is concealed by the 

 pigment granules. B, two cells seen in profile ; a, the outer or posterior 

 part containing scarcely any pigment. 



f Fig. 13. Ramified pigment-cells, from the tissue of the choroid 

 coat of the eye ; magnified 350 diameters (after Kolliker). a, cells with 

 pigment ; b } colourless fusiform cells. 



E 



