ELEMENTARY TISSUES. 



In all these cases the dark colour is due to the presence of 

 so-called pigment- cells. 



Pigment-cells 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.* 



Flj. i 3 .f 



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 w r hich 

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

 the deep layer of the epidermis, or the so-called rete- 

 mucosmn. (See chapter on the Skin.) 



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

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

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

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

 part containing scarcely any pigment. 



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

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

 pigment ; b, colourless fusiform cells. 



