42 ELEMENTARY TISSUES. 



fat serves such purposes may be mentioned the palms of the 

 hands, and soles of the feet, and the orbits. 



4. In the long bones, fatty tissue, in the form known as 

 marrow, serves to fill up the medullary canal, and to support 

 the small bloodvessels which are distributed from it to the 

 inner part of the substance of the bone. 



Pigment. 



In various parts of the body there exists a considerable 

 quantity of dark pigmentary matter, e. g., in the choroid coat 

 of the eye, at the back of the iris, in the skin, &c. In all these 

 cases the dark color 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 colorless, the 

 dark tint being produced by small dark granules heaped closely 

 together, and more or less concealing the nucleus, itself color- 



FIG. 12. FIG. 1?. 



FIG. 12. Pigment-cells from the choroid; magnified 370 diameters (Henle). A, 

 cells still cohering, seen on their surface ; 6, nucleus indistinctly seen. In the other 

 cells the nucleus is concealed by the pigment-granules. 



FIG. 13. Ramified pigment-cells, from the tissue of the choroid coat of the eye ; 

 magnified 350 diameters (after Kolliker). a, cells with pigment ; 6, colorless fusi- 

 form cells. 



less, which each cell contains. The dark tint of the skin, in 

 those of dark complexion and in the colored races, is seated 

 chiefly in the epidermis, and depends on the presence of pig- 

 ment-cells, which, except in the presence of the dark granules 

 in their interior, closely resemble the colorless cells with which 

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



