PIGMENT-CELLS. 131 



ISOLATED CELLS : CELLS NOT COALESCING TO FOKM TISSUES. 



Under the head of isolated cells, Carpenter includes the white 

 and the colored blood-corpuscles, epidermis and epithelium, the 

 cells containing the spermatozoids of the semen, and absorbing and 

 secreting cells. 



Epidermis and epithelium, however, perform their functions as a 

 distinct tissue, and will therefore be included in the classification of 

 the tissues proper. Secreting cells will be described in connection 

 with the various glands containing them; and the blood-cells of 

 both kinds, and the spermatophori, will be considered in connection 

 with the fluids of which they respectively constitute a part. 



The only kind of cells to be considered here, as being normally 

 scattered in the interstices of the tissues, and not forming a tissue 

 by themselves, is the pigment-cell; after which the various forms of 

 cancer-cells will be described, as constituting one of the most im- 

 portant of the pathological developments. 



The isolated cells in the nervous centres will be described in the 

 chapter on "Nerve-tissue." 



I. PIGMENT-CELLS. 



Pigment-cells derive their peculiarities from the fact that the 

 granules they contain are colored, consisting of the immediate prin- 

 ciple melanine, described on page 103. As this principle abounds 

 in carbon, neither chlorine nor strong acids remove the color of the 

 granules. The latter are often found lying among the cells, as well 

 as within them. They are also among the minutest objects in 

 nature, being often less than 33(3^ of an inch in diameter. 



It has been shown that melanine is probably derived from haema- 

 tine, and, like the latter, has iron associated with it ; the pigment of 

 the choroid coat of the eye containing .254 per cent, of this metal. 



Distribution of Pigment cells. 



In the human body, the distribution of pigment-cells is quite 

 limited. In the eye, they are found on the inner surface of the 

 choroid membrane, on the ciliary processes and the iris (uvei), and 

 between the choroid and the sclerotica. They also exist in the skin 

 of the perineum and of the genital organs, especially of the scrotum r 

 and in the areola of the mammary gland. Pigment-granules also 



