TISSUES OF THE BODY. 



145 



rounded, and usually appear darker the smaller they are, in any one 

 individual. They are probably crystalline. The tint of the mole- 

 cules is by no means exactly the same in different mammals. In man, 

 where the granules are small, it is seen to be brownish black, but in 

 many of our mammalia, as in the pig and calf, it is jet black. The 

 size of these particles remains always considerably below 0-0023 mm. 

 Answering to their minuteness, they show, on becoming free in water, the 

 liveliest molecular motion, a phenomenon which may, however, be 

 remarked in the substance of uninjured cells when strongly swollen 

 under the action of imbibed water. 



The pigmentary epithelium itself (fig. 138) appears on the choroid as a 

 simple bed of closely crowded cells, of a beautiful polyhedral, usually 

 hexagonal figure, running at times through whole groups (a) with the 

 greatest regularity. Ard yet they may be found indefinitely angular ; and 

 some unusually large cells are frequently octagonal (b). The diameter of 

 most of these cells is on an average O'OOl 4 0*0204 mm., and their thick- 

 ness 0-0090 mm. 



The quantity of molecules of melanin contained in the transparent thick 

 and tenacious contents of the cell is by no means everywhere the 

 same. We meet some cells (and they are the 

 most suitable for examination) in which 

 the amount of black elementary particles 

 is but small, so that the nuclei and mem- 

 branes, always very delicate, may easily 

 be distinguished. In such specimens the 

 nucleus is found to be 0-0055-0-0075 mm. 

 in size, either round or more or less oval, and 

 always smooth-edged. It has usually one or 

 more nucleoli. But much oftener the amount of molecules in the pig- 

 ment cells is far more considerable, so that the nucleus only glistens 

 through as a clear speck. Should the particles remain somewhat distant 

 from the external surface of the cell-body, such groups of cells appear at 

 first sight as though separated by narrow intervals of transparent inter- 

 cellular substance. Finally, cells are encountered, 

 in which, such is their richness in pigmentary par- 

 ticles, the nucleus is completely hidden. 



A side view of these pigment cells (easily 

 obtainable, owing to the small amount of flatten- 

 ing present in the structure) shows that only in 

 one-half of them, namely, that directed towards 

 the retina, do these melanin granules occur, a 

 transparent contents occupying the other half. 

 The nucleus is situated in the latter, or at the 

 junction of the clear and dark portions (fig. 

 139, b). 



In conclusion, we may remark that cells with 

 two nuclei are also encountered here, but are of 

 rare occurrence. 



At the boundaries of the choroid, near the processus ciliares, the lami- 

 nated cells are smaller and less clearly polygonal, while they have become 

 "far richer in pigment, so that the nucleus can only be rendered visible in 

 general by squeezing the cell-body. The lining of the back of the iris is 

 of precisely similar nature. 



Fig. 138. So-called pigment cells 

 from the choroidea of sheep. 

 At a, a mosaic of hexagonal cells ; 

 at b, & larger octagonal 



Fig.139. Cells from the choroid 

 of the calf, a, cell with two 

 nuclei; b, a side view of ordi- 

 nary cells, moderately filled 

 with pigment ; c, some which 

 are only supplied with but a 

 scanty amount of pigment 

 particles; from the neighbour- 

 hood of the tapetum. 



