260 THE SOLIDS. 



The pigment cells of the posterior face of the iris and 

 ciliary processes are smaller than those of the choroid, are 

 for the most part round, or approach that form, and so 

 filled with corpuscles that the nucleus and margin of the 

 cell is not usually visible. 



In the skin the pigment cells are placed between the 

 dermis and epidermis ; they do not there form a layer of 

 equal thickness, but accumulate in the depressions left be- 

 tween the papillae, forming many superimposed layers, 

 while on these they are spread out in a single thin layer, and 

 are often much scattered. It is to this circumstance, as well 

 as the thickness of the epidermis, and the state of repletion 

 of the cells, that the varying shades of the colour of the 

 skin of the same individual depend. In the negro the cells 

 resemble much in form those of the choroid, being either 

 perfectly hexagonal, polygonal, or irregularly rounded ; the 

 nucleus can be well seen in those cells which are less filled. 



Amongst the white race, the pigment cells in those situa- 

 tions in which they occur beneath the skin are fewer in 

 number, smaller, more rounded, and frequently resemble 

 little masses of corpuscles rather than distinct cells ; never- 

 theless it is even here sometimes possible to distinguish the 

 nucleus and cell wall. 



There exists between the internal face of the sclerotic 

 and the external of the choroid a fibrous tissue of a brown 

 colour ; this, when these two coats of the eye are separated, 

 remains attached in part to the one and in part to the other ; 

 that, however, which adheres to the sclerotic has received a 

 distinct name, ^and is called lamina fusca. Now the colour of 

 this layer is due to the presence, scattered amongst the fibres, 

 of pigment cells of a very peculiar form and construction ; they 

 are mostly very irregular in size and shape, are marked with 

 a clear central spot, which indicates the locality of the nu- 

 cleus, and from the margin of many of them proceed fila- 

 mentous processes of variable number and size, and the 

 extreme points of which are mostly colourless, and are not 

 dissolved by acetic acid. 



Pigment cells of analogous construction exist on the ex- 



