LECTURE XVIII 
PIGMENT. 
I NOW arrive at the description of the last variety of 
permanent cellular tissue occurring in the human sub- 
ject, pigment. This consists either of a solid or fluid 
matter, contained within a cell-wall. When speaking of 
pigment in vegetables, I stated that, in some few cases 
as in Ferns , we had colour belonging to the cell- wall 
itself, but in all other instances, the various and beau- 
tiful hues of flowers were produced by a fluid colouring- 
matter occurring within the cells. We have likewise a 
striking example of fluid colouring-matter in animal 
cells, in the blood discs, but in animal tissues generally 
the pigmental matter occurs in the form of granules. 
The most striking examples of pigment occur in the eye, 
upon the choroid coat of which is a distinct layer of 
hexagonal cells, most probably epithelial, termed the 
