ORGANS OF PERCEPTION. 175 
project into the cavity of the eye-ball, and form a broad 
radiated ring. These projecting lamin are termed Cili- 
ary Processes. 
After forming the ciliary ligament, the choroides still 
proceeds along the concave surface of the sclerotica, and 
parallel with it, until its junction with the cornea, when it 
forms another projection into the cavity of the eye-ball, 
termed the Uvea. It is here continuous, and constitutes 
an annular veil, perforated in the centre. Its anterior sur- 
face is covered, and intimately connected, with a membrane 
of a spongy, fibrous texture. This membrane is term- 
ed the Jris, and is well known as the seat of the colour of 
the eye. It is sensible to the impressions of light, and readily 
expands or contracts, according to circumstances ;_ thus, 
enlarging or diminishing the size of the central perforation. 
The central aperture is termed the Pupil, and varies great- 
ly inits dimensions and forms in different animals. 
On the central surface of the choroides, including the c- 
liary processes and uvea, there is a slight villosity, to which 
there adheres a mucous pigment. This is usually dark 
coloured, and termed the Pigmentum nigrum, but, in 
some cases, it is light coloured approaching to white. This 
pigment is insoluble in water, but soluble in alkalies and 
the stronger acids. .LEoro.p GMELIN considers. that it 
approaches the nature of indigo. On the temporal side of 
the bottom of the cavity of the eye, there is a small space 
destitute of this pigment, through which the colours of the 
membrana Ruyschiana appear. This spot is termed the 
T'apetum, and is peculiar to quadrupeds. 
(2.) Humours of the Eye.—The cavity of the eye, form- 
ed by the coats which have been described, contains Fluids 
differmg in consistency, form, and situation. ‘They are di- 
vided into three kinds, termed the Vitreous, Crystalline, 
and Aqueous Fumours. 
