ORGANS OF PERCEPTION. 175 



project into the cavity of the eye-ball, and form a broad 

 radiated ring. These projecting laminae 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 Iris, 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 in its dimensions and forms in different animals. 



On the central surface of the choroides, including the ci- 

 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. LEOPOLD 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 

 Tapetum, 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 

 differing in consistency, form, and situation. They are di- 

 vided into three kinds, termed the Vitreous, Crystalline, 

 awl Aqucou ; 



