ORGANS OF PERCEPTION. 177 
the cornea ; and the posterior, between the uvea and cili- 
ary ligament, which is very small, and whose existence is 
even denied by some, The aqueous humour agrees with 
the vitreous in chemical composition, but it is less viscid. It 
is not lodged in cells, and hence it readily flows out when 
the cornea is punctured *. 
3. Nerves of the Eye.—The position of the optic thala- 
mi, and their relation to the cerebral portion of the brain, 
have been already explained. From these thalami, the 
optic nerves take their rise. They are two in number, and 
in some animals, the nerve which proceeds to the right eye, 
originates in the left side, and the nerve of the left eye ori- 
ginates in the right. Each nerve is divided, internally, 
into a great number of canals, formed by the neurilema, 
which contain the medullary matter. This structure is dis- 
played when the medullary matter is removed by macera- 
tion, and the nerve inflated and dried. 
The optic nerve penetrates the sclerotica and the cho- 
roid coat, and becomes expanded on the concave surface of 
the latter, in the form of a delicate transparent membrane, 
without, however, adhering to any part of it. It extends 
to the ciliary ligament, terminating at the base of the pro- 
cesses. The internal surface, next the vitreous humour, is 
of a firmer consistence than the external, contiguous to the 
choroides, as it contains numerous minute bloodvessels. In 
some animals these surfaces can be separated, and the 
dense central Jamina is termed the Arachnoid. 
The retina is perhaps the most delicately sensible mem- 
brane of the animal frame. It is readily affected by the 
rays of light, which, when too intense, excite very pain- 
ful sensations, 

* The relative position of the different parts of the eye, is displayed 
most distinctly when the eye-ball is in a frozen state. 
VOL, I. M 
