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 lamina 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 



