OF THE SENSE OF SIGHT. 113 



called the pupil, and which in man is circular, varies 

 perpetually with the amount of light to which the eye is 

 exposed. This circular membrane is muscular, or at least 

 fibrous and contractile, and is abundantly supplied with 

 nerves. 



Continuous with the iris, ck ' cr b 



and extending backwards 

 beneath the sclerotic, is the 

 dark vascular membrane, 

 called choroid, connected 

 with the pigment of the 

 eye ; and within it the re- 

 tina, or membrane in which 

 the optic nerve terminates. 



The humours which these 

 membranes contain and cir- 

 cumscribe are in succes- ' g . ' v c 'b 

 sion, proceeding from before 



backwards, the aqueous Fig ' ^ Disced * f the Eye 



humour, the lens, and the 

 vitreous humour. 



The optic nerve enters the eyeball towards the back part, 

 passing through the sclerotic and choroid, and terminates in 

 the retina, which is generally viewed as an expansion of this 

 nerve. But other nerves enter the eyeball, such as the 

 ciliary, coming partly from the fifth pair and partly from a 

 ganglion, the ophthalmic, with which the fifth and third com- 

 municate. From this ganglion, though small, most of the 

 ciliary nerves proceed into the interior of the eye to supply 

 the ciliary circle and the iris. 



The ciliary ligament or circle is a peculiar ring connecting 

 the choroid, iris, and sclerotic to each other. Its nature and 

 functions have not yet been determined, but it is well sup- 

 plied with nerves in animals of strong vision, as in the eagle 

 and vulture. The humours are colourless and transparent, 

 and the same remark applies to all the parts situated between 



* c, the transparent cornea ; s, sclerotic ; ', portion of the sclerotic turned 

 back to display portions of the membranes situated beneath it ; ch, the cho- 

 rpid ; v, the retina ; n, the optic nerve ; ca, anterior chamber of the eye, 

 situated between the cornea and the iris, and communicating with the pos- 

 terior chamber by the opening in the iris, called the pupil ; these chambers 

 contain the aqueous humour ; i, the iris ; p, the pupil, an opening in the iris 

 by which the rays of light pass into the deeper chambers ot the eye ; cr, the 

 crystalline humour or lens ; pc, ciliary processes ; v, vitreous humour ; 

 b b, a portion of the conjunctiva. 



I 



