THE SENSE OF SIGHT. 159 



The outer surface of the vitreous is covered by a delicate transparent mem- 

 brane, termed the hyaloid membrane, which serves to maintain its globular 

 form. 



The aqueous humor found in the anterior chamber of the eye is a clear 

 alkaline fluid, having a specific gravity of 1.003-1.009. It is secreted most 

 probably by the blood vessels of the iris and ciliary processes. It passes 

 from the interior of the eye, through the canal of Schlemm and the meshes 

 at the base of the iris, into the anterior circular vein. 



The crystalline lens enclosed within its capsule, is a transparent bi-con- 

 vex body, situated just behind the iris and resting in the depression in the 

 anterior part of the vitreous. The two convexities are not quite alike, the 

 curvature of the posterior surface being slightly greater than that of the an- 

 terior. The lens measures about one-third of an inch in the transverse 

 diameter and one-fifth of an inch in the antero-posterior diameter. 



The suspensory ligament, by which the lens is held in position, is a firm 

 transparent membrane, united to the ciliary processes. A short distance 

 beyond its origin, it splits into two layers, the anterior of which is inserted 

 into the capsule of the lens and blends with it ; the posterior passing inward 

 behind the lens, becomes united to its capsule. The anterior layer pre- 

 sents a series of foldings, Zone of Zinn, which are inserted into the intervals 

 of the folds of the ciliary processes. The triangular space between the two 

 layers is the canal of Petit. 



Blood vessels and Nerves. The structures composing the eyeball are 

 supplied with blood by the long and short ciliary arteries, branches of the 

 ophthalmic ; they pierce the sclerotic at various points and are ultimately 

 distributed to all tissues within the ball. 



The nerve supply comes largely from the ophthalmic or ciliary ganglion. 

 This is a small body, situated in the posterior part of the orbit; it receives 

 motor fibres from a branch of the motor- oculi, or third nerve; a sensory 

 branch from the ophthalmic division of the fifth nerve and fibres from the 

 cavernous plexus of the sympathetic. From the anterior border of the 

 ganglion proceed the ciliary nerves which, entering the eyeball, endow its 

 structures with motion and sensation. 



The Eyeball a Living Camera Obscura. The eyeball may be com- 

 pared in a general way to a camera obscura. The anatomical arrangement 

 of its structures reveal many points of similarity. The sclerotic and choroid 

 may be compared with the walls of the chamber; the combined refractive 

 media, cornea, aqueous humor, lens, and vitreous humor, to the lens for 

 focusing the rays of light ; the retina to the sensitive plate receiving the 



