SENSE OF SIGHT. 217 



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

 alkaline fluid, having a specific gravity of 1003-1009. 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, inclosed within its capsule, is a transparent biconvex 

 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 anterior. The 

 lens measures about J of an inch in the transverse diameter and of an inch 

 in the anteroposterior 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 be- 

 yond 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 be- 

 hind the lens, becomes united to its capsule. The anterior layer presents 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 

 opthalmic; they pierce the sclerotic at various points and are ultimately dis- 

 tributed to all tissues within the ball. 



The nerve-supply comes largely from the opthalmic or ciliary ganglion. 

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

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

 from the opthalmic division of the fifth nerve, and fibers 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 mo- 

 tion and sensation. 



The Eyeball a Living Camera Obscura. The eyeball may be compared 

 in a general way to a camera obscura. The anatomic arrangement of its 

 structures reveals 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 image formed at 

 the focal point; the iris, to the diaphragm, which, by cutting off the marginal 

 rays, prevents spheric aberration and at the same time regulates the amount 

 of light entering the eye; the ciliary muscle, to the adjusting screw, by which 

 distinct images are thrown upon the retina in spite of varying distances of the 



