242 HUMAN PHYSIOLOGY. 



refracting media; it is globular in form and constitutes about four 

 fifths of the ball ; it is hollowed out anteriorly for the reception of 

 the crystalline lens. The outer surface of the vitreous is covered by 

 a delicate, transparent membrane, 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 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 

 bicovex body, situated just behind the iris and resting in the depres- 

 sion 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 Ys 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 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 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 fibers from a branch of the motor oculi, or 

 third nerve ; a sensory branch from the ophthalmic 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 motion and 

 sensation. 



