772 



PHYSIOLOGY. 



The organ of sight, the eye, is constructed upon the principles of 

 the camera obscura. In the latter the collecting lens unites the light 

 impressions at the back of the apparatus to form upon the ground- 

 glass plate a diminished and reversed image of external objects. 



Structure. The eye is composed of three concentric coats (scle- 

 rotic, choroid, and retina), the aqueous and vitreous hurnqg-s and the 

 crystalline lens. 



Fig. 321. Diagram of a Horizontal Section through the Human Eye. 



(YEO.) 



1, Cornea. 2, Sclerotic. 3, Choroid. 4. Ciliary processes. 5, Suspensory 

 ligament of lens. 6, So-called posterior chamber between iris and lens. 7, 

 Iris. 8, Optic nerve. 8', Entrance of cerebral artery of retina. 8", Central 

 depression of retina, or yellow spot. 9, Anterior limit of retina. 10, Hyaline 

 membrane. 11, Aqueous chamber. 12, Crystalline lens. 13, Vitreous humor. 

 14, Circular venous sinus which lies around the cornea, a-a, Antero-posterior 

 axis of bulb. &-&, Transverse axis of bulb. 



The first, or outside, coat of the eye is opaque in all of its parts 

 except a small anterior segment. This area, which is about one-sixth 

 of the entire circumference, is perfectly transparent. The dense, 

 opaque part is known as the sclerotic; the transparent portion is the 

 cornea, which is the most anterior portion of the sclerotic. 



The sclerotic is thickest behind, in the neighborhood of the part 

 pierced by the optic nerve, which is placed about a tenth of an inch 

 inside of the antero-posterior axis. The sclerotic thins a little as it 



