104 ANIMAL MECHANISM. 



color is that of gum arable, or ground glass : nothing can 

 be more delicate, being too tender to bear its own weight. 

 In fact, it is the expansion of the optic nerve, the imme- 

 diate seat of vision. To see it well, an eye should be 

 taken to pieces in a tumbler of water. 



Explanation of Figure 3, FIG. 3. 



from dissection of a human eye, 



the organ being represented of the 



proper size. 



a The optic nerve. 



bb The Sclerotic coat cut and 



turned outward, 

 c A circular portion of the Scle- 



rotica, being a rim of the white 



of the eye, cut, and turned up- 

 ward, having in its embrace the 



cornea. 



d The cornea, 

 ee One half the iris, in its place, 



the other half being removed. 

 /The Pupil, soon to be described, with the crystalline lens in its 



place. 

 g The Ciliary circle, or second vertical partition, within the eye, 



behind the iris. 

 h h Choroid coat, 

 i The Ciliary processes, or ruffle like plaits of the ciliary circle, yet 



to be explained. A small portion of the iris is cut away to show 



them 



A- A portion of the ins cut and turned back. 

 I The floating points of the ciliary processes, also turned back. 

 m The middle smooth part of the retina, seen by cutting a hole 



through the choroid coat. 

 n The roots of the ciliary processes, to which the black paint, 



secreted by the tapetum or inner surface of the choroides, adheres. 

 o The ciliary processes inserted into the capsule, or sack which 



contains the crystalline lens. 



THE CORNEA. 



Anteriorly, that clear, shining wall, resembling a watch 

 crystal, which finishes the membranous box, is called 

 the cornea. Simple as this thin crystal appears, it is 

 infinitely curious in structure. It is made of thin, pelliv- 

 cid plates, one over another, held together by a spongy 

 elastic substance. By maceration in water, a few hours, 

 the sponge will absorb it, to such a degree, that the plate? 



