OF SIGHT. 49 



through the eye, from before backwards, and will give an 

 dea of the relative position of these different parts. 



72.. The outer coat is called the sclerotic, ( b ;) it is a 

 thick, firm, white membrane, having its anterior portion 

 transparent. This transparent segment, which seems set in 

 the opaque portion, like a watch-glass in its rim, is called the 

 cornea, (f.) 



73. The inside of the sclerotic is lined by a thin, dark- 

 colored membrane, the choroid, (c.) It becomes detached 

 from the sclerotic when it reaches the edge of the cornea, 

 and forms a curtain behind it. This curtain gives to the eye 

 its peculiar color, and is called the iris, (g.) The iris read- 

 ily contracts and dilates, so as to enlarge or diminish an open- 

 ing at its centre, the pupil, according as more or less light 

 is desired. Sometimes the pupil is circular, as in man, the 

 dog, the monkey ; sometimes in the form of a vertical ellipse, 

 as in the cat ; or it is elongated sidewise, as in the sheep. 



74. The third membrane is the retina, (d.) It is formed 

 by the optic nerve, which enters the back part of the eye, by 

 an opening through both the sclerotic and choroid coats, and 

 expands upon the interior into a whitish and most delicate 

 membrane. It is upon the retina that the images of objects 

 are received, and produce impressions, which are conveyed 

 by the nerve to the brain. 



75. The fluids which occupy the cavity of the eye are of 

 different densities. Behind, and directly opposite to the pupil, 

 is placed a spheroidal body, called the crystalline lens, (e.) 

 It is tolerably firm, perfectly transparent, and composed of 

 layers of unequal density, the interior being always more 

 compact than the exterior. Its form varies in different classes 

 of animals. In general, it is more convex in aquatic than in 

 land animals ; whilst with the cornea it is directly the con- 

 trary, being flat in the former, and convex in the latter. 



76. By means of the iris, the cavity, (i,} in front of the crys- 



5 



