OF SIGHT. 25 



72. The outer coat is called the sclerotic (Z>) ; it is a thick, 

 firm, white membrane, having its anterior portion transpa- 

 rent. 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 readily 

 contracts and dilates, so as to enlarge or diminish an 

 opening 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 ob- 

 jects are received, and produce impressions, which are con- 

 veyed 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 com- 

 posed 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 contrary, being flat in the former, and con- 

 vex in the latter. 



76. By means of the iris ; the cavity in front of the crys- 



3 



