SENSE ORGANS. 287 



B. The Eye. 



Remove one of the eyes by cutting across the eye muscles 

 and the optic nerve. Snip off the remains of the muscles, and 

 divide the eye into an inner and an outer half by cutting all 

 round it equatorially. Place the two halves under water in a 

 dissecting dish. 



The walls of the eye consist of three concentric layers, 

 and within its cavity, near the outer side, is the spherical 

 lens about the size of a pea. 



1. The sclerotic, or outermost coat of the eye, is thin and 



cartilaginous. It invests the whole of the eye 

 except the part covered by the cornea. 



2. The cornea is an oval patch on the outer surface of the 



eye, through which the light enters. It is about 

 half an inch long and a quarter of an inch broad, 

 and is almost flat. It consists of transparent con- 

 nective tissue, and is continuous at its edges with 

 the sclerotic. 



3. The choroid is a black opaque membrane, lying within 



the sclerotic. It adheres closely to the sclerotic, 

 and is firmly attached to the line of union of 

 sclerotic and cornea. Beyond this line it is con- 

 tinued into the iris, which lies free behind the 

 cornea, and is perforated in its centre by an elon- 

 gated oval slit, the pupil, through which light passes 

 to the interior of the eye. 



The inner surface of the choroid, especially in 

 the inner half of the eye, is covered by a glistening 

 silvery membrane, the tapetum. 



4. The retina is a delicate membrane lying on the inner 



surface of the choroid, and readily separating from 

 it except at the point of entrance of the optic nerve, 

 where it adheres firmly, and is continuous with the 

 nerve. It is the only part of the eye sensitive to 

 light. 



