SENSE ORGANS. 279 



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 connective 

 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 continued into the 

 iris, which lies free behind the cornea, and is per- 

 forated in its centre by an elongated oval slit, the 

 pupil, through which light passes to tlie 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. 



5. The lens is a soUd spherical body, suspended in the eye 



immediately behind the pupil. During life it is trans- 

 parent, and serves to bring the rays of light to a focus 

 on the retina. 



