THE EYE 253 



B. The Eye. 



Remove one of the eyes by cutting across the eye-muscles 

 and 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 



The eye is hollow ; its walls 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. It 

 consists of transparent connective tissue, and is con- 

 tinuous at its edges with the sclerotic. 



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



the sclerotic and cornea. It adheres closely to the 

 sclerotic, and is firmly attached to the line of union 

 of sclerotic and cornea, but is riot attached to the 

 cornea itself. Opposite the middle of the cornea it 

 is perforated by an elongated oval slit, the pupil, 

 through which alone light can get 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 soft pulpy membrane lying on the inner 



surface of the choroid, and readily separable from it 

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

 which it adheres firmly, and with which it is con- 

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

 light. 



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



immediately behind the pupil. During life it is 



