\l\ 



111!. I. VI. AND THE SENSK OF SIGHT 



207 



rttina may be considered as a thin cup-like expansion of the 

 optic nerve. The large cavity between the crytsalline lens in 

 front and the retina behind is filled by a clear semi-fluid sub- 

 stance called the vitreous humour. The space between 

 the cornea and the iris, called the anterior chamber of the eye, 

 is in like manner full of fluid, which, in this case, is thin and 

 watery, and called the aqueous Tmmour. 



Drive the sharp point of a fine pair of scissors through the 

 cornea of the bullock's eye close to its edge and cut it away with 

 a circular cut all round. A few drops of aqueous humour will 

 run out. Notice that 

 the cornea is a fairly 

 thick but perfectly trans- 

 parent membrane. It 

 also appears now as a 

 flat membrane, its bul- 

 ging outwards in the eye 

 being maintained by the 

 aqueous humour. With 

 the point of the scissors 

 raise the pupil edge of 

 the iris ; notice that it is 

 lying on the crystalline 

 lens but not attached to 

 it. Put the point of tin- 

 scissors under the iris 

 and make two radiating 

 cuts outwards through 

 the iris and its attach- 

 ment to the coats of the eyeball. Turn the small piece of 

 the iris which you have thus cut from the rest outwards 

 and notice its attachment at the edge of the cornea, and 

 under it the black ciliary processes projecting inwards. The 

 ciliary processes are still attached to the lens by the sus- 

 pensory ligamtnt, so that you cannot turn the whole piece 

 outwards without tearing something. Tear or cut a part of the 

 suspensory ligament ; by gentle squeezing the lens will then 

 come away from the rest of the eyeball. It is a perfectly 

 transparent body, convex both in front and behind, though more 

 strongly so behind. IMace it over printed paper and notice its 



Fir,. 97. View of the iris and ciliary processes 



from behind. 

 a, circular fibres, and fr, radiating fibres of the 



iris ; f, ciliary processes lightly shaded on ; 



</, choroid darkly -haded. 



