986 A MANUAL OF PHYSIOLOGY 



out. Since its refractive index is nearly the same as that of water, 

 it is scarcely observed when immersed, and the interior of the eye 

 can be easily seen through it. 



The optic disc can now be again studied, with the stump of the 

 optic nerve entering it and the retinal vessels piercing the disc. In 

 the centre of the retina is the yellow spot. 



In the anterior portion of the eyeball note the crystalline lens, 

 and at its circumference the radiating folds of the choroid called the 

 ciliary processes. Closely covering the ciliary processes, the anterior 

 border of the retina forms the ora serrata, a plaited arrangement 

 like an old-time ruff. 



Now complete the separation of the anterior and posterior por- 

 tions of the eyeball. Remove the vitreous humour, noting that it 

 is attached to the ciliary processes and the posterior surface of the 

 capsule of the lens by its enveloping membrane, the hyaloid mem- 

 brane. With scissors snip through the corneo-sclerotic junction at 

 one point down to the border of the lens, and observe the suspensory 

 ligament passing from the ciliary body chiefly towards the anterior 

 surface of the lens, where it blends with the lens capsule. Open 

 the anterior chamber of the eye by an incision through the cornea 

 in front of its junction with the sclerotic. It is filled with the clear, 

 watery, aqueous humour. Note the pigmented iris projecting in 

 front of the lens. 



Remove the sclerotic and cornea for some distance along their 

 line of junction, using gentle pressure with the edge of a fine knife 

 to separate the junction from the attached border of the iris. The 

 ciliary muscle, forming a pale, narrow ring around the eye at the 

 corneo-sclerotic junction will be thus exposed. Its external surface 

 is closely adherent to the sclerotic, and its internal blends with the 

 ciliary body. The circumference of the iris is attached at its anterior 

 border. Posteriorly it passes into the choroid. 



Take out the lens and observe the curvature of its anterior and 

 posterior surfaces. Determine which has the greater curvature. 

 In the excised eye the lens will, of course, be in the condition of 

 relaxed accommodation. 



2. Formation of Inverted Image on the Retina. Fix the eye of 

 an ox or of a dog or rabbit, after careful removal of part of the 

 posterior surface of the sclerotic, in one end of a blackened tube, 

 with the cornea in front. A tube made by rolling up a piece of 

 thick brown paper will do. Place a candle in front of the eye. 

 Look through the other end of the tube, and observe the inverted 

 image of the candle formed on the retina. Move the candle until 

 the image is as sharp as possible. Now bring between the candle 

 and the eye a concave lens. The image becomes blurred, the 

 candle must be put farther away to render it distinct, and perhaps 

 no position of the candle can be found which will give a sharp image. 

 If the lens is convex, the candle must be brought nearer, and a sharp 

 image can always be formed by bringing it near enough. If both 

 a convex and a concave glass be placed in front of the eye, they will 

 partially or wholly neutralize each other. Instead of the candle 

 a window may be looked at. If the eye of an albino rabbit can 

 be obtained, it is not necessary to remove a part of the sclerotic. 



3. Helmholtz's Phakoscope (Fig. 434). This instrument is em- 

 ployed in studying the changes that take place in the curvature of 

 the lens during accommodation. It is to be used in a dark room. 

 A candle is placed in front of the two prisms P, P'. The observer 



