PRACTICAL EXERCISES 1059 



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 portions 

 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 membrane. 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 througn 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. 451). 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 looks through 

 the hole B; the observed eye is placed at a hole opposite the hole A. 

 The candle or the observed eye is moved till the observer sees three 

 pairs of images, one pair, the brightest of all, reflected from the anterior 

 surface of the cornea; another, the largest of the three, but dim, re- 

 flected from the anterior surface of the lens; and a third pair, the 

 smallest of all, reflected from the posterior surface of the lens (Fig. 402, 



