I05 8 THE SENSES 



PRACTICAL EXERCISES ON CHAPTER XVIII. 

 VISION. 



i. Dissection of the Eye. The student may profitably refresh his 

 memory on the anatomy of the eye by dissecting a fresh eye that of 

 a large animal like an ox is preferable, but the eye of a sheep or dog 

 may also be used. The eye is removed from the orbit by cutting 

 through the conjunctiva where it is reflected on to the eyelids, care- 

 fully severing the extrinsic muscles and scooping the eyeball out of the 

 mass of loose connective tissue and fat in which it is embedded, and 

 which serves as a cushion to protect it from injury during its move- 

 ments. Observe the transparent cornea in front, blending at its pos- 

 terior border with the opaque sclerotic, which is covered by a layer of 

 conjunctiva reflected from the lids. On clearing the fat cautiously 

 away, the tendinous insertions of the external or extrinsic muscles of 

 the eyeball into the anterior part of the sclerotic will be seen. Identify 

 the various muscles (p. 1023). 



Immerse the eye in water in a small glass dish, with the cornea 

 uppermost. The interior can now be seen, because the refractive 

 index of the cornea being nearly the same as that of water, the light is 

 only very slightly refracted there. The same effect is produced when 

 a cover-slip is placed over the cornea in the air ; a plane surface being 

 substituted for the curved anterior surface of the cornea, its refraction 

 is abolished. Observe in the fundus of the eye the optic disc, eccentric- 

 ally placed in the retina, and the retinal vessels radiating out from it. 

 A portion of the fundus shows brilliant iridescent colours in many 

 animals (the tapetum lucid um). This portion is abruptly bounded by 

 a line a little above the optic disc. The appearance is due to a peculiar 

 arrangement of the connective-tissue (including elastic) fibres in this 

 part of the choroid. 



Pinch up with forceps a small portion of the sclerotic a little posterior 

 to its junction with the cornea, and clip it away with fine, blunt- 

 pointed scissors, being careful not to penetrate the choroid layer, which 

 lies immediately beneath the sclerotic. Extend the incision through 

 the sclerotic backwards, and then transversely, and peel off strips of 

 the sclerotic from behind forwards. The lower surface of the sclerotic 

 (the so-called lamina fusca) is dark, owing to the presence in it of the 

 same pigment which is so abundant in the choroid coat. Go on re- 

 moving the sclerotic piecemeal until a considerable area of the dark 

 choroid layer is exposed with the ciliary nerves passing forward on its 

 surface towards the iris. One or other of the long ciliary arteries may 

 also be seen coursing between the sclerotic and choroid if the sclerotic 

 happens to have been removed at its position. On the anterior part of 

 the choroid may be observed some pale fibres passing backwards from 

 the corneo-sclerotic junction. They are the meridional fibres of the 

 ciliary muscle (p. 982). 



The eye being immersed in water, remove cautiously with the forceps 

 and scissors the portion of the choroid exposed. The retina is now seen 

 as a pale membrane, transparent when quite fresh, but becoming whitish 

 soon after death. Cut through sclerotic, choroid, and retina about half- 

 way round the eyeball, a little posterior to the corneo-sclerotic junction. 

 The vitreous humour will bulge out. Since its refractive index is 

 nearly th~ same as that of water, it is scarcely observed when im- 

 mersed, a id 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 



