PRACTICAL EXERCISES 985 



PRACTICAL EXERCISES ON CHAPTER XIII. 



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 eye- 

 lids, carefully 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 movements. Observe the transparent cornea in 

 front, blending at its posterior 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. 952). 



Immerse the eye in water in a small glas3 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 pro- 

 duced 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, eccentrically 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 lucidum). 

 This portion is abruptly bounded by a line a little above the optic 

 disc. The appearance is due to a peculiar arrangement of the con- 

 nective-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 removing 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. 907). 



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 



