CHAPTER VI I. 



DISSECTION OF THE EYEBALL. 



Directions. — Let the student procure three or four eyes of the horse, or, 

 failing these, of the ox. They should be excised from the orbit immedi- 

 ately after death, and as much as possible of the optic nerve should be 

 preserved in connection with the eye. While an assistant holds the eye 

 without squeezing it, the dissector should clean the optic nerve and the 

 outer surface of the sclerotic with forceps and a sharp scalpel. One of 

 the eyes so prepared should be completely frozen in a mixture of ice and 

 salt, and it should then be bisected vertically with a large knife or fine 

 saw. While still frozen, the section to which the optic nerve is attached 

 should be fastened by a strong pin to a layer of solid paraffin at the 

 bottom of a wide and shallow basin. It should be fastened with the cut 

 surface upwards, the pin being passed vertically from the centre of that 

 surface ; and the vessel should then be filled with water. The remaining 

 segment should be laid on the freezing mixture, with its cut surface' 

 upwards. By an examination of both segments, the student should 

 make out the following points : — 



The Globe or Ball of the eye approaches the spherical in form, as is 

 expressed by these designations. On closer inspection, however, it will 

 appear to be made up of two combined portions from spheres of different 

 sizes. The posterior portion, forming about five-sixths of the ball, is a 

 sphere of comparatively large size with a small segment cut off it in 

 front ; and at this point there is applied to it the anterior portion, which, 

 being a segment of a smaller sphere, projects at the front of the ball 

 with a greater convexity than the posterior portion. 



The eyeball consists of concentrically arranged coats, and of refracting 

 media enclosed within these coats. The coats are three in number, viz., 



(1) an external protective tunic made up of the sclerotic and cornea, 



(2) a middle vascular and pigmentary tunic — the choroid, (3) an internal 

 nervous layer — the retina. The sclerotic is the white opaque part of 

 the outer tunic, of which it forms about the posterior five-sixths, being 

 co-extensive with the larger sphere already mentioned. The cornea 

 forms the remaining one-sixth of the outer tunic, being co-extensive 

 with the segment of the smaller sphere. It is distinguished from the 

 sclerotic by being colourless and transparent. The choroid coat will be 

 recognised as the black layer lying subjacent to the sclerotic. It does 



