CHAPTER YIL 



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 

 immediately 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 o\iter 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 

 sm-face 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 Avill 

 appear to be made iip of two combined portions fi-om 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 thi-ee 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 



