246 ELEMENTARY BIOLOGY. [CHAP. 



and which forms the outer envelope of the eye- 

 ball on its sides and back. In some parts the 

 sclerotic is semi-transparent and allows the pig- 

 mented choroid coat (3. g) to be more or less 

 distinctly seen through it. 



b. Prick the cornea with the point of a sharp scalpel, 

 taking care not to injure the iris ; note the clear 

 aqueous humour which spirts out, the cornea at 

 the same time collapsing. 



c. Seize the cut edge of the cornea with a fine pair 

 of forceps and, with sharp scissors, carefully cut 

 through it all round at the line of junction with 

 the sclerotic. The convex anterior surface of the 

 transparent crystalline lens will now be seen pro- 

 jecting through the pupil. 



d. Place the cork in a vessel of convenient size 

 and add enough water to cover the eye. Then, 

 with sharp scissors, cut away the iris, and so 

 expose all the anterior surface of the lens. 

 Passing the point of a scalpel under one edge of 

 the lens, gently tilt it out and examine it. 



a. The crystalline lens of the frog is nearly sphe- 

 rical but somewhat thicker from side to side 

 than antero-posteriorly. Its anterior surface 

 (that which projects into the pupil) is also less- 

 convex than its posterior surface. 



e. The cavity of the posterior chamber of the eye 

 is now exposed. It is filled with a gelatinous 

 transparent mass, the vitreous humour, which 

 can be seen if the water, in which the eye is 

 being dissected, be poured away. 



