

XIII.] THE FROG. 237 



eye, pin it to a piece of loaded cork, with the corneal 

 surface upwards. 



a. Notice the more opaque coat (sclerotic), with 

 which the margin of the cornea is continuous, 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 pigmented 

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

 seen through it. 



1). 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 trans- 

 parent mass, the vitreous humour, which can be seen 



