236 ELEMENTARY BIOLOGY. [XIII. 



roof, and this is more prominent when the eye-ball is 

 retracted. 



a. Gently touch the eye and observe how it is closed, 

 by the pulling over it of the lower transparent 

 eye-lid. The upper eye-lid is very small and 

 hardly moveable. 



b. When the eye is open, observe the parts exposed- 

 a. The transparent cornea covering all its exposed 



surface. 



/3. Through the cornea is seen the iris, a mem- 

 brane coloured by brown and golden pigment, 

 the latter forming a very brilliant ring around 

 the inner margin of the iris. The lower mar- 

 gin of this ring is interrupted at one point, 

 the yellow pigment being there absent, and 

 from the break a faint dark line can be traced 

 downwards through the rest of the lower part of 

 the iris. 



7. The elliptical opening or pupil in the middle 

 of the iris with its long axis directed antero- 

 posteriorly. 



2. Kill the frog (by chloroform or by pithing), and care- 

 fully dissect away the parts from around the eye-ball, 

 cutting away, with the rest, the part of the upper jaw- 

 bone which forms the lower boundary of the socket of 

 the eye-ball, or the orbital cavity. 



a. As the surrounding tissues are cleared away 

 from the eye-ball notice the small muscles 

 which are inserted into it. 



ft. At the back of the eye-ball and passing into it 

 will be found the optic nerve. 



3. Divide the optic nerve and, having thus detached the 



