XIIL] THE FROG. 245 



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

 posed. 



a. The transparent cornea covering all its exposed 

 surface. 



/?. 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. 



y. 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. 



/?. 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 

 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. 



