I.] THE FROG. 113 



b. When the eye is open, observe the following — 



a. The cornea; the transparent covering for its 

 exposed surface. 



/?. The iris, a membrane, seen through a, coloured 

 by brown and golden pigment, the latter form- 

 ing a very brilliant ring around its inner 

 margin. Below it is interrupted by a faint 

 dark \ line which can be traced downwards 

 through the whole iris. 



y. The pupil ; elliptical and lying within the 

 iris, its long axis directed antero-posteriorly. 



ii. The eye-muscles and their nerves. These can be 

 most satisfactorily made out, notwithstanding their 

 small size, in a frog preserved in chromic acid as 

 directed in Sect. K. 4. Examine under a hand 

 lens. 



a. Dorsal aspect. Remove the head from the body, 

 cutting well back so as to include the first two 

 vertebrae, and pin down in as small a dish as 

 may be convenient. Dissect off the integument 

 of the top of the head together with the upper 

 eyelids, but be very careful to leave the lower 

 lids uninjured. 

 a. The eyeball ; completely filling the orbit. 

 p. The superior oblique muscle; ribbon-shaped 

 and passing obliquely forwards between the 

 antero-dorsal face of a. and the orbit. 

 y. The superior rectus ; a trifle larger than /3. 

 and arising from the dorsal face of the eye- 

 ball. It passes backwards and inwards, to be 

 attached to the cranial wall. 

 M. 8 



