58 THE FROG 
XWVil, Poe -Bye; 
Review your observations on the external anatomy of the eye 
of a living frog (Section II, 1). The following work may be 
done on a preserved specimen, though a freshly killed specimen is 
better. 
Remove an eye-ball by cutting around it through the lids, and 
severing the muscles and the optic nerve. The muscles that move 
the eye-ball will not be considered here, since they may be studied 
to better advantage in a larger vertebrate (¢. g., an elasmobranch 
or a mammal). Observe how the lids are connected with the 
eye-ball by means of a membrane called the conjunctiva; clean 
away the fragments of the lids from the eye-ball. 
Observe carefully the shape of the eye-ball. In the living frog 
it is nearly spherical but slightly top-shaped; in preserved 
material it may be slightly flattened on the outer side where it is 
covered by the cornea. Make an outline (8 cm. in diameter) 
showing the shape of the eye-ball as viewed from its side (1. @., 
viewed at right angles to the axis passing from the center of the 
pupil through the center of the eye). This outline is later to be 
filled in to show the structure of the eye. 
The sclerotic is the firm outer wall of the eye, composed of 
hyaline cartilage and dense white connective tissue; it covers all 
that part of the eye-ball not covered by the cornea. The optic 
nerve may be seen piercing the sclerotic to enter the eye-ball on 
its proximal side. 
With scissors divide the eye into two unequal parts by an in- 
cision passing a little to one side of the pupil and the optic nerve, 
so as to lay open completely the interior of the eye. Observe the 
sudden collapse of the eye-ball as soon as its fluid contents are 
allowed to escape. Remove the other eye-ball and divide it into 
two parts by an incision separating proximal and distal halves. 
Place the parts of both eye-balls in water and observe the follow- 
ing features: 
1. The crystalline lens is a solid body, transparent in life, 
situated just behind the iris and attached to the iris by its outer 
margin. It is more convex on its inner than on its outer surface. 
2. The anterior chamber of the eye is the space between the 
