THE FROG II 
brane known as the cornea. The iris is a pigmented ring situated 
behind the cornea and showing through; it acts as a diaphragm, 
limiting the amount of light that enters the eye. The pupil, 
through which light enters the eye, is an oval aperture surrounded 
by the iris. Watch the eye carefully to see if the pupil changes in 
size through the expansion and contraction of the iris. 
2. The tympanic membrane of the ear, a short distance be- 
hind the eye. 
3. The brow spot, a very small light spot midway between the 
anterior ends of the eyes. The brow spot is the vestige of a 
median eye which was functional in ancestral forms. 
4. The mouth. This is kept tightly closed, except in the act of 
taking food. 
5. The anus (posterior opening of the digestive tube). It is 
slightly dorsal in position. 
6. The anterior nares, or external openings of the nasal 
cavities. 
7. The fore limbs. There are three divisions, upper-arm, 
fore-arm and hand (manus). The wrist, forming the proximal 
portion of the manus, is scarcely distinguishable externally. In 
the male there is a thickening along the inner edge of the first 
finger or digit, which is especially prominent during the breeding 
season. 
8. The hind limbs. There are three divisions, thigh, leg and 
foot (pes). Certain bones of the proximal part of the foot (the 
ankle) are greatly elongated; of what advantage is this to the 
frog? What is the function of the web of the hind foot? 
Ill. Tse MovutH Cavity. 
For the following study both recently killed and preserved 
specimens are desirable. 
Open the mouth of a frog to its fullest extent and observe: 
I. On the roof: 
(a) The teeth. 
(1) Maxillary teeth on the edge of the upper jaw. 
(2) Vomerine teeth, two groups on the fore part of the roof. 
Count them with the aid of a dissecting needle; about how many 
teeth in each group? Is the number constant in different frogs? 
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