upper and lower eyelids of the same thickness? With which lid does it wink? Close its eye?
Observe the large oval ear drum or tympanum. What is its direction from the eye? (Fig. 251.) The mouth? Is there a projecting ear? Does the frog hear well? What reason for your answer? As in the human ear, a tube (the Eustachian tube) leads from the mouth to the inner side of the tympanum.
How many nostrils? (Fig. 251.) Are they near together or separated? Large or small? A bristle passed into the nostril comes into the mouth not far back in the roof. Why must it differ from a fish in this?
How do the fore and hind legs differ? How many toes on the fore foot or hand? On the hind foot? On which foot is one of the toes rudimentary? Why is the fore limb of no assistance in propelling the body in jumping? Do the toes turn in or out? (Fig. 250.) How does the frog give direction to the jump? What would be the disadvantage of always jumping straight forward when fleeing? Which legs are more useful in alighting?
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Fig. 252.—Skeleton of Frog.
Divisions of the Limbs.—Distinguish the upper arm, forearm, and hand in the fore limb (Figs. 252 and 253). Compare with skeleton of man (Fig. 399). Do the arms of a man and a frog both have one bone in the upper arm and two in the forearm? Both have several closely joined bones in the wrist and
