Fig. 251.—Head of Frog.
Describe the mouth. Observe the extent of the mouth opening (Fig. [251]). Are teeth present in the upper jaw? The lower jaw? Are the teeth sharp or dull? Does the frog chew its food? Is the tongue slender or thick? (Fig. [251].) Is it attached to the front or the back of the mouth? In what direction does the free end extend when the tongue lies flat? Is the end pointed or lobed? How far out will the tongue stretch? For what is it used? Why is it better for the teeth to be in the upper jaw rather than in the lower jaw? That the teeth are of little service is shown by the fact that the toad with similar habits of eating has no teeth. Will a toad catch and swallow a bullet or a pebble rolled before it? The toad is accustomed to living food, hence it prefers a moving insect to a still one.
The Senses.—Compare the eyes with the eyes of a fish in respect to position and parts. Are the eyes protruding or deep-set? Touch the eye of a live frog. Can it be retracted? What is the shape of the pupil? The colour of the iris? Is the eye bright or dull? What probably gave rise to the superstition that a toad had a jewel in its head? Is there a third eyelid? Are the 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?
Fig. 252.—Skeleton of Frog.
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?
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 five separate bones in the palm. Do any of the frog’s fingers have three joints? Compare also the leg of man and the hind leg of the frog (Figs. [253] and [399]). Does the thigh have one bone in each? The shank of man has two bones, shin and splint bone. Do you see a groove near the end in the shank bone of a frog (Fig. [252]), indicating that it was formed by the union of a shin and a splint bone? The first two of the five bones of the ankle are elongated, giving the hind leg the appearance of having an extra joint (Fig. [253]). The foot consists of six digits, one of which, like the thumb on the fore limb, is rudimentary. The five developed toes give the five digits of the typical vertebrate foot. Besides the five bones corresponding to the instep, the toes have two, three, or four bones each. How is the hind foot specialized for swimming? Which joint of the leg contains most muscle? (Fig. [254].) Find other bones of the frog analogous in position and similar in form to bones in the human skeleton.