=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 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. [Illustration: FIG. 253.--SKELETON OF FROG.] [Illustration: FIG. 254.--LEG MUSCLES OF FROG.] Is the =skin= of a frog tight or loose? Does it have any appendages corresponding to scales, feathers, or hair of other vertebrates? Is the skin rough or smooth? The toad is furnished with glands in the skin which are sometimes swollen; they form a bitter secretion, and may be, to some extent, a protection. Yet birds and snakes do not hesitate to swallow toads whole. Show how both upper and under surfaces of frog illustrate protective coloration. All batrachians have large and _numerous blood vessels in the skin_ by which gases are exchanged with the air, the skin being almost equal to _a third lung_. That the skin may function in this way, it must not become dry. Using this fact, account for certain habits of toads as well as frogs. If a frog is kept in the dark or on a dark surface, _its skin will become darker_ than if kept in the light or on a white dish. Try this experiment, comparing two frogs. This power of changing color is believed to be due to the diminution in size of certain pigment cells by contraction, and enlargement from relaxation. This power is possessed to a certain degree not only by batrachians but also by many fishes and reptiles. The chameleon, or green lizard of the Gulf states, surpasses all other animals in this respect (Fig. 280). What advantage from this power? [Illustration: FIG. 255.--DIGESTIVE CANAL OF FROG. _Mh_, mouth; _Z_, tongue pulled outward; _S_, opening to larynx; _Oe_, gullet; _M_, stomach; _D_, intestine; _P_, pancreas; _L_, liver; _G_, gall bladder; _R_, rectum; _Hb_, bladder; _Cl_, cloaca; _A_, vent.]