capillaries. Does the frog breathe with mouth open or closed? Does the frog have any ribs for expanding the chest? What part of the head expands and contracts? Is this motion repeated at a slow or rapid rate? Regularly or irregularly? There are valves in the nostrils for opening and closing them. Is there any indication of opening and closing as the throat expands and contracts? The mouth and throat (pharynx) are filled with air each time the throat swells, and the exchange of gases (which gases?) takes place continually through their walls and the walls of the lungs. At intervals the air is forced through the glottis into the lungs. After a short time it is expelled from the lungs by the muscular abdominal walls, which press upon the abdominal organs, and so upon the lungs. Immediately the air is forced back into the lungs, so that they are kept filled. In some species the lungs regularly expand at every second contraction of the throat. This is shown by a slight outward motion at the sides. Does the motion of the throat cease when the frog is under water? Why would the frog be unable to breathe (except through the skin) if its mouth were propped open? Why does the fact that the breathing is so slow as to almost cease when hibernating, aid the frog in going through the winter without starving? (Chap. I.) Why must frogs and toads keep their skins moist? Which looks more like a clod? Why?
The Heart and Circulation.—What is the shape of the heart?
(Fig. 257.) Observe the two auricles in front and the conical
ventricle behind them. The great arterial trunk from the ventricle
passes forward beyond the auricles; it divides into two branches
which turn to the right and left (Fig. 257). Each branch immediately
subdivides into three arteries (Fig. 257), one going to
the head, one to the lungs and skin, and a third, the largest,
