B ATRAC HI A 135 



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 ? Regu- 

 larly 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 con- 

 traction of the throat. This is shown by a slight out- 

 ward 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 hibernat- 

 ing, 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 im- 

 mediately subdivides into three arteries (Fig. 257), one going to 

 the head, one to the lungs and skin, and a third, the largest, 



