34 



PHYSIOLOGY 



might be studied to advantage in a cat or a dog, for the 

 conditions are similar. 



How does the air get inside the air cell \ Each little 

 air cell has a tiny tube opening into it which in turn 

 opens into a larger tube and this into a still larger tube 

 until we find that the whole mass of air cells in each lung 

 opens into one large tube (Fig. 19). The tubes from 



the two lungs unite to form 

 the trachea or windpipe 

 (Fig. 20). Air that finds 

 its way into the windpipe 

 thus has free access to each 

 little sac. As the behavior 

 of the lungs, as a whole, is 

 only the concerted behavior 

 of all the cells, we may 

 think of each lung as a hol- 

 low bag, a single enlarged 

 air ceU (Fig. 25, L). 



What makes the air go 

 into the lungs ? It is again 

 a case of air pressure. If 

 the pressure of the air is 

 greater outside than inside 

 the air rushes through the 

 windpipe into the lungs ; if 

 it is greater inside than 

 This occurs at fairly regu- 

 lar intervals and is due to a rhythmical change in the 

 pressure of the air within the lungs. 



Respiratory Muscles. — What causes the alternate 



Fig. 19.— Air cells and bronchial tubes. 



outside the air rushes out. 



