362 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



and 244. For greater clearness there is a space between 

 the lungs and the walls. In reality they are in contact. 



This description is necessary to make clear the me- 

 chanics of breathing, which we now proceed to explain. 



Breathing consists in alternate expansion and con- 

 traction of the thoracic cavity by means of the respira- 



Fig. 243. — Vertical section through 

 the lungs, showing the relation 

 to the pleura : tr, trachea ; 6r, 

 bronchi ; cpl, costal pleura ; ppl, 

 pulmonic pleura. 



Fig. 244. — Horizontal section through 

 the thorax : tr, trachea ; H, heart ; 

 pc, pericardium ; ppl, pulmonic 

 pleura ; cpl, costal pleura. 



tory muscles. The lungs are perfectly passive in the 

 operation. To illustrate : Suppose we take a hand 

 bellows and arrange a bladder within so as to connect 

 with the nozzle, with no opening into the bellows, but 

 only into the bladder, through the nozzle. Now, on 

 expanding the bellows the air rushes in through the 

 nozzle into the bladder and expands it, but no air enters 

 the cavity of the bellows. On shutting up the bellows 

 the air is again squeezed out from the bladder through 

 the nozzle. In all these movements the exterior sur- 

 face of the bladder and the interior surface of the bel- 

 lows never break contact. But if there be an opening on 

 the side of the bellows the air will rush in there and not 

 fill the bladder. 



Application to Breathing. — So the lungs are 

 placed in the thorax as the bladder in the bellows. If 



