THE RABBIT. 37 



produce a vacuum in the pleiiral cavities around the lungs 

 But the walls of the air-sacs are highly elastic, and as the 

 pressure outside them is diminished the pressure of the air 

 inside causes them to expand, so that the lungs enlarge as 

 rapidly as do the pleural cavities. As they do so the 

 pressure of the air within diminishes, and if the air-passages 

 are open, air rushes in from the outer atmosphere until the 

 pressure is equalized. Its course is through the anterior 

 or external nares (nostrils), through the nasal passage to the 

 throat, and then through the glottis to larynx, trachea and 

 lungs. The whole action of inspiration is essentially that 

 of a suction pump. 



/ 6. Expiration. In expiration the ribs are pulled back, 

 and the diaphragm relaxes and is pressed forward by the abdo- 

 minal viscera, which have been somewhat compressed during 

 inspiration. Thus the pleural cavities are reduced in size 

 and the excess of air is forced out again from the lungs by 

 the same path as it entered. Moreover, the walls of the 

 air-sacs are very elastic and themselves tend to return to 

 a smaller size, as a simple experiment will show. When a 

 rabbit dies under chloroform, its breathing always stops at 

 the end of an inspiration. Hence its lungs remain dis- 

 tended. But if, after opening the abdominal cavity and 

 exposing the posterior face of the diaphragm, you prick a 

 hole on one side of this, air rushes at once into the pleural 

 cavity of that side and the lung can be seen to collapse 

 abruptly to a much smaller bulk. The lung of the other 

 side does not collapse until that side of the diaphragm is 

 pricked, because the two plenral chambers are perfectly 

 separate. This is an experiment that any student can 

 easily perform for himself. 



