MECHANISM OF KESPIEATION. 253 



from the debris of the solids results, must at all times be 

 indispensable. 



The mechanism of respiration in the higher animals is so 

 contrived as alternately to draw into the body and expel from 

 it a certain amount of atmospheric air, nearly a fourth part 

 of which is the oxygen required. The respirations in a minute 

 amount to about a fourth part of the number of pulsations of 

 the heart. Care must be taken not to misunderstand " draw 

 in " as used above. The air is not drawn in by what is some- 

 times called a vis afronte, as we draw in water from a well 

 with a bucket, but is drawn in as the water is drawn into the 

 common pump when the handle is depressed. In short, the 

 atmospheric pressure, being about fifteen pounds to the square 

 inch on objects at the earth's surface, whenever the slightest 

 rarification takes place in a cavity communicating with the 

 atmosphere, the atmospheric air must instantly begin to enter 

 that cavity, and go on entering it so long as the rarification, 

 however minute, is continued. 



The lung is, in fact, a bag divided into many millions of 

 small sacs or vesicles, to which there is but one narrow entrance 

 namely, the orifice of the larynx ; but every one of these little 

 sacs comports itself in respiration exactly as if it were one 

 large sac of the same magnitude as the lung itself, and like it 

 at all times during health, filling exactly the cavity of the 

 chest in which it is lodged, and communicating with the at- 

 mosphere, like the lung, only by the narrow aperture of the 

 larynx. The cavity of the thorax in which this sac is lodged, 

 and which it entirely fills, is capable of undergoing alternately 

 a very considerable enlargement, and a like contraction, in its 

 dimensions, owing to the mobility of the walls which bound 

 the cavity. Thus, when the walls of the thorax (the ribs and 

 midriff) are at rest, the sac within contains air having exactly 

 the same density as the air of the surrounding atmosphere, 



