358 THE HUMAN BODY. 



of things will then answer to that naturally found in the? 

 chest. If the bottle were now increased in size without 

 letting air into it, the bag would expand still more, so as to 

 fill it, and in so doing would receive air from outside 

 through b', and if the bottle then returned to its original 

 size, its walls would press on the bag and cause it to shrink 

 and expel some of its air through I. Exactly the same 

 must of course happen, under similar circumstances, in the 

 chest, the windpipe answering to the tube 1} through which 

 air enters or leaves the elastic sac. 



The Respiratory Movements. The air taken into the- 

 lungs soon becomes laden in them with carbon dioxide, and. 

 at the same time loses much of its oxygen; these inter- 

 changes take place mainly in the deep recesses of the- 

 alveoli, far from the exterior and only communicating with, 

 it through a long tract of narrow tubes. The alveolar air, 

 thus become unfit to any longer convert venous blood into 

 arterial, could only very slowly be renewed by gaseous dif- 

 fusion with the atmosphere through the long air-passages 

 not nearly fast enough for the requirements of the Body, 

 as one learns by the sensation of suffocation which fol- 

 lows holding the breath for a short time with mouth and 

 larynx open. Consequently added on to the lungs is a 

 respiratory mechanism, by which the air within them is 

 periodically mixed with fresh air taken from the outside, 

 and also the air in the alveoli is stirred up so as to bring; 

 fresh layers of it in contact with the walls of the air-cells. 

 This mixing is brought about by the breathing movements, 

 consisting of regularly alternating inspirations, during- 

 which the chest cavity is enlarged and fresh air enters the 

 lungs, and expirations, in which the cavity is diminished 

 and air expelled from the lungs. When the chest is enlarged 

 the air the lungs contain immediately distends them so as. 

 to fill the larger space; in so doing it become rarefied and 

 less dense than the external air; and since gases flow from 

 points of greater to those of less pressure, some outside air 

 at once flows in by the air-passages and enters the lungs. 

 In expiration the reverse takes place. The chest cavity, 

 diminishing, presses on the lungs and makes the air inside 



