RESPIRATION: THE MECHANISM OF BREATHING 353 



that on its exterior. At last, when all the air is sucked out of the 

 bottle and the stop-cock on c closed, the bag, if sufficiently dis- 

 tensible, will be expanded so as to completely fill the bottle and 

 press against its inside, and the state 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 ex- 

 pand 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, FlG 116 __ Dla _ 

 its walls would press on the bag and cause it to e ram illustrating the 



, . , , . . A . ,. , 7 T, pressure relation- 



shrink and expel some of its air through b. Ex- ships of the lungs in 



actly the same must of course happen, under the 1 



similar circumstances, in the chest, the windpipe answering to 



the tube b through which air enters or leaves this elastic sac. 



The Respiratory Movements. The air taken into the lungs 

 soon becomes laden in them with carbon dioxid, and at the same 

 time loses much of its oxygen; these interchanges 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 any longer to convert 

 venous blood into arterial, could only very slowly be renewed by 

 gaseous diffusion 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 follows 

 holding the breath for a short time with mouth and larynx open. 

 Consequently cooperating with the lungs is a respiratory mechan- 

 ism, 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 move- 

 ments, consisting of regularly alternating inspirations, during 

 which the chest cavity is enlarged and fresh air enters jthejungs, 

 and expirations, irf which the cavity is diminished r 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 spacei 

 in so doing it becomes 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 



