THE RESPIRATORY MECHANISM. 385 



below into the bottle. When the stop-cock, c, is open 

 the air will enter the bottle and press there on the out- 

 side of the bag, as well as on its inside through b. The bag 

 will therefore collapse, as the lungs do when the chest -cavity 

 is opened. But if some air be sucked out through c the pres- 

 sure of that remaining in the bottle will diminish, and of that 

 inside thebag will be unchanged, and the bag will thus be blown 

 up, because the atmospheric pressure on its interior will not be 

 balanced by 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 distensible, will be expanded so as to com- 

 pletely 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 expand still more, so as to 

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

 6; 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 b. Exactly the same must of 

 course happen, under similar circumstances, in the chest, the 

 windpipe answering to the tube b 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 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 

 to any longer convert venous blood into arterial, could only 

 very slowly be renewed by gaseous diffusion with the atmos- 

 phere through the long air-passages not nearly fast enough 

 for the requirements of the Body, as one learns by the sensa- 

 tion of suffocation which follows holding the breath for a 

 short time with mouth and larynx open. Consequently co- 

 operating with 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 in- 

 spirations^ during which the chest cavity is enlarged and 

 fresh air enters the lungs, and expirations, in which the cav- 



