254 PHYSIOLOGY AT THE FAEM. 



the elasticity of the walls of the sac being sufficiently over- 

 come by the pressure of the atmosphere acting through the 

 aperture of the larynx, so as to reach into every corner of the 

 cavity of the thorax in which the sac is lodged. It is easy to 

 conceive a sac with walls so highly elastic that the ordinary 

 atmospheric pressure should not be sufficient so completely to 

 overcome that high elasticity as to permit the sac to protrude 

 exactly into every corner of the cavity of the thorax. In such 

 a case there would be a vacuum between the inner surface of 

 the cavity of the thorax and the outer surface of the sac. Such 

 a state of things would render the effort of inspiration much 

 more difficult ; that is, there would be required for it a much 

 greater mechanical force. But such is not the case in any 

 known animal. In the ordinary case the elasticity of the air- 

 bag or air-bags of the lungs is never so great as to permit any 

 approach to a vacuum between the inner surface of the walls 

 of the chest and the outer surface of the wall of the lungs. 

 Whenever, then, the cavity of the chest by the movement of 

 its walls becomes in the slightest degree enlarged, the air-bag 

 within becomes proportionately enlarged, so as always to fill 

 the cavity of the chest, even when brought to its largest 

 capacity. Whenever the chest enlarges in a given time by an 

 increment greater than corresponds to the bulk of air that can 

 pass through the larynx into the lung in the same time, then 

 it is manifest the air in the lung or air-bag must be rarefied 

 before the entrance of new air into the lung. And in propor- 

 tion as the enlargement of the chest in a given time exceeds 

 the measure of air which can enter the lung in that time, is 

 the amount of rarefaction on the body of air already in the 

 lung before inspiration begins, or previous to the addition of 

 new air from without. Such rarefaction of the air in the lung 

 when inspiration begins is retarded by the degree in which the 

 lung or air-bag is elastic ; and in proportion as such resistance 



