THE THORAX AND RESriRATION. 237 



to his centre of gravity. The force of this argu- 

 ment will be understood when we remember that 

 the air contained within the lungs, after a man has 

 made an inspiration, amounts to three hundred 

 and thirty cubic inches. 



Looking to the means of guarding life, nothing 

 can be more important than the condition of the 

 lungs, in respect to the atmospheric air within 

 them. The sensibility, and the rapid contraction 

 of the glottis, which is at the mouth of the respira- 

 tory tube, is for the purpose of arresting any for- 

 eign matter afloat in the atmosphere, which might 

 be drawn in by the stream of inspired air, and so 

 reach the recesses of the lungs. But were this all, 

 the oflice would be but half performed. The 

 foreign body would be arrested ; but how would 

 it be expelled if it lodged ? In common expiration 

 the air is never expelled altogether from the lungs : 

 there is enough retained to be propelled against 

 this foreign body, and to eject it. And, but for 

 this, the sensibility of the glottis, and the actions of 

 the expiratory muscles, would be in vain ; we 

 should be suffocated by the slightest husk of seed, 

 or subject to deep inflammation by the collection of 

 foreign matter drawn into the air-tubes. 



We may here observe, that the instinctive ac- 

 tions for the protection of the body are calculated, 

 if we may say so, for the natural condition of 

 man. The manufacturer is sometimes removed 

 from that condition ; and our invention must be 

 taxed, ont only to maintain the purity of the at- 



