246 NATURAL THEOLOGY. 



ing upon a cavity of the size of the mouth would 

 be equal to the weight of seventy pounds pressing 

 upon a cavity of the dimensions of the chest.* 



Let us see how beautifully this hydraulic prin- 

 ciple is introduced to give strength in the common 

 actions of the body. We have remarked that the 

 extension of the superficies of the thorax is neces- 

 sary to the powerful action of the muscles which 

 lie upon it; and these are the muscles of the arms. 

 We must all have observed, too, that in prepara- 

 tion for a great effort, we draw the breath and 

 expand the chest. The start into exertion, and of 

 surprise, in man and animals, is this instinctive 

 act. But unless there were some other means of 

 preserving the lungs distended, the action of those 

 muscles which should be thrown upon the arms, 

 would be wasted in keeping the chest expanded. 

 It is here, then, that the principle which we have 

 noticed is brought into play. The chink of the 

 glottis, which the reader has already understood 

 to be the top of that tube which descends into the 

 lungs, is closed by a muscle not weighing a thou- 



* The action of one who uses the blow-pipe is rather curious. 

 The mouth is distended with air, and the passage at the back of 

 the mouth closed; the man breathes through the nostrils, but, 

 from time to time, admits a portion of air into the mouth in expi- 

 ration. The pressure into the blow-pipe is from the distension 

 and consequent elasticity of the cheeks, occasionally assisted by 

 the buccinator muscle, or trumpeter's muscle, so called because it 

 compresses the distended checks. In this way the stream of air 

 through the blow-pipe is kept up uninterruptedly, whilst the man 

 breathes freely through his nostrils. 



