238 NATURAL THEOLOGY. 



mosphere in which he works, in a chemical sense, 

 but to arrest, or convey away, the small portions 

 of material which may be thrown off by the ope- 

 rations of the flax-dresser, for example, in heck- 

 ling, or of the cutler, whose occupation it is to 

 grind the steel after the instrument is forged, or 

 of the stone-cutter, &c., and so to prevent those 

 particles being inhaled. The length of the pas- 

 sages which lead to the lungs, the sensibility and 

 muscular apparatus bestowed upon them, and the 

 mucous secretions thrown into them, are the natural 

 means by which foreign matter is arrested and 

 thrown out. But in these artificial conditions of 

 men, insoluble particles are continually floating in 

 the atmosphere which they breathe ; these are 

 drawn in and lodge in the lungs, and irritate to 

 disease. 



The reader will find that the following extract, 

 from a paper upon the actions of the windpipe, 

 illustrates the present subject.* 



" We read that the trachea is formed of imper- 

 fect hoops of cartilages joined by membranes, and 

 that it is flat on the back part for these reasons : 

 that it may be a rigid and free tube for respiring 

 the air: that it may accommodate itself to the 

 motions of the head and neck ; and that it may 

 yield in the act of swallowing to the distended 

 oesophagus, and permit the morsel to descend. 

 This is perfectly correct : but there is a grand 



* Philosophical Transactions. 



