138 THE ENGINES OF THE HUMAN BODY 



We have only to see a ray of sunshine break through 

 the air of the room in which we sit and live to realise 

 how crowded our breath must be with fine particles of 

 dust and soot. The nose filters off many of these parti- 

 cles, but multitudes are carried into the main passages, 

 and not a few reach even the final respiratory chambers. 

 It is very manifest that the air passages will become 

 clogged — even become choked — just as certainly as a 

 chimney will become filled with soot unless there is 

 some arrangement for keeping it repeatedly swept. 

 Chimneys lined with automatic sweeps have not yet been 

 thought of, but a contrivance of this kind has been 

 adopted by Nature for keeping clear and clean the air- 

 ways to the respiratory chambers. Conceive a chimney 

 set with boot brushes, so that their hairs or bristles form 

 a continuous lining on which the soot from the smoke 

 is constantly falling, and conceive, too, that the bristles 

 are in constant movement, waving every particle of soot 

 which falls on them in one direction — namely, from fire- 

 place to the vent, — then you have some idea of the con- 

 trivance which Nature has adopted to keep the breath 

 passages clear. The bristles or cilia which line the air 

 passages are so delicate and small that they require a 

 strong microscope to bring them within the range of 

 our vision ; the backs in which the ciliary brushes are 

 set and which keep the bristles in motion are micro- 

 scopic brick-like corpuscles with which the air passages 

 are paved. Cilia cannot work unless they are kept moist ; 

 hence everywhere along the walls of the respiratory 

 passages we meet with the mouths of small glands or 

 workshops at which a clear sticky substance called mucus 

 is being constantly thrown out to meet the needs of 

 the cilia. 



The layer of ciliated lining ceases suddenly as the 

 respiratory chambers are approached and entered. The 

 chambers have a thin delicate lining composed of living 

 units which we may name pavement corpuscles. Stray 

 microscopic particles of dust or soot may occasionally 

 reach the respiratory chambers, sometimes carrying disease 



