RESPIRATORY CHAMBERS 139 



germs with them. For their removal Nature calls in 

 wandering microscopic scavengers known to medical men 

 as phagocytes ; they are noted for their hearty indis- 

 criminate appetites. More frequently the disease-causing 

 germs light on the main passageways and give rise to 

 colds — afflictions from which internal-combustion engines 

 are happily free. When a severe cold sets in, the ciliated 

 corpuscles which line the trachea and bronchi become 

 damaged by the invasion of germs ; the mucous glands 

 flood the passages ; in a tussle with the germs, millions 

 of scavengers fall victims, so that the air passages become 

 clogged with a great accumulation of matter. In such 

 states the respiratory bellows have to be called in to keep 

 the air-ways clear ; they are thrown into fits of convulsive 

 action ; this is a round-about way of saying that when we 

 have a bad cold, we have to cough a great deal. 



So far, we have been speaking only of the passage- 

 ways leading to the essential elements of the lungs — the 

 respiratory chambers. We must now look closely at a 

 small set of chambers. In fig. 34 a minute piece of 

 lung, such as could be covered by the eye of a darning 

 needle, has been taken out and magnified. A little 

 terminal pipe or bronchiole is seen leading to a set or 

 cluster of air chambers or sacs. The bronchiole is just 

 large enough to allow the finest sewing-needle to pass 

 along it. Each chamber or sac is rather irregular in 

 shape ; its walls are exceedingly thin and delicate, but 

 highly elastic, so that when blown up, and the air is 

 again allowed to escape, it returns to its original size 

 and shape. These are the sacs which are dilated when 

 we take a breath ; it is into these microscopic chambers 

 that the air then rushes. One can hear it enter if the 

 ear is closely applied to the wall of the chest in the 

 act of inspiration. The noise is no more than a gentle 

 rustle if the lungs are healthy. The air sacs are very 

 small, only about T \yth of an inch in length and -/<yth 

 in width. In our two lungs there are about six millions 

 of them, and therefore the charge of air which each 

 holds is a very minute one. When we have taken the 



