BREATHING.] PHYSIOLOGY, 89 



hollow bladders, stuck on to the smallest hollow 

 twigs, and made of some delicate, but strong and 

 exceedingly elastic, substance. If you blew down 

 the trunk you might stretch and swell out all the 

 hollow leaves ; when you left off blowing they would 

 all fall together, and shrink up again. 



Around such a framework of hollow branches 

 called bronchial-tubes, and hollow elastic bladders 

 called air-cells, is wrapped the intricate network of 

 pulmonary arteries, veins, and capillaries, in such a 

 way that each air-cell, each little bladder, is covered 

 by the finest and most close-set network of capillaries, 

 very much as a child's india-rubber ball is covered 

 round with a network of string. Very thin are the 

 walls of the air-cell, so thin that the blood in the 

 capillary is separated from the air in the air-cell by 

 the thinnest possible sheet of finest membrane. As 

 the dark purple blood rushes through the crowded 

 network, its carbonic acid escapes through this thin 

 membrane, from the blood into the air, and oxygen 

 slips from the air into the blood. 



Thus the dark purple venous blood coming along 

 the pulmonary artery, as it glides ii> the pulmonary 

 capillaries along the outside of the inflated air-cells, 

 by loss of carbonic acid and gai% of oxygen is changed 

 into the bright scarlet blood of the pulmonary 

 veins. ^ ^ . . ; 



This then is the mystery of our constant need of air. 

 The flesh of the body of whatever kind, 

 everywhere all over the body, breathes 

 blood, making pure arterial blood venous 

 and impure, all over the body except in the 

 lungs, where the blood itself breathes air, 



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