170 STRUCTURE OF THE LUNGS, 



The substance of the lungs consists of bronchial 

 tubes, air-cells, blood-vessels, nerves, and cellular 

 membrane, or parenchyma. The first are merely 

 continuations and subdivisions of the windpipe, and 

 serve to convey the external air to the air-cells 

 of the lungs. The air-cells constitute the chief part 

 of the pulmonary tissue, and are in one sense the 

 terminations of the smaller branches of the bron- 

 chial tubes. When fully distended, they are so nu- 

 merous as in appearance to constitute almost the 

 whole lung. They are of various sizes, from the 

 20th to the 100th of an inch in diameter, and are 

 lined with an exceedingly fine thin membrane, on 

 which the minute capillary branches of the pulmo- 

 nary arteries and veins are copiously ramified ; and 

 it is while circulating in the small vessels of this 

 membrane, and there exposed to the air, that the 

 blood undergoes the change from the venous to the 

 arterial state. So prodigiously numerous are these 

 air-cells that the aggregate extent of their lining 

 membrane in man has been computed to exceed a 

 surface of 20,000 square inches. 



It may be thought that the interposition of such 

 a membrane must have the effect of preventing any 

 action of the air upon the blood. But, in addition 

 to the proof to the contrary drawn from observa- 

 tion, it has been ascertained by experiment that 

 even the thick and firm texture of bladder is insuf- 

 ficient to prevent the occurrence of the change; 

 venous blood confined in a bladder speedily becorn 

 ing of a florid red, like arterial blood. 



Blood-vessels necessarily form a large constituent 

 portion of the substance of the lungs. Besides the 

 arteries and veins which the lungs possess in com- 

 mon with other parts for the purposes of nutrition, 

 they have, as we have seen, the large pulmonary 

 arteries and veins, dividing everywhere through 

 their substance into innumerable branches, convey- 

 ing the whole blood of the body to and from the air- 



