OF RESPIRATION. 85 



both their cartilaginous rings and muscular coat, until 

 their extreme divisions terminate in those cells which 

 form the chief part of the substance of the lungs and 

 alternately receive and emit the air we breathe. 



138. The shape and magnitude * of the air cells are 

 various. The former is generally polyedrical. The 

 latter, in regard to surface, is scarcely to be defined :*f- 

 though, indeed, the capacity of the lungs of an adult, 

 during a strong inspiration, is about 120 cubic inches. 

 The immense size to which the lungs may be inflated, 

 when the chest has been opened, has no relation to our 

 present subject. 



139. The cells are invested and connected by the 

 common but delicate mucous web, the general vincu- 

 lum of the body, and must be carefully distinguished 

 from it. In healthy and very recent lungs, I have found 

 the cells so unconnected that they \vere distended in 

 one insulated spot by air cautiously inflated into a fine 

 branch of the bronchia?, while neither the neighbouring 

 cells nor the cellular membrane, which lies between 

 the cells, admitted a single portion. If air is forcibly 

 thrown in, the air cells are ruptured and confounded 

 with the cellular membrane, and both parts distended. 



140. The mucous web surrounding the air cells of 

 the lungs is supplied with innumerable blood vessels- 

 divisions of the pulmonary artery and four pulmonary 

 veins, the branches of which accompany the ramifica- 

 tions of the bronchia 1 ,! and, after repeated division, 



* Keil, indulging his luxuriant iatro-maUieuiatical genius, assigned more 

 Than 1744,000,000 cells to each lung. 



f Liebcrkiihn, with equal exaggeration, made the surface of the cells 

 '> l.'iOO square feet. 



* Existacliius, Tab. xxvii. fig. 13. 



