182 APPLIED PHYSIOLOGY 



dibula are enlarged, whilst the alveoli lining them are 

 widened and flattened out. Keith's observations* show 

 that the infundibula in the anterior and superficial parts 

 of the lungs are larger and more easily distended than 

 those which occupy the deeper and posterior parts, 

 and it is to this, perhaps, that their greater liability to 

 suffer from the overdistension of emphysema is to be 

 attributed. 



The alveoli, or air cells, are the respiratory units. 

 Their walls contain a large quantity of elastic tissue, to 

 which the contractility of the lungs is due. They also 

 contain a dense network of capillaries, the meshes of 

 which are no larger than the diameter of the individual 

 twigs. In these the blood is spread out and separated 

 from the air in the alveoli only by the walls of the 

 capillaries and by a single layer of endothelial plates, not 

 even provided with nuclei, and through which inter- 

 change between the blood and the lungs can readily take 

 place. In the meshes of the capillary network, however, 

 where interchange does not occur, one finds smaller and 

 nucleated cells, from which the larger plates appear to 

 be continually renewed. It has been calculated that the 

 total area of the alveolar surface of the lung amounts to 

 no less than 90 square metres, or 100 times the body 

 surface, from which one can realize how perfect the 

 arrangements for bringing the blood and the air into 

 proximity are. Here, however, as in all the organs of 

 the body, ample provision has been made for accidental 

 contingencies, and the fact that life can be maintained 



* 'Why Does Phthisis Attack the Apex of the Lung?' (London 

 Hosp. Gaz., 1904, x. 99). 



