RESPIRATORY MEMBRANE. 283 



or vesicles (Fig. 74, c, c). The alveoli join together, forming 

 lobules, which are easily distinguished on the surface of the 

 lung in a system of network (division lines of the lobules), 

 and the lobules themselves, uniting, form the lobes of the 



Fig. 74. Lobule of the lung in man.* 



lung. The alveoli are, therefore, very numerous ; their num- 

 ber has been estimated approximately as seventeen or eighteen 

 hundred millions. 



I. STRUCTURE OF THE RESPIRATORY MEMBRANE. ARRANGE- 

 MENT OF ITS PARTS. 



THE pulmonary alveolus constitutes essentially the respir- 

 atory surface : it consists of epithelium and a substratum of 

 connective tissue. 



1. The pulmonary epithelium is formed of epithelial layers, 

 extremely delicate and not readily observed, arranged in a 

 single row, and frequently at a considerable distance from 

 each other. 1 In the normal state its elements exhibit very 



1 See Ch. Schmidt, " De PEpithe'lium Pulmonaire. " These de 

 Strasbourg, 1866, No. 931. 



The existence of the pulmonary epithelium was, for a long time, 

 disputed. Villemin was one of its most ardent opponents, which 

 is not to be wondered at when we consider the elaborate process of 



* ft, Termination of the bronchial tube. 6, Cavity of the lobule, c, c, c, r, 

 Air-cells or vesicles. (Dal ton, " Human Physiology.") This sac or pouch 

 exactly represents the whole lung of a frog. 



