No. I.] 



THE STRUCTURE OF THE LUNG. 



173 



have seen that the lung becomes more complex by a continuous 

 formation of septa, and the process of buckling forms only a 

 secondary role (Fig. 4). Now the budding begins to become 

 more important and the septum-formation is secondary. This 

 we must keep clearly in mind when we compare the develop- 

 ment of the various lungs. 



Mammalian lung. — In the study of the mammalian lung I 

 have made use of specimens taken from the rat, rabbit, cat, 

 dog, sheep and man ; not only during adult life but also during 

 the growing period and in foetal life. 



Mammalian lung. 



Terminal bronchus of mammalian lung. 



S, air-sac ; C, air-cell ; A , atrium ; B, terminal bronchus ; V, vestibule ; P, air-sac passage. 

 Tlie artery is shaded, and the vein is in outline. 



The last division of the bronchus, or terminal bronchus, 

 before breaking up into the parenchyma of the lung, bears on 

 its distal extremity a club-shaped expansion as shown in Fig. 

 1 1 . From this expansion arise a number of passages which 

 widen out into secondary expansions, and from the latter other 

 passages lead out which open into central cavities set about 

 with small irregular cells (Figs. 19 to 25). A good idea of 

 this arrangement may be obtained if we compare it to a 

 Pompeian house. The passage leading off from the terminal 



