170 MILLER. [Vol. VIII. 



All the lungs examined thus far have a large central cavity, 

 the inner surface of which is smooth or more or less sub- 

 divided ; the bronchus opens into the anterior end, but is 

 not continued into the interior. The central cavity is not yet 

 divided into special cavities by partitions extending across it. 

 The diagram in cut 3 shows the relation of this lung to that 

 of lower animals. In many respects it is a continuation of the 

 lung of Necturus and that of the frog. Even if we do view 

 the snake's lung as degenerated, we can not fail to see that in 

 that portion which is more complex it is slightly in advance 

 of that of the frog. In the former the air-cells are already 

 grouped in bunches forming the air-sacs. This grouping is 

 more marked in other reptiles. 



In some of the lizards (Heloderma suspectum) we find that 

 this central cavity is divided into several smaller ones by means 

 of partitions (Fig. 5). These smaller cavities or air-sacs are 

 subdivided into air-cells, just as we have seen in the case of 

 the frog's lung. The bronchus is continued into the lung 

 (which it enters at its inner and anterior sixth) until it reaches 

 the wall which divides the posterior sac from those lying in 

 front of it ; here it ends as a circular opening. This posterior 

 sac is simpler in its structure than those lying anterior to it, 

 just as the posterior end of the snake's lung was found to be 

 simpler than the anterior. The subdivision of the inner surface 

 of the lung begins at the anterior end and gradually proceeds 

 backward. In general the whole lung can be viewed as some- 

 what more advanced than that of the snake. The air-sacs 

 have encroached upon the central cavity until nothing but a 

 small tube or semi-bronchus is left. I am not inclined to 

 call this a bronchus, because it is perforated all along its walls 

 with the openings of the air-sacs. It might be considered as 

 a single atrium or third air-cavity which communicates with 

 the bronchus. This third cavity is best seen in the crocodile. 



The lungs of the turtles furnish beautiful gradations in their 

 subdivisions into sacs, and in the complex division of these sacs. 

 The posterior sac may be smooth like that of the Necturus 

 lung, subdivided as in the frog's lung (Fig. 6), or it may attain 

 the same degree of subdivision as the other sacs, a condition 



