No. I.] 



THE STRUCTURE OF THE LUNG. 



169 



which smaller septa are found, to form a second and more 

 delicate network. In this manner the inner surface is given a 

 honeycombed appearance. We have in these two animals the 

 extremes of the lungs of the amphibians; one smooth and the 

 other honeycombed, but between them we find all degrees of 

 division of the inner surface. The accompanying cuts give the 

 general arrangement of the interior of the lungs of Necturus 

 and of the frog. In the first the walls are perfectly smooth, 

 while in the frog the septa, which are destined to break the 







~,s 



Lung of snake. Lung of Heloderma. 



S, air-sac ; B, bronchus. 



lung into compartments in the higher animals, are already 

 beginning to form. 



Reptiliaji lung. — In the snake we find a single, much 

 elongated lung {Fig. 3), perfectly smooth externally, and inter- 

 nally for the posterior two thirds of its length. The inner 

 surface of the anterior third becomes more and more divided 

 until we reach the anterior sixth, where we have the inner sur- 

 face divided into many small sacs which communicate with the 

 central cavity by nearly circular openings (Fig. 4). 



