No. I.] 



THE STRUCTURE OF THE LONG. 



171 



which is found in Macrochelys. In Heloderma and the turtles 

 each air-sac communicates with the bronchus by an oval or 

 circular opening in its walls ; this opening is the only one con- 

 nected with the sac. 



In the crocodile (Fig. 7) and alligator the bronchus enters 

 the lung near its center, and passes somewhat obliquely into 

 the lung until it reaches the junction of the lower and middle 

 third ; here it breaks up into eight to fifteen tubular passages. 

 These tubular passages are studded with a great many air-sacs, 



Lung of crocodile. 



Lung of bird. 



S, air-sac ; A , atrium ; B, bronchus ; BB, air-bag. 



as shown in cut 5. In these animals the lung for the first time 

 gives the structure as it is found in mammals. There are 

 many air-sacs which communicate with a common cavity, or 

 atrium, all of which in turn communicate with a single 

 terminal bronchus. A single lobule of the mammalian lung is 

 simply enlarged to form the lung of the crocodile ; the lung 

 of the former is only a conglomerate of that of the latter. 

 Avian hmg. — ln many respects the bird's lung varies con- 



