mUller] the air-sacs of the pigeon 373 



parabronchia of different bronchial branches communicate with each 

 other in this manner. With one exception the ento- and ectobronchia 

 do not divide into branches of equal value, but end blindly, gradually 

 thinning out, or terminating in the air-sacs. 



The Ostia 



Besides the entrance for the bronchus and the already mentioned 

 ostium posterius, each lung possesses four other ostia, the ostium 

 cervicale, claviculare, intermedium anterius, and intermedium poste- 

 rius. The ostium cervicale (prsebronchiale, Gadow ; figs. 9, 10, and 12, 

 OC) is penetrated by the distal branch of the ectobronchium primum. 

 It lies between the bronchus and the apex of the lung, and sup- 

 plies the cervical sac. The ostium claviculare (subbronchiale, 

 Gadow; figs. 9 and 12, OCL) is penetrated by the ectobronchium 

 primum itself. It lies in front of the entrance of the bronchus into 

 the lung, and supplies the interclavicular sac. The ostium inter- 

 medium anterius (figs. 9 and 12, OIA) is penetrated by the ecto- 

 bronchium tertium. It is situated behind and medially from the en- 

 trance of the bronchus and supplies the saccus intermedins anterior. 

 The ostium intermedium (figs. 8, 9, 10, and 12, IOP) is penetrated 

 by a special lateral branch of the mesobronchium. It lies near the 

 ostium posterius, and supplies the saccus intermedius posterior. 



Concerning the shape of the ostia, it may be remarked that the 

 transition from the lung to the air-sac is somewhat gradual. Eberth 

 (1863, p. 436) has stated that the ciliated epithelium of the air- 

 passages is continued through the ostia posteriora into the abdominal 

 air-sacs. Recent authors have confirmed this, and it is only a ques- 

 tion of how far into the air-sacs the ciliated epithelium extends. 

 Eberth (1863, p. 436) reports that he has seen at these ostia oblique 

 and radial muscle fibers. I was unable to verify this, and found no 

 trace of a true muscular sphincter at these ostia. 



The Air-sacs 



True Air-sacs and Other Air-spaces 



I here regard as true air-sacs only those air-filled spaces in the 

 body that arise ontogenetically from branches of the bronchi. The 

 bronchial branches that form the air-sacs develop rapidly, and early 

 invade the body cavity. Besides these true air-sacs, other air-spaces 

 occur. Such are the tracheal labyrinths of ducks and emus (Gadow, 

 1890), and the pneumatic cavities of the bones of the head. 



