RESPIRATORY SYSTEM 73 



ramifies, a wide branch descending to the subbroiichial 

 ostium. The third entobronchium runs backwards and 

 gives off a number of branches. Close to its origin from 

 the bronchus it opens into the anterior intermediate air sac 

 by the anterior intermediate ostium. The fourth ento- 

 bronchium runs parallel with this ; it gives off branches 

 from its ventral wall, but ends caecally. In addition to the 

 entobronchia there are the ectobronchia. These are six or 

 seven branches given off laterally and dorsally from the 

 mesobronchium. These various bronchia are in communi- 

 cation with each other, so that the substance of the lung is 

 a meshwork. 



As a rule there are ten air sacs in birds, which are 

 arranged in five pairs, five, in fact, arising from each lung. 

 In front of the windpipe are the praebronchial air sacs ; 

 below the trachea are the subbroiichial air sacs ; the oblique 

 septa, which have been described elsewhere (p. 38), enclose, 

 in the duck and all other birds except the Apteryx, two air 

 sacs, the anterior and posterior intermediate sacs. The 

 abdominal air sacs lie among the intestines, and are fed by 

 an ostium which is at the extreme posterior end of the lung ; 

 they have been, as HUXLEY has expressed it, pushed out 

 from the space enclosed by the oblique septa like a hernia. 

 In Apteryx, quite exceptionally, these air sacs are not so 

 pushed out, but lie within the area enclosed by the oblique 

 septa. The only differences that have been noticed in birds, 

 apart from those that have been already mentioned, appear 

 to consist in the number of the intermediate air sacs and in 

 the condition of the praebronchial. WELDON ' has described 

 the breaking up of the pripbronchial in the storks into a 

 number of sacs, at least five in number, and the complete 

 fusion of the subbroiichial sacs into a single one. The 

 breaking up of the praebroiichial sacs is carried to a more 

 complete extent in Chauna. In some birds there are three 

 instead of two intermediate air sacs. I have observed this 

 in Podargns. In many Accipitres the abdominal air sacs are 



1 ' On some Points in the Anatomy of PJicenicopterus and its Allies,' P. Z. S. 

 1883, p. 640. 



