74 STRUCTURE AND CLASSIFICATION OF BIRDS 



peculiar in that one of the walls of the sacs has got to be 

 firmly adherent to the ventral parietes, and the walls, of the 

 two sacs enclose between them the intestines, which have 

 thus the appearance of being enclosed in a special compart- 

 ment of the coelom. In one or two Aocipitres there is the 

 same subdivision of the intermediate air sacs that I have 

 referred to in Podargus. 



These air sacs communicate with subsidiary spaces lying 

 among the viscera, between the muscles, in the skin 

 (particularly in Ghauna and several Steganopodes), and 

 with the bones. The skull, however, is aerated by a set of 

 spaces which are not connected with the trachea and lungs, 

 but with the Eustachian tubes and the nasal chambers. 



The literature relating to the lungs and air sacs is large. 

 In addition to the memoirs already quoted the following 

 bear upon the matter : — — 



F. BiGNON, ' Sur les Cellules Aeriennes du Cr^ne,' &c., 

 C. B. Soc. Biol. 1887, p. 36. 



Idem, ' Eecherches sur les Cavit^s Aeriennes Cervico- 

 cephaliques chez les Psitacides,' Bull. Soc. Zool. France, 

 xiii. 1888, p. 180. 



Idem, ' Note sur les Eeservoirs Aeriens de I'Urubu 

 (Cathartes atra),' G. B. Soc. Biol. (9), i. 1889, p. 39. 



Idem, ' Contribution a I'Etude de la Pneumaticite,' &c., 

 Mem. Soc. Zool. France, ii. 1889, p. 260. 



Milne-Edwards, ' Observations sur I'Appareil Eespira- 

 toire de quelques Giseaux,' Ann. Sci. Nat. (5), iii. 1865, 

 p. 137, and ibid. 1867, p. 12. 



Idem, ' Sur les Sacs Eespiratoires du Galao rhinoceros,' 

 G. R. 1885, p. 833. 



E. EiCALBi, ' Alcune ricerche sulla struttura istologica 

 dellesacche aerifere,' &c.,Atti Soc. Tosc. Sci. Nat. (vi. 1885, 

 p. 249). 



H. FiLHOL, ' Sur la Constitution du Diaphragme des 

 Eudyptes,' Bull. Soc. Philom. (7), vi. 188^, p. 235. 



N. GuiLLOT, ' Memoire sur I'Appareil de la Eespiration 

 dans les Oiseaux,' Ann. Sci. Nat. (3), V. 1846, p. 25. 



E. Selbnka, ' Beitrage zur Entwicklungsgeshcichte d. 



