374 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



The Nomenclature of the Parts of the Air- sac- system 



In order to avoid confusion I will restrict the term sac to the large 

 primary air-spaces, and not use it, as is usually done, also for the 

 processes arising from these primary spaces. These processes I will 

 call diverticula, and instead of saying axillary sac, subscapular sac, 

 etc., say axillary diverticulum, subscapular diverticulum, etc. If 

 this terminology be used, the names indicate the morphological value 

 of the part of the air-sac system, for example, that the axillary 

 diverticulum and the interclavicular sac are not structures of the same 

 morphological value and homologous. The sac is an enlargement 

 of a bronchial branch ; the diverticulum an appendage of the sac. 

 Air-spaces that take their origin from diverticula I will call out- 

 growths or secondary diverticula. According to this nomenclature 

 the pigeon has five pairs of large (primary) air-sacs. Besides these, 

 other true (primary) sacs may exceptionally occur. Gadow (1891, 

 p. 751) mentions numerous openings of the secondary bronchi at the 

 surface of the lung which lead into such sacs. I found such sacs 

 only very rarely, in a few individuals and in varying localities, 

 mostly near the ribs. Having no regularity in their arrangement or 

 occurrence, they are, in the pigeon at least, only to be considered as 

 abnormal structures, and both on that account and because of their 

 small size, can hardly be regarded as possessing any great functional 

 importance. 



Relations of the Air-sacs of Birds to Similar Structures in Reptiles 



In the chameleon and some other reptiles bronchial branches ex- 

 tending beyond the lungs are observed. These are homologous with 

 the true (primary) air-sacs of birds. At an early stage of develop- 

 ment the reptilian lung is similar to that of birds. If, as Milani 

 ( 1897, p. 47) remarks, we compare a bird's lung with that of Thalas- 

 sochelys carctta, and disregard the adaptive peculiarities of this 

 organ in the latter, we find that the chief air duct of the bird's lung, 

 although shorter, essentially corresponds, so far as it is supported 

 by cartilage, with the intrapulmonary bronchus of the lung of 

 Thalassochelys, and that there are structures in the lung of the latter 

 homologous with the bronchial tubes and the parabronchia in the 

 lung of the former. 



