368 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



parts of the bones could then be recognized from without by means 

 of their blue color. 



In order to obtain an idea of the degree of expansion of the air- 

 sacs during the various phases of breathing, I strangled several 

 pigeons, some during inspiration and some during expiration, so 

 suddenly that no more air could pass either in or out, and then 

 hardened them by long immersion in formol. 



The Air-sac-system tn General 



In pigeons and other flying birds there are numerous hollow, air- 

 filled spaces, the air-sacs, some of which are very large. Their size 

 varies with the respiratory and other movements. All are connected 

 either directly or indirectly with the bronchi, and through them and 

 the trachea with the outer air. The air passes from the bucco- 

 pharyngeal cavity (fig. 12, CO), 1 into the larynx (fig. 12, L,), the 

 trachea (figs. 2, 7, 11, 12, TR), the syrinx (fig. 12, S), and the two 

 bronchi (figs. 9, 10, 12, BR). From the latter two tubes branch off 

 to the air-sacs, of which 10 groups, forming 5 pairs, can be distin- 

 guished : 



1. Cervical sacs (in the region of the nape of the neck). 



2. Interclavicular sacs (adherent to the syrinx and situated ven- 

 trally, laterally, and proximally from it). 



3. Anterior intermediate sacs. 



4. Posterior intermediate sacs, 

 (in the thoracic cavity). 



5. Abdominal sacs (in the abdominal cavity). 



Each of the 10 groups consists of a main sac, from which numer- 

 ous diverticula arise. The latter, as well as the main sacs, possess 

 many secondary diverticula (sacculi). 



Before considering the details of the air-sacs it will be well to 

 determine the localities where they are situated and their relations 

 to the diaphragmatic membranes. 



The Diaphragmatic Membranes 



The open communication between the cavities of the lungs and 

 the air-sacs makes diaphragmatic breathing, as it occurs in mammals, 

 impossible in birds. The diaphragm of birds is quite different in 

 structure and function from that of mammals. It is composed of 

 two parts, viz., the pulmonary diaphragm, which separates the air- 

 sacs from the lungs and effects the expansion of the latter in a man- 



1 The figures referred to are on plates xi.v-xux. 



