THE AIR SACS IN BIRDS. 317 



others near its entrance into the lung, whicli pass directly 

 to the ventral surface of the lung, and there open into other 

 ail' sacs. Of these there are f oiu-. Two, the anterior and the 

 posterior thoracic, lie on the ventral face of the lung in the 

 thorax. The other two are situated in front of its anterior 

 end, and are extra-thoracic. The external and superior is 

 the cervical, the internal and infei'ior, the interclavicular. 

 This last unites into one cavity with its fellow of the oppo- 

 site lung. Thus there are altogether nine air sacs ; two 

 posterior or abdominal, fova: thoracic, two cervical, and 

 one interclavicular. Other large canals given off fi'om the 

 bronchus do not end in air-sacs, but those which pass from 

 the inner side of the bronchus, run along the ventral sur- 

 face, and those on the outer side, along the dorsal surface, 

 of the lung. Here they give off, at right angles, series of 

 secondary canals, and these similarly emit still smaller ter- 

 tiary canals ; and thus the whole substance of the lung be- 

 comes interpenetrated by tubuli, the walls of the finest of 

 which are minutely sacculated. The different systems of 

 tubuli are placed in communication by perforations in their 

 walls. 



In most birds, the air-sacs (except the anterior and pos- 

 terior thoracic, which never communicate with any cavity 

 but that of the hmgs) are in connection with a more or less 

 extensively ramified system of air passages, which may ex- 

 tend through a great many of the bones, and even give off 

 subcutaneous sacs. Thus the interclavicular air-sac gene- 

 rally sends a prolongation into each axilla, which opens 

 into the proximal end of the humerus, and causes the 

 cavity of that bone to be full of air. When the sterniim, 

 the ribs, and the bones of the pectoral girdle are pneumatic, 

 they also receive their air from the interclavicular air- sacs. 

 The cervical air-sacs may send prolongations along the ver- 

 tebral canal of each side, which supply the bodies of the 

 cervical vertebrse, and communicate with elongated air- 

 chambers in the spinal canal itself. When the dorsal ver- 

 tebrte are pneumatic, they communicate with the system of 

 the cervical air-sacs. The abdominal air-sacs send pro- 



