390 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



often found the injected gelatine or paraffine under the skin of 

 pigeons, but I found that in every such case the wall of an air-sac 

 had been ruptured, and the mass had then effused. It may be 

 possible that such injuries of the air-sac wall sometimes occur in 

 the living animal without fatal result by the rupture of an extra- 

 thoracic diverticulum. I twice found pneumatized bones which had 

 been broken, and had healed. 



The Pneumaticity op the Bones 



The air-sacs, their diverticula and outgrowths, send processes into 

 many bones. These expand there by pushing aside the marrow, 

 the bones thus becoming pneumatized. The sacci cervicales pneu- 

 matize the cervical and dorsal vertebrae and the ribs. The latter 

 are sometimes also pneumatized directly from the bronchial branches 

 of the lungs. The intrathoracic diverticula of the saccus inter- 

 clavicularis pneumatize the sternum and the sterno-costal bones ; its 

 subscapular diverticula pneumatize the coracoids, the furculum, and 

 the scapulas, and its axillary diverticula pneumatize the humeri. The 

 sacci intermedii anteriores and posteriores do not pneumatize any 

 bones. The sacci abdominales pneumatize the bones of the pelvis 

 and the lumbar and the sacral vertebrae. 



All the bones of the neck and trunk, as well as the humeri, are 

 pneumatized. All bones situated distally from the humerus in the 

 anterior extremity and all the bones of the posterior extremity are 

 filled with marrow and not pneumatized. The bones of the skull are 

 partly pneumatized, not by the pulmonary air-sacs, but from the 

 nasal air-spaces. All air-sacs that possess diverticula also pneumatize 

 bones. The sacci intermedii anteriores and posteriores have no 

 diverticula, and they alone pneumatize no bones. 



The pneumatic bones are strikingly different from the medullated 

 ones. Since they are free from marrow and fat, they are lighter in 

 color and usually so transparent that their internal structure, the 

 lamellae and trabecular, can be seen through their walls. The walls 

 of such bones are thinner, more compact, and more brittle than those 

 of mecUdlated bones, a part of the spongiosa and the innermost 

 layer of the compact substance of the wall appearing to have been re- 

 absorbed. The delicate membranous wall of the air-sac is applied 

 immediately to the nearly smooth inner surfaces of the bone. 



The Pneumatic Foramina of the Bones 



In each pneumatic bone there is found a pneumatic foramen 

 through which an air-sac sends a process into the bone. The situa- 



