82 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



the bones are always filled with marrow, but often as 

 it grows to maturity the marrow is absorbed, leaving 

 only a thin dry-looking lining, and the delicate mem- 

 brane of the air-sacks extends into the cavity. Thus 



Fig. 24.— Section of (a) femur of Ostrich ; (l>) skull of Carinate Bird. E, external 

 opening to ear. The bronchial membrane lines all the small cavities in the bones. 



whenever a bone is hollow (if we except certain 

 parts of the skull), the cavity connects with the lungs 

 and is lined with the bronchial membrane. When the 

 cavity in the bone is large, thin plates separate from 



