82. THE STRUCTURE AND LIFE OF BIRDS cuwap. 
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 
Fic. 24.—Section of (a) femur of Ostrich ; (4) skull of Carinate Bird. §, external 
opening toear. ‘Lhe 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 
