OSTEOLOGY AND ARTHROLOGY 



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of the animal is greatly lessened. This condition is secured 

 by the pneumaticity. The bone consists of a cover of 

 compact tissue, which, instead of enclosing marrow, is 



lb 10 n 



EA.C. 



Fig. 21. — Skeleton of a Bird (Vulture): Left Surface. 



I, Cranium ; 2, face ; 3, cervical vertebrae ; 4, spinous processes of 

 the dorsal vertebrae ; 5, coccygeal vertebrae ; 6, sternum ; 7, keel ; 8, 

 superior ribs; 9, inferior ribs; 10, clavicle; 11, coracoid bone (for the 

 details of the skeleton of the shoulder, see Fig. 18) ; 12, humerus; 13, 

 radius ; 14, ulna ; 15, carpus ; 16, hand (for details of the skeleton of 

 this region, see Fig. 31); 17, ilium; 18, ischium; 19, pubis (for the details 

 of the pelvis, see Fig. 46) ; 20, femur ; 21, tibia ; 22, fibula ; 23, osseous 

 nodule, which some anatomists think represents the calcaneum ; it is 

 the sole vestige of the tarsus ; 24, metatarsus ; 25, foot ; 26, first toe (for 

 the details of the skeleton of the foot, see Fig. 48). 



hollowed out by cavities which contain air, and communicate 

 with special pouches, the air-sacs, which are appendages of 

 the lungs.* 



* The presence of air in the bones does not seem to be always associated 

 with the pov.'er of flight ; as a matter of fact, we find air spaces in the bones 

 of some birds which do not fly (E. J. Marey, ' The Flight of Birds,' Paris, 

 1890, p. 51). 



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