OSSEOUS SYSTEM. 69 



the Crows, Shrikes, the humerus is the least hollow. The femur 

 is far more generally filled with medullary tissue, but in the Diurnal 

 birds of Prey it is permeated by air as well as the sternum, verte- 

 brae, ribs, and pelvic bones. In the Owls, the femora of which con- 

 tain marrow, the cranium is very much elevated by the large and 

 wide air-cells of the diploe. In the Pelicans very many of the bones 

 receive air. The air openings are developed to the greatest extent, 

 however, in the genus Buceros, where, in addition to the cranial and 

 Maxillary bones, the cervical vertebra?, the pelvis, the caudal verte- 

 brae (but not the sternum and ribs), and all the bones of the extremi- 

 ties, even to the phalanges and toes, are permeated with air. As re- 

 gards their situation, the air-openings are frequently characteristic of 

 different genera. Thus, e. g. in the Vulture and Falcon, the air 

 opening in the femur is placed anteriorly and superiorly beneath its 

 head, as is also the case in the Stork and Picariae, but in the Ostrich, 

 the Blackbird, and Thrush, it is situated posteriorly ; in Buceros 

 melanoleucos there are two pneumatic openings, one superiorly and 

 in front of the femur, the second inferiorly and posteriorly. The Os- 

 trich has the humerus permeable to the air ; in the Cassowary, how- 

 ever, it is full of marrow, which is the case also in most of the Scan- 

 sores, Gallinse, Grallae, and Palmipedes. In the Pelican, the Pea- 

 cock, Bustard, this bone is again found permeable to air. In young 

 birds those bones which at a later period admit air, are filled with 

 marrow, which first of all becomes gradually absorbed. Those bones 

 in the birds' skeleton which convey air differ at the first glance from 

 the rest by their greater whiteness, and more compact cancellated 

 structure ; they contain also more earthy constituents. 



The Cranium of Birds is peculiar in the early union of its sepa- 

 rate bones and the complete obliteration of their connecting sutures, 

 so that a complete bony case is formed, which encloses the brain, 

 and leaves free only the occipital foramen and the openings for the 

 exit of the nerves. In very young birds, however, the several 

 bones and their sutures may be distinguished. The occipital bone 

 consists, as in most Amphibia, of four primary elements, the body, 

 the two condyloid, and the superior occipital. The articulating 

 condyle consists of a single round tubercle. The foramen magnum 

 is sometimes perpepdicular, as in the Gallinse, Grallae, and Palmi- 

 pedes, e. g. the Goose, but more frequently horizontal, in which 

 case the posterior part of the skull projects in a vaulted form be- 

 hind it, as in the Rapaces and Passeres, the Woodpeckers, and 

 many of the Grallatorial birds. In the Snipe especially it lies very 



