144 BIRD GALLERY. 
the Gannets and Screamers, these sacs also penetrate between the muscles 
and beneath the skin. The bones which contain these air-sacs are 
hollow or pneumatic and consequently have no marrow. In the Albatroses, 
Gannets and Pelicans, which possess great powers of flight, almost every 
bone in the body becomes pneumatic, but the Swifts and Swallows, which 
possess equal powers, have the long bones filled with marrow, as is also 
the case in Penguins, Grebes, Divers, and the smaller Petrels. 
Skeleton [Plate XXV. figs. 5-7].—As regards the details of the 
structure of the Birds’ skeleton, we can only refer here to those points 
which are either generally characteristic of the Class or which are 
strikingly correlated to the peculiarities of their life. The bones of the 
cranium (Pl. XXV. fig. 51) become united (coalesce) early in life, 

Head of a Falcon (HMierofalco islandus) to show (1) impervious nostrils, 
and (2) tooth-like process of the bill. 
Fig. 9: 

Head of the Black Turkey-Vulture (Catharistes urubu) to show (1) pervious nostrils. 
about the period when growth ceases, so that the sutures between the 
bones, which are persistent for so long a period in the Mammalian and 
also in the Reptilian skull, disappear entirely. As in Reptiles, the skull 
is joined with the neck by means of a single hinge or condyle (fig. 61), 
the orbits are of very large size in accordance with the great development 
of the eye (fig. 54). The facial bones are more or less prolonged and 
united to form the beak, which is covered with a horny sheath, the edges 
of which may be notched (Barbets and Falcons) (fig. 82) or serrated 
(Mergansers), but teeth are invariably absent in living forms. The 
external nostrils are either pervious (fig. 91) or separated from one 
