Page:Natural History, Birds.djvu/15

2 it is ready for exclusion; when, by means of a horny, pointed scale, attached temporarily to the tip of its beak, it succeeds in breaking the eggshell, and forcing its passage into liberty. Some species, as those of the gallinaceous and swimming tribes, are able to run about, and pick up their own food, immediately after their escape from the egg; but more commonly the newly-excluded young are callow and helpless, for several days, unable to quit the nest; and are assiduously fed and tended by the parent birds, with a care which has become proverbial.

In the general structure of their skeleton, Birds agree with Quadrupeds, but in many important particulars it is modified to suit widely different habits, and another sphere of action. Birds are flying animals: their sphere of activity is the air; and their whole organization (we speak not of exceptions) is adapted to a continued suspension in so subtle a medium, and to rapid motion through it. The skeleton unites lightness with firmness; the great bones of the limbs, and many of those of the body, are hollow reservoirs of air communicating with the lungs. Various irregular membranous sacs which can be filled with air, are also distributed about the body; some internally, others between the muscles and the skin, down the throat and chest, or along the tendons of the shoulder: and these communicate with each other, and with the lungs. These