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The numerous Class of vertebrated animals which we are about to consider, differs from that of the Mammalia, in that the young are not born alive, but are produced from eggs; which consist of a living point, attached to a globular sac of nutriment, called the yolk, surrounded by a layer of albumen, the glaire, and inclosed in two series of membrane, and a hard calcareous shell. For the development of the vital point into a living and active chick, it is needful that it should receive the stimulus of warmth; and this is, in general, supplied from the body of the parent bird, during the process of brooding or incubation; while it is retained, in most cases, by means of nests, in which the eggs are deposited, and which are composed of substances more or less calculated to resist the rapid abstraction of heat by the surrounding atmosphere. During the process of incubation, which lasts only a few weeks, the yolk is gradually absorbed into the body of the inclosed chick, forming its sustenance, until
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 newlyexcluded 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