2 BIRDS. 



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 egg- 

 shell, and forcing its passage into liberty. Some 

 species, as those of the gallinaceous and swim- 

 ming 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 im- 

 portant 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 sus- 

 pension in so subtle a medium, and to rapid 

 motion through it. The skeleton unites light- 

 ness with firmness ; the great bones of the limbs, 

 and many of those of the body, are hollow reser- 

 voirs of air communicating with the lungs. Va- 

 rious 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 communi- 

 cate with each other, and with the lungs. These 



