BIRDS. 38 



rt 



extremity, though destined exclusively to support the wing, 

 retain the same divisions, and are composed of the usual 

 elements: and the general form of the body is that best cal- 

 culated to glide through the air with the least resistance. 

 As birds swallow their food entire, there is no necessity for 

 any part of the bulky apparatus of hard and solid teeth, large 

 muscles and heavy jaws which are required by most quad- 

 rupeds: hence the head admits of being greatly reduced in 

 its, dimensions; and the form of the beak, which is drawn to 

 a point, and cuts the opposing air, tends to facilitate the pro- 

 gress of the bird in its flight. 



In the conformation of the body, also, every circumstance 

 that could contribute to give it lightness has been sedulously 

 studied. The general size of birds is considerably smaller 

 than quadrupeds of corresponding habits. No where has 

 Nature attempted to endow a huge ponderous animal, like 

 the fabled Pegasus, with the power of flight. Great con- 

 densation has been given to the osseous substance,'^' in order 

 that the greatest degree of strength might be procured with 

 the same weight of solid materials; and the mechanical ad- 

 vantage derived from their being disposed in the circum- 

 ference rather than in central masses, has been obtained to 

 the utmost extent. The horny material, of which the stems 

 of the feathers are constructed, arc, in like manner, formed 

 into hollow cylinders, which, compared with their weight, 

 are exceedingly strong. A similar shape has been given to 

 the cylindrical bones, which are fashioned into tubes with 

 dense but thin sides: most of the other bones have likewise 

 been made hollow, and instead of their cavities being filled 

 with marrow, they contain only air.t Thus, the whole ske- 

 leton is rendered remarkably light: that of the Pelicanus 



* Ossification not only proceeds more rapidly, bat is also carried to a 

 greater extent in this class of animals than in any other; as a proof of which, 

 the tendons, especially those of the muscles of the legs, are frequently ossi- 

 fied. 



•j- In the bat there is no provision of this kind for lightening the boneSy 

 and we find them containing marrow, as in other mammalia, and not air. 



