556 THE MECHANICAL FUNCTIONS. 



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 advantage derived 

 from their being disposed in the circumference 

 rather than in central masses, has been obtained 

 to the utmost extent. The horny material, of 

 which the stems of the feathers are constructed, 

 are, in like manner, formed into hollow cylin- 

 ders, 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 skeleton 

 is rendered remarkably light : that of the Peli- 

 caims onocrotalus, for instance, or white Pelican, 

 which is five feet in length, was found by the 

 Parisian Academicians to weigh only twenty- 

 three ounces, while the entire bird weighed 

 nearly twenty-five pounds. The cavities in the 

 bones communicate with large air cells, which 

 are distributed in various parts of the body, and 



* Ossification not only proceeds more rapidly, but 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 ossified. 



f In the bat there is no provision of this kind for lightening 

 the bones, and we find them containing marrow, as in other 

 mammalia, and not air. 



