260 THE MECHANICAL FUNCTIONS. 



composition: for while cartilage isjormed almost wholly of 

 albumen, the animal basis of bone is almost entirely resol- 

 vable into gelatin. 



Thus may a bone be analyzed into its two constituent 

 parts: by the process first described we obtain its earth de- 

 prived of its animal constituent; by the second, we obtain 

 its membranous basis free from earth. The first of these 

 gives it hardness; the second, tenacity: and thus, by the in- 

 timate combination of these elements, two qualities, which, 

 in masses of homogeneous and unorganized matter, are 

 scarcely compatible with one another, are skilfully united. 



The mechanical structure of bone is no less worthy of ad- 

 miration, as evincing the skill with which every part is 

 adapted to its destined uses. The animal membrane, which, 

 as we have seen, is the bed in which the calcareous phos- 

 phate is deposited, partakes of the reticular structure belong- 

 ino- to the ordinary cellular texture; and a bone, when mi- 

 nutely examined, exhibits also the same appearance of plates 

 intermixed vvith fibres. In the outer compact portion, in-^ 

 deed, the fibrous arrangement of the particles is not so easi- 

 ly distinguished: but it may be detected in young bones 

 while they are becoming ossified: and also in bones that 

 have been long exposed to the weather, or long macerated 

 in water. The interior of most bones, in the higher classes 

 of animals, presents distinctly the appearance of irregular 

 cavities, resulting from the partial separation of the plates, 

 and their mutual crossings, and fibrous connexions. 



The different mechanical purposes for which bones are 

 employed in the animal economy require them to be of dif- 

 ferent forms. Where a part is intended to have compact- 

 ness and strength, with a very limited degree of motion, it 

 is divided into a great number of small pieces, united toge- 

 ther by ligaments, and the separate bones are short and com- 

 pressed, approaching more or less to a cubical shape. Of 

 such is the column of the spine composed, as also the joints 

 of the wrist and ankle. Where the principal object is either 

 extensive protection, or the provision of broad surfaces for 

 the attachment of muscles, we find the osseous structure ex- 



