54 SKELETON. 



under different circumstances in the three species of bones. The 

 compact has an unusual thickness in the bodies or diaphyses of 

 the long bones, and is accumulated in quantities particularly 

 great in their middle, which, from its position, is more exposed 

 than their extremities, to fracture from falls, blows, and violent 

 muscular efforts. But as this texture approaches the extremi- 

 ties of the bones it is reduced to a very thin lamina, merely suf- 

 ficient to enclose the cellular structure and to furnish a smooth 

 articular face for the joints. The cellular structure, on the con- 

 trary, in the long bones, is most abundant in their extremities, 

 constituting their bulk there, and is least so in their bodies. It 

 is so scattered at the latter place as to leave a cylindrical canal 

 in their middle, almost uninterrupted for some inches. This 

 canal, cellular in its periphery, has its more interior parts tra- 

 versed in every direction by an extremely delicate filamentous 

 bony matter, which, from the fineness of its threads and the 

 wide intervals between them, has been, not unaptly, compared 

 to the meshes of a net, and is, therefore, spoken of especially 

 under the name of the reticulated structure or tissue of the bones, 

 in contra-distinction to the cellular. It is formed on the same 

 principle with the latter; and though the term, from that circum- 

 stance, has been rejected, upon high authority, as superfluous, it 

 appears worthy of retention, as it expresses a fact of some im- 

 portance, Too weak to contribute in an appreciable degree to 

 the strength of the bone, the reticulated tissue seems principally 

 useful in supporting the marrow and in giving attachinent to its 

 membrane. The extremities of this cylindrical canal gradually 

 disappear by becoming more and more cellular. 



In the fiat bones, the compact structure forms only their sur* 

 face or periphery, and is of inconsiderable but generally uniform 

 thickness; the space within is filled up with the cellular struc? 

 ture, which is rather more laminated than it is in the long 

 bones. 



In the thick or round bones, the compact structure forms their 

 periphery also; but, generally, it is thinner than in the flat: their 

 interior is likewise filled up by the cellular structure, and does 

 not present differences of importance, from the ends of the long 

 bones. 



A simple experiment on any of the cylindrical bones will prove 

 that the tumefaction of their extremities does not add proporr 



