472 OFOLDAGE 



confidercd, in their original ftate, as fmall tubes 

 lined, both vvithm and without, with a thin 

 membrane : This double membrane furnifhes 

 the oiTeous matter ; for the fmall interval be- 

 tween the internal and external neriofleum is 

 foon converted into a bony plate. Some idea 

 of the produdlion and growth of bones may be 

 formed, by comparing them with the manner 

 in which wood and the more folid parts of ve- 

 getables are produced. We fhall take, for ex- 

 ample, the fig-tree or the alder, which are at 

 firft hollow in the middle, like the thigh and 

 other hollow bones of the body. When a bud, 

 that is to form a branch, begins to extend, it is 

 only a foft dudlile matter, which, by extenfion, 

 becomes a {lender herbaceous tube filled with 

 pith.' The external and internal furfaces of 

 this tube are covered with a fibrous membrane, 

 as well as the internal partitions by which the 

 cavity is divided. Thefe membranes, however 

 ihin, are compofed of feveral plates of fibres 

 lying above each other, which are flill foft, 

 but gradually harden by depofiting the fap 

 which they abforb for their nourifhment ; and 

 by this means a woody plate is formed, du- 

 ring the firll: year, between the tv/o membranes, 

 which is more or lefs thick in proportion to the 

 quantity of fap that has been depolited between 

 the external and internal membranes. But, 

 though each of thefe membranes become woody 

 internally, their external furfaces remain foft 



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