474 O F O L p A G E 



like the herbaceous tubes above defcribed. This 

 tube is Ihut at both ends by a pulpy fubftance, 

 and its external and internal lurfaces are cover- 

 ed with two membranes compofed of feveral 

 layers of foft and dud:ile fibres. In proportion 

 as this tube receives nutritious juices, the two 

 extremities extend and recede from the middle 

 point, which always preferves the fame ftation. 

 The extremities cannot extend without reading: 

 againft this middle point ; and the parts which 

 are neareft it begin firft to acquire folidity. The 

 iirft bony plate, like the firft plate of wood, is 

 produced in the interval which feparates the 

 two membranes or perioftea. But the offifica- 

 tibn commences in the middle, and gradually 

 extends to the extremities, which remain foft 

 long after the middle parts are converted into 

 bone. The middle parts of bones, therefore, 

 being firft oiTified, it is impoffible that they 

 fhould afterwards expand equally with thofe 

 parts which remain longer in a foft and dudile 

 ftate. This is the reafon why bones are always 

 thinneft in the middle, and thickeft at the ex- 

 tremities. But, independent of this difference 

 between the longitudinal growth of bones and 

 of wood, the analogy between their increafe in 

 thicknefs is very ftriking : For the firft bony 

 plate is produced from the internal part of the 

 periofteum ; and, after the formation of this 

 plate between the two perioftea, two other plates 

 are foon formed, one on each fide of the firil, 



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