USE OF THE SUTURES. ^^^ 



a general principle, tliat where two parts of owe bone are fepa- 

 rated from each other by an intervening cartilage, or tzvo 

 dirtindl bones merely by periofteum, at that part ofleous ma- 

 terials are added to increaie their length or extend their fuper- 

 fices. This we fiiall find takes place, whether the junftion 

 be effe6led by comparatively fmooih furfaces, as between the 

 body of" a bone and its epiphyfis ; or between the bones of 

 the fkull by jagged futures. Hence it appears that the bones 

 of the body generally are increafed in length or extent, not by 

 a uniform extenfion of the whole fubfiance, but by an ad- 

 tlition of bony matter in fome particular part. 



Thus the body of a cylindrical bone is lengthened by ad- Cylindrical 

 dilioiv to ,each end. This we might conclude would be the '^""ssarelergth;- 

 •cafe, from confidering the part in which its oflification com- ditions at^e«h 

 mences : as this commences in a middle point and proceeds end. 

 to each extremity, it is natural to fuppofe that its growth fiill 

 goes on in the fame dire61ion, or continues at the extremities. 

 ■That this is the cafe we know, not by reafoning alone, but 

 ■by a direCi experiment. Mr. Hunter funk two fmall pieces of 

 lead in the middle of the tibia, or fliin bone of a pig, and 

 tneafured accurately the difiance between them : on examining 

 the animal fome time afterwards, it appeared * that though the 

 bone had increafed coniiderably in lenglh, the pieces of lead 

 {\\\l remained at the fame diftance from each other that they 

 were before. From this experiment we learn, that a cylin- 

 drical bone is not extended in its middle, but is lengthened 

 by addition to its extremities, where the body of the bone is 

 joined to its epiphyfis; the chief intention of the epiphyfis 

 being to allow the intervention of a vafcular organ, which 

 may conveniently depofit bony materials, without interfering 

 with the joint itielf. 



As cylindrical bones are lengthened at their extreme parts, ^j^^ ^^^^ ^^^^ 

 we are led by analogy to conclude, that the fame general plan cefs appears to 

 is purfued in the extenfion of the flat bones of the body : and '/^ges^if flf/''" 

 although we have no dirc6t experiment by which this has been bones. 

 proved, there are circumflances which leave little doubt but 

 they are extended by addition to their edges. Thus to take 

 the parietal bone as an example ; as oflification begins in a 

 central point and extends towards the circumference, it, is 

 probable that to the completion of the procefs, it continues to ' 



go on in the fanrie direaion; and the fame circumflance taking 



place 



