258 Use of the Sutures in the Skulls of Animals. 

 the bones of the skull by jagged sutures. Hence it appears 

 that the bones of the body generally are increased in length 

 or extent, not by a uniform extension of the whole sub- 

 stance, but by an addition of bony matter in some particular 

 part. 



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

 dition to each end. This we might conclude would be the 

 case, from considering the part in which its ossification 

 commences: as this commences in a middle point and pro- 

 ceeds to each extremity, it is natural to suppose that its 

 growth still {loes on in the same direction, or continues at 

 the extremities. That this is the case we know, not by 

 reasoning alone, but by a direct experiment. Mr. Hunter 

 sunk two small pieces of lead in the middle of the tibia or 

 shin bone of a pig, and measured accurately the distance be- 

 tween them : on examining the animal some time after- 

 wards, it appeared, that though tJie bone had increased con- 

 siderably in length, the pieces of lead still remained at the 

 same distance from each other that they were before. From 

 this experiment w'c learn, that a cylindrical bone is not ex- 

 tended in its middle, but is lengthened by addition to its ex- 

 tremities, where the body of the bone is joined to its epi- 

 physis ; the chief intention of the epiphysis being to allow 

 the intervention of a vascular organ, which may convenient- 

 ly deposit bony materials, without interfering with the joint, 

 itself. 



As cylindrical bones arc lengthened at their extreme parts, 

 we are led by analogy to conclude, that the same general 

 plan is pursued in the extension of the flat bones of the 

 body': and although we have no direct experiment by which 

 this has been prt;ved, there are circumstances which leave 

 iittlc doubt but they are extended by addition to their edges. 

 Thus, to take the parietal bone as an example j as ossification 

 begins in .i central point and extends towards the circumfe- 

 rence, it is probable that, to the completion of the process, it 

 continues to go on in the same direction ; and the same 

 circumstance taking place in every bone of the cranium, it 

 is probable, th;.t even after the whole of the brain is incased 



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