DEVELOPMENT OP BONE 



279 



out that these layers are formed, not (as usually stated) in a cartilaginous 

 matrix, but in the substance of a membrane that intervenes between the proper 

 periosteum and the surface of the bone, consisting of fibres and granular cells, 

 and exactly resembling that in which the flat bones of the roof of the skull are 

 developed (Fig. 64). The basis of this sub-periosteal tissue has been shown by 



Fig. 71. 



Fig. 72. 



Ossification of foetal humerus, Subperiosteal layer from the extremity of the bony 



natural size, the upper half di- shaft of the ossifying tibia. The cartilage and more open 



Tided longitudinally; a, cartilage, bony tissue, have been scraped off from the inside of the 



with vascular canals ; fc, termina- crust except at a, where a dark shade indicates a few 



tion of bony deposit in the shaft. vertical osseous areolae out of focus and indistinctly seen. 



The part a, &, of the crust is ossified, between 6 and c are 

 the clear reticulated fibres into which the earthy deposit 

 is advancing. Magnified 150 diameters. 



Scherer not to be chondrin, but glutin ; consequently it has no title whatever to 

 the designation of cartilage, which some have applied to it. The Haversian 

 canals, too, of these new layers, are formed in the same manner as those of the 

 tabular bones of the skull ; the osseous matter being not only laid on in strata 

 parallel to the surface, but also being deposited around processes of the vascular 

 membranous tissue, which extend obliquely from the surface into the substance 

 of the shaft ; the channels, in which these membranous processes lie, becoming 

 narrowed by the deposition of concentric osseous laminae, and at last remaining 

 as the Haversian canals. Whilst this new deposition is taking place on the 

 exterior of the shaft, absorption of the inner and older layers goes on ; so that 

 the central cavity is proportionably enlarged. The increase of the bone in length 

 appears due to the growth of the cartilage between the shaft and the epiphyses, 

 so long as this remains unconsolidated by ossific deposit ; and this state continues, 

 until the bone has acquired nearly its full dimensions. What further increase 

 it gains, seems chiefly if not entirely due to the progressive ossification of the 

 articular cartilage covering the extremities; which progressively diminishes in 

 thickness during the whole of life, and which in old age sometimes appears to 

 have been almost completely converted into bone. 



