OSSIFICATION IN CARTILAGE. 279 



the periosteum into the bone, pierce these circumferential lamellae, but are not at 

 first surrounded with concentric lamellae, and do not therefore lie in true Haversian 

 canals. 1 These canals become formed later by absorption taking place around 

 the blood-vessels for some little distance, succeeded by a re-deposition of concentric 

 lamellae within the Haversian spaces thus formed. 



Immediately before the occurrence of the lamellar deposition under the 

 periosteum, the young bone undergoes a process of absorption from the inside. 

 The medullary canal becomes thereby enlarged, and the medullary spaces, 

 particularly those near the medullary canal, partaking of this absorption and 

 enlargement, the result is that at about this period there is less bony matter in 

 a section of the shaft than there was immediately before. To the change in 

 question Schwalbe has given the name " osteoporosis ; " it is followed by a re- 

 deposition of osseous lamellae both on the wall of the medullary canal, and on the 

 walls of the medullary spaces of the embryonic osseous tissue. 



Since the cartilage grows in every dimension by interstitial expansion, the bone 

 which is invading it (endochondral bone) becomes gradually wider as the ossification 

 advances. It is narrowest near the centre of the shaft where the process began, and 

 widens gradually towards the ends ; it has therefore somewhat of an hour-glass shape 

 (fig. 313, i.e.). The cylindrical form of the shaft is maintained, however, by the thick- 

 ness of the periosteal bone being greater at the centre (where the deposition of bone 

 first began, and has been longest proceeding) than at the ends. Hence it gradually 

 diminishes to a thin layer immediately investing that part of the cartilage into 

 which the calcification is extending, so that the intramembranous subperiosteal 

 ossification on the outside, may be said to closely accompany, if it does not even 

 precede,' the calcification of the cartilage in the interior. Either this investment of 

 periosteal bone, or the calcification of the cartilage, seems to hinder the lateral 

 expansion of that part of the cartilage in which the calcification is proceeding ; but 

 immediately beyond, the interstitial expansion mentioned takes place. By the time 

 that the ossification has advanced to the extremities of the shaft, the enlarged and 

 expanded end of the cartilage has extended itself over the subperiosteal layer, so 

 that this, with the accompanying osteoblastic tissue, now seems to lie in a groove 

 or notch (fig. 313, p) in the cartilaginous head of the bone (Ranvier). This groove 

 is filled with the same tissue as that which underlies the rest of the periosteum, 

 namely, a vascular tissue with branched cells and osteoblasts and osteogenic fibres. 

 The latter are prolonged from the periosteal bone, and have for the most part a 

 longitudinal direction (fig. 319). The tissue which fills this periosteal notch 

 appears to become gradually converted by a metaplastic process into cartilage in 

 the same way as the superficial part of a rib cartilage is formed by conversion of 

 the deeper layers of the perichondrium (see p. 253). Thus, besides the interstitial 

 growth and expansion of the cartilaginous end, there is a constant new formation 

 of cartilage going on at its surface. 



Blood-vessels extend from the newly-formed osseous tissue beyond it into the 

 cartilage. The vessels are lodged in excavations or branching canals in the 

 cartilage (fig. 313), which also contain granular corpuscles (? osteoblasts). Other 

 vascular canals enter the cartilage from its outer surface, and conduct vessels into it 

 directly from the perichondrium. 



The formation of osseous tissue, having thus proceeded for some time in the 

 shaft, ;at length begins in the extremities of the bone from one or more independent 

 centres, and extends through the cartilage, leaving, however, a thick superficial layer 

 of it unossified, which permanently covers the articular ends of the bone. The 



1 Even in the adult bone it may often be noticed that the blood-vessels which pierce the superficial 

 lamellae are not enclosed by Haversian systems (see pp. 257 8). 



