GROWTH OF BOXE 103 



probably derived by descent from similar cells equally abundant beneath the 

 ])criosteum. Tlie cells or coriJuscles in question, in whatever way produced, are 

 disposed in a layer or layers upon the walls of the secondary or medullary spaces, 

 in immediate contact with the new osteogenic deposit, which here, as in the 

 intramembranous ossification, they probably produce (figs. CO, 02). Here too the 

 osteogenic substance is finely reticular, and retains that character when calcified ; 

 for the secondaiy bony deposit is fonned in layers made up of finely reticulating 

 fibres, like the lamellaiof perfect bone shown in fig. 47. On a careful inspection, 

 and with a certain adjustment of the light, fine striajmay be seen in many parts 

 indicating the obliquely decussating filjres of the newly formed laminaj. The 

 structure in some measure reminds us of the secondary dej^osit inside the oblong 

 cells of the wood of coniferous trees, in which the ligneous matter is aiTanged in 

 fibres, or rather in fine lines, nmning obliquely round the wall of the cell and 

 crossing one another in alternate laj'ers. 



The lacunjB are fonned, as described in the intramembranous ossification, by 

 some of the granular coiiiuscles becoming embedded in the osteogenic substance, 

 and inclosed in a cavity fonned round them by its further deposit (fig. (J2.(l). Lacunas 

 fonned from cartilage-cells exist but very- scantily. Examples occur in articular 

 cartilage, and in that of the pubic symphj-sis. when, as commonly happens in 

 mature life, the part of these tissues adjoining the lione is encroachetl on by a 

 species of ossification, as noticed at page '.»7. The ossifying process in this case 

 is mere calcification of the cartilage, and stellate lacuna;, not intercommunicating 

 ))y canaliculi, remain in the partially os.sified cells. "When this hard tissue 

 is decalcified by an acid, the original cella and cartilaginous matrix become 

 apparent. 



As ossification advances towards the ends of the bone, the portion as 

 yet cartilaginous continues to <i;vo\\ at the same time, and increases in 

 every dimension. Tlie part already osseous increases also in circum- 

 ference ; the medullary canal, of -which for some time there is no 

 appearance, begins to be excavated in the interior by absorption, and 

 the sides of the shaft acquire compactness and solidity. The increase 

 in girth is brought about by deposition of bone at the surface under- 

 neath the periosteum. It was at one time supposed that a formation 

 of cartilage precedes the bone also in this situation ; but such is not 

 the case, for the vascular soft tissue in immediate contact with the 

 surface of the growing bone is not cartilage, but a soft substance con- 

 taining fibres and osteoblasts ; in fact, the increase takes place by intra- 

 membranous ossification, and accordingly the Haversian canals of the 

 shaft are formed in the same way as those of the tabular bones of the 

 skull, — that is, the osseous matter is not only laid on in strata parallel 

 to the surface of the bone, but is deposited around processes of the 

 vascular membranous tissue which extend from the surface obliquely 

 into the substance of the shaft (fig. 62) ; and the canals in which 

 these vascular processes lie, becoming narrowed by the deposition of 

 concentric osseous lamina, eventually remain as the Haversian canals. 



Ossification having thus proceeded for some time in the shaft, at 

 length begins in the extremities of the bone from one or more inde- 

 pendent centres, and extends through the cartilage, leaving, however, 

 a thick superficial layer of it unossified, which permanently covers the 

 articular end of the bone. The epiphyses thus formed continue long 

 separated from the shaft or diaphysis by an intervening portion of 

 cartilage, which is at last ossified, and the bone is then consolidated. 



Growth and absorption of Bone. — The time of final junction 

 of the epiphyses is ditfereut in different bones ; in many it does 

 not arrive until the body has reached its full stature. Meanwhile 



