DEVELOPMENT OF BONE. 987 



the soft ossifying blastema and its contained cells. Hence the explanation of 

 the concentric laminae of the perfect bone. The indistinctly fibrous matrix of 

 the ossifying blastema also becomes saturated with coalescing osseous gran- 

 ules, so as to appear perfectly transparent and homogeneous, instead of opaque 

 and granular, like the primary bone. The entangled nucleated cells, the walls 

 of which also become calcified, are believed to be converted into stellate cavi- 

 ties, by the partial intrusion of the calcifying process into them, and so to 

 form the greater part of, or all, the bone lacunae, whilst their radiating 

 branches, multiplying, become very fine, and joining those of adjacent cells, 

 form the eanalicufi. The primary opaque granular bone contains no lacunas. 

 Some of the areolee, or spots of solution, absorption, and new deposit, reduced 

 in size by the successive formation of concentric laminae of secondary bone 

 around them, ultimately constitute certain of the Haversian canals ; whilst 

 others, even after they are lined by secondary bone, continue to widen by 

 further solution and absorption of the surrounding bone, whether primary or 

 secondary, and so form the larger nutrient foramina and canals, the small and 

 large cancelli, and even the chief medullary cavity of a long bone. Indeed, 

 so long as the bones themselves continue to grow, and even, but less evidently, 

 throughout life, such processes of solution, absorption, and re-deposit are re- 

 peated, in the secondary bone, over and over again, as it is modified, by age, 

 in size, form, and structure. 



In the intra-membranous mode of origin of bone, there first appears, in con- 

 nection with some previously developed fibrous membrane or precursory mem- 

 brane, which afterwards forms the fibrous periosteum, a soft matrix, or blas- 

 tema, which contains largish nucleated granular corpuscles, and soon puts on 

 an indistinctly fibrous aspect ; it resembles, indeed, imperfectly developed 

 fibrous connective tissue. This fibrous matrix and the walls of its included 

 cells become opaque by granular calcification. This occurring in successive 

 strata forms the bone tissue ; whilst the cavities of the cells, becoming stel- 

 late, with fine radiating and communicating branches, constitute the lacunce 

 and canaliculi. This process of calcification occurs from the first in a reticu- 

 lar manner. The fine spicula of the network increase in thickness, by the 

 formation around them of a soft transparent osteogenic substance (Mailer), 

 which ultimately becomes bone. The granular corpuscles, or osteoblasts, are 

 embedded in the new-formed bone. The interspaces, meshes, or areolse, form 

 softer spots, the constituent cells of which develop into blood-cells, bloodves- 

 sels, and areolar tissue, and, after changes in their parietes, such as those 

 already described in speaking of the secondary bone tissue of bones preceded 

 by precursory cartilage and primary bone, constitute the canals of Havers, 

 the larger vascular foramina and canals, and the cancelli of the cancellated 

 bony tissue. 



Although, therefore, the intra-cartilaginous and intra-membranous modes 

 of formation of bone differ from each other, the development of the secondary 

 bone, which goes on, pari passu, with the absorption of the primary cartilage- 

 formed bone, is analogous to the intra-membranous mode of ossification. 

 The soft ossifying blastema, in the one case, and the precursory membrane, in 

 the other, alike resemble an imperfectly formed fibrous connective tissue, and 

 undergo similar changes. Chemical considerations also support this analogy, 

 for, on removing the earthy matter from primary bone, its organic substance 

 yields chondrin, whilst the animal basis of the secondary bone, and of the 

 intra-membranous-formed bone, gives gelatin on being boiled. 



The intra-membranous mode of ossification is solely concerned in the form- 

 ation of the bones of the face, and of those of the upper part and sides of the 

 cranium. The upper and lower jaw bones, the palate bones and vomer, the 

 malar, nasal, and lachrymal bones, are included in this category ; so also are 

 the frontals and parietals, the squamous part of the temporals, the part of the 

 occipital above the occipital eminence, and certain slight portions of the sphe- 

 noid. In the case of some of the cranial bones thus formed, the precursory 

 membrane is developed outside a corresponding primordial cartilage, which is 

 said to be afterwards absorbed. The cranial bones begin by one or more flat 

 radiating centres of ossification, which spread out as the membrane advances, 

 and go on increasing in thickness, until, at length, they meet at the different 



