OSSIFICATION IX CARTILAGE. 97 



bone itself, but must not be confounded with fibres which may pre-exist 

 in the membranous tissue in which the bone is growing. 



The granular corpuscles or cells everywhere cover, in a dense layer, 

 the osteogenic substance, and lie in its meshes ; most probably they 

 yield or excrete that substance ; hence the name " osteoblasts" has been 

 assigned to them. 



But some of the granular cells are involved in the ossifying matrix, 

 and eventually inclosed in lacunte. Single cells may accordingly be 

 seen partially sunk in the recent osteogenic deposit, which then gradu- 

 ally grows over them and buries them in its substance ; and the cavity 

 in which the corpuscle is thus enclosed becomes a lacuna. 



With regard to the formation of the canaliculi. some obseiTers state that, when 

 such a coi-puscle is as yet but half sunk in the growing substance, processes may 

 be seen passing from the imbedded side into fine clefts of the matrix, which 

 close in around them and become the canaliculi : and that as the inclosui'e of the 

 corpuscle is completed, canaliculi are in like manner fonned in the rest of its 

 circumference. It is also supposed that the canaliculi are afterwards extended by 

 absoi"ption, so as to anastomose with those of neighboui'ing lacuna?. 



As the bone extends in circumference, it also increases in thickness ; 

 the vacuities between the bony spicula become narrowed or disappear, 

 and at a more advanced period the tabular bones of the cranium are 

 tolerably compact towards the centre, although their edges are still 

 formed of slender radiating processes. At tliis time also numerous 

 furrows are grooved on the surfiicc of the bone in a similar radiating 

 manner, and towards the centre these are continued into complete tubes 

 or canals in the older and denser part, which run in the same direction. 

 The canals, as well as the grooves, which become converted into canals, 

 contain blood-vessels supported by processes of the investing membrane, 

 and are lined with osteoblasts, which deposit concentric layers of bone 

 inside these channels ; and, when thus surrounded with concentric 

 laminre, these tubular cavities arc in fact Haversian canals. 



It may here be observed that in earlier stages, such as that sho'mi in fig. .54, 

 vessels may be seen in the soft tissue, some twice or thi'ee times the size of a 

 blood-capillary, others considerably more, but all Mrith only a homogeneous coat 

 with cells upon it here and there, and without a muscular layer. 



Ossification in cartilage. — It has already been stated that, in by 

 far the greater number of bones, the primitive soft cellular matter of 

 which they originally consist is very quickly succeeded by cartilage, in 

 which the ossification begins. One of the long bones taken from a very 

 small embryo, just before ossification has commenced in it, is observed 

 to be distinctly cartilaginous. In the tibia of a sheep, for example, at 

 a time when the whole embryo is not more than an inch and a quarter 

 in length, we can plainly see that the substance consists of cartilage- 

 cells imbedded in a pellucid matrix. These cells, which can scarcely 

 be said to be collected into groups, are much larger in the middle part 

 of the shaft where ossification afterwards commences, and there also 

 they are mostly placed with their long diameter across the direction of 

 the bone : towards the ends they are much smaller and closer together, 

 and the cartilage there is less transparent. As it enlarges, the cartilage 

 acquires firmer consistence ; it represents in figure the future bone, 

 though of course much smaller in size, and it is surrounded with a 



VOL, II. II 



