DEVELOPMENT OF BONE. 275 



so many involutions of the external surface, allow the vessels to come into nearer 

 relation with the interior parts of the cartilaginous structure, than they would 

 otherwise do. They are especially developed at certain points, which are to be 

 the centres of the ossifying process ; and it is always observable, that the vascu- 

 larity is greatest at the zone in which the conversion of cartilage into bone is 

 actually taking place, and that the cartilage cells are there arranged in a some- 

 what radiating manner around them (Fig. 66). During the extension of the 

 vascular canals into the Cartilaginous matrix, certain changes are taking place 

 in the substance of the latter, which are preparatory to its conversion into Bone. 

 Instead of single isolated cells, or groups of two, three, or four, such as we have 

 seen to be characteristic of ordinary Cartilage (Fig. 51), we find, as we approach 

 the plane of ossification in a vertical section, clusters made up of a larger number 

 arranged in a linear manner (Figs. 67, 68) ; which seem to be formed by a con- 

 tinuance of the same multiplying process as that formerly described ( 104). 

 And when we pass still nearer, we see that these clusters are composed of a yet 

 greater number of cells, which are arranged in long rows, whose direction cor- 

 responds to the longitudinal axis of the bone ; these clusters are still separated 

 by intercellular substance ; and it is in this, that the ossific matter is first depo- 

 sited. If we separate the cartilaginous and the osseous substance at this stage 

 of the process, we find that the ends of the rows of cartilage cells are received 

 into deep, narrow cups of bone, formed by the calcification of the intercellular 

 substance between them (Fig. 69). Thus the Bone first formed in the cartila- 



Fig. 69. 



First Osseous network formed in the intercellular substance of Cartilage, around a vascular canal, as in Fig 66. 



ginous matrix, is seen to consist of a series of lamellae of a somewhat cylindrical 

 form ; inclosing oblong areolae, or short tubular cavities, within which the piles 

 of cartilage cells yet lie; and it thus corresponds closely with thereticular struc- 

 ture, which first makes its appearance in the intra-membranous form of the 

 process. So far, it would appear that the bloodvessels are not directly concerned 

 in the operation; for although they advance to the near neighborhood of the 

 first ossific deposit, they do not make their way into its substance, or even into 

 the intervening areolae. 



265. This state of things, however, speedily gives place to another. On 

 examining the subjacent portion, in which the ossification has advanced further, 

 it is found that the original closed cavities have coalesced to a certain extent 

 (probably by the absorption of their walls), both laterally and longitudinally 

 (Fig. 70) ; and that they now form a cancellated texture (Fig. 67, d), the areolse 

 of which (e, e) receive numerous bloodvessels, prolonged into them from the 

 previously-ossified portion. The groups of cartilage cells which originally occu- 



