276 BONE OR OSSEOUS TISSUE. 



their walls (which were at first formed only by the remains of the walls of the 

 primary areolse and therefore only by calcified cartilage-matrix) begin to be 

 thickened by the deposition of layers of new bone, and this deposition increases 

 gradually towards the middle of the shaft (compare fig. 317, c and d). The lacunae 

 first appear in this deposit, there are of course none in the calcified cartilage. 

 Moreover as layer after layer is deposited upon the walls of the medullary spaces 

 these become gradually narrowed into inter-communicating channels, which contain 

 little more than a blood-vessel and some jelly-like embryonic connective tissue (foetal 

 marrow) with a few osteoblasts applied to the bone. 



In the end, some of the enlarged cavities and their walls remain to form the 

 cancellated tissue, but much of this structure is afterwards removed by absorption, 

 to give place to the medullary canal of the shaft. In many of the cavities the 

 walls of the coalesced primary areolae may long be distinguished, like little arches, 

 forming by their union a sort of festooned outline, upon which the new bony 

 laminae are deposited (see fig. 318, and fig. 305, c). 



In some of the smaller bones it may happen that the calcified cartilage is 

 completely absorbed from the centre of the shaft before any new deposition of bone 

 takes place. This is the case in the phalanges (fig. 314). 



The primary osseous matter forming the original thin walls of the areolae, and 

 produced by calcification of the cartilaginous matrix, is decidedly granular, and 

 has a dark appearance ; the subsequent or secondary deposit on the other hand is 

 quite transparent, and of a uniform, homogeneous aspect. This secondary deposit 

 begins to cover the granular bone a very short distance below the surface of 

 ossification (see fig. 317), and, as already stated, increases in thickness further down. 



Close to the limit of advancing ossification, the blood-vessels terminate in capil- 

 lary loops (see figs. 313, 317), which are often somewhat dilated. It is supposed by 

 Ranvier that these vascular loops by their growth directly produce absorption of the 

 cartilage, but it is more probable that this is caused by the agency of some of the 

 cells which accompany the blood-vessels. The absorption of the walls of the 

 primary areofee (calcified cartilage-matrix) seems to be effected by certain large 

 cells (fig. 317, /, /) which from their function have been termed by Kolliker, 

 ostocJasts, and which are found wherever bone is being eaten away : we shall 

 return to them further on. The secondary bone which thickens the walls of the 

 medullary spaces is no doubt formed by the osteoblasts. 



"With regard to the destination of the cartilage-cells, two opposite views have 

 been taken by histologists. According to one, which was that adopted by H. 

 Miiller, and has received most adherence, the capsules are opened by absorption, and 

 the cells are converted, after undergoing division, into osteoblasts. According to 

 the other, the cartilage-cells themselves become removed by absorption, and take no 

 part, directly or indirectly, in the production of the secondary bone. The latter 

 view of the matter was taken by Loven, and it was also regarded by Sharpey as in 

 all probability the more correct. 



It is difficult to decide between these views. All that can be said is that the 

 line of demarcation between the cartilage-cells and the osteoblastic tissue is 

 exceedingly abrupt (fig. 317), and that the latter often, if not always, terminates 

 either by a dilated vascular loop, or it may be by a developing capillary filled with 

 blood corpuscles. Except that they are generally much shrunken and irregular in 

 form (at least after death or the action of reagents), the cartilage-cells show no 

 absorption and no distinct evidence of division, and it may be remarked that this 

 is also the case when, as sometimes happens, they have not disappeared before the 

 advancing subperiosteal tissue, but remain for a time still occupying an untouched 

 primary areola (see fig. 313, c). 



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



