256 MANUAL OF HISTOLOGY. 



substance, and the consequent acquisition of new spaces for the growing 

 bony tissue, which lays down additional layers of progressively increas- 

 ing thickness, at the same time that the canaliculi are being more and 

 more developed, the new osseous substance takes the place of the pre- 

 existing cartilage very extensively. That a residue of the original calci- 

 ned tissue may persist in the interior of fully-matured bone appears 

 certain, although at present we know nothing definite as to the extent 

 to which it may do so (Tomes and de Morgan, Mutter}. 



That a rapid and extensive re-absorption takes place in the formed bone 

 will be seen later on. But apart from this re-solution of calcified osteoid 

 tissue on a large scale, there is besides another hidden process in osseous 

 tissue, by which older portions of the latter are dissolved, and new masses 

 laid down in their place. This is first of all borne witness to by the 

 nature of the medullary cells in old spongy bone, as compared to those of 

 the same parts in younger individuals. And that there is an incessant 

 disappearance of material of the same kind at a late period has been 

 seen when discussing at 141, fig. 234, the formation of the Haversian 

 spaces, and their being re-lined subsequently by new lamella of bony sub- 

 stanoe. 



A direct transformation, however, of cartilage into osseous tissue does 

 likewise occur, though as a rare exception. In such cases we remark 

 certain jagged cartilage lacunae in the calcified tissue (fig. 247, Z>), which 

 have arisen from a peculiar mode of thickening of layers on the internal 

 surface of the capsules. Later on the granular calcification becomes 

 diffuse (c), the jagged processes of adjacent cells unite to form passages; 

 in short, bone corpuscles and canaliculi (c) are produced. In these the 

 cells lie in twos or threes. The frontal protuberance of calves and 

 tracheal rings of birds afford the best examples of transformations of this 

 kind (Gegenbaur). In rachitic bone, also, as has long been known, 

 isolated spots of this kind are to be found with the same transitions 

 going on in them. In the antlers of deer undergoing ossification similar 

 changes probably take place to a more marked extent. 



148. 



There still remains for our consideration the formation of osseous 

 tissue in parts of the body where cartilage is not previously laid down, 

 to be again dissolved in order to make way for the former. Under this 

 head we shall have to discuss, first, the origin of bone /row the periosteum, 

 and again the ossification of the so-called secondary bones. 



The first of these, a process very extensively met with, and in its 

 beginnings frequently preceding the ossification which takes place in 

 cartilage, is the source of the increase in thickness of bones. 



Holding still to the example of the cylindrical bones, we know by ex- 

 perience that the latter increase with the growth of the body, not only 

 in length, but also considerably in thickness. The increase in length, 

 we may here mention, is -a continuation of the process treated of in the 

 foregoing section : it takes place, namely, at the expense of the epiphysis 

 and articular cartilages, whose deeper portions become calcified and then 

 dissolved to make room for the advance of osseous substance. During 

 this time the cartilage also grows upwards by division of its cells and 

 accumulation of its ground substance. The increase in thickness takes 

 place in the following way : New layers of bone are formed under the 

 clothing periosteum, which envelope the mass within in a series of tubes. 



