CONNECTIVE TISSUE. 



237 



times has been : How does bone arise from cartilage ? A satis- 

 factory answer to this question was impossible so long as the 

 minute structure of cartilage was unknown, and, indeed, a full 

 understanding of the process of ossification is of a very recent 

 date. 



Through the researches of Rathke, Reichert, Kolliker, and 

 others, we know that there are bones which do not develop from 

 cartilage, but from fibrous connective tissue, formerly thought 

 to be a u blastema.' 7 All bones of the skeleton arise from pre- 

 existing cartilage, except the flat skull-bones viz. : the squam- 

 ous portion of the occipital and temporal bones, the parietal, 

 frontal, and portions of the sphenoid bone, and the nasal, 

 lachrymal, vomer ? malar, palatine, and upper maxillary bones. 

 The clavicle was in former times thought to be destitute of a 

 cartilaginous basis, but recently it was found to be cartilaginous, 

 at least at its extremities (M. Kassowitz). There is, however, a 

 great similarity between the formation of the so-called " carti- 

 laginous" bones and that of bones termed " covering. " In all 

 cartilaginous bones the formation of bone proceeds simultane- 

 ously both from the cartilage and from the perichondrium, the 

 fibrous investing membrane. 



The ossification of cartilage was first studied. In former 

 times it was believed that, by the deposition of lime-salts, the 

 cartilage was directly transformed into bone the "cartilage 

 cells" directly converted into "bone-cells." Observers were 

 much puzzled over the formation of the u canaliculi," and Kol- 

 liker, in 1852, imagined that he had settled the matter by assum- 

 ing that the cartilage-cells were transformed into bone-cells 

 by a thickening of their walls, with a simultaneous formation of 

 canaliculi, similar to the pore-canals of "wood-cells" of plants. 



A new era was inaugurated in 1858 by H. Miiller, * who, after 

 very careful researches, came to the conclusion that the cartilage 

 first breaks down into medullary tissue, and from this tissue bone 

 is developed. H. Miiller, however, admitted that a direct ossifi- 

 cation of cartilage (metaplasia of authors) may also occur. 

 Although he was ignorant of the fact that the whole medullary 

 tissue giving rise to bone is an offspring of cartilage, the great 

 merits of this accurate observer must always be recognized. 

 To-day we know that a direct transformation of cartilage or fibrous 



* " Ueber die Entwicklung der Knochensubstanz, nebst Bemerkungen 

 iiber den Ban rachitischer Kiiochen." Zeitschr. f. Wissensch. Zoologie. 

 Bd. ix. 



