CARTILAGE AND BONE 63 



by some modern authors. A priori, it seems to be in the highest 

 degree improbable that such highly differentiated cells as those of 

 cartilage should be able to so entirely change their form and 

 function. All analogy would lead us to suppose that, as a rule, 

 highly differentiated tissues become very limited in their powers of 

 growth, and can only produce more tissue of a nature similar to 

 their own. Moreover, there is much evidence against this theory 

 of conversion. 



More than a century and a half ago, Nesbitt (1736) denied, on 

 general grounds, the genetic connection between bone and cartilage. 

 Describing two species of ossification yielding the same bony 

 substance, one in membrane and the other in cartilage, he pointed 

 out that in the latter case the connection between the bone and the 

 cartilage is loose. It was not till the middle of the last century 

 that Nesbitt's speculations were placed on a sound basis of 

 observation by Sharpey, the founder of the modern theory of 

 the development of bone in cartilage by substitution. Sharpey 

 showed that membrane-bones are developed in connective tissue 

 without being preceded by cartilage ; that, in the case of cartilage- 

 bones, the outer layers may be deposited beneath the periosteum 

 without the intervention of cartilage ; and finally, that, even in the 

 case of true ' emlochondral ' ossification, the bone is not formed by 

 the conversion of previously existing cartilage, but by the invasion, 

 so to speak, of bone-forming tissue from without. Blood-vessels 

 penetrate into the cartilage, the cartilage-cells become peculiarly 

 modified, the original matrix becomes calcified, then destroyed and 

 absorbed, and finally it is replaced by a new deposit secreted by 

 cells brought in with the vessels (Fig. 43). 



These views of Sharpey, now almost universally adopted and 

 much extended, met at first with g7-eat opposition. The triumph 

 of the new theory of ' substitution ' abroad was greatly 

 helped by the careful researches of H. Miiller in 1858, and of 

 Gegenbaur in 1864. It is now established that through the 

 work of active immigrating cells, which accompany the fine blood- 

 vessels, channels are eaten away or dissolved in the cartilage 

 matrix. Other immigrating cells, called 'osteoblasts' by Gegen- 

 baur, deposit the new matrix or lamellae of ossein on the walls of 

 the cavities so formed. The osteoblasts become surrounded by 

 their own secretion, and so converted into bone-cells united to each 

 other only by fine threads of protoplasm (Fig. 43). The typical 

 Haversian systems are not well marked at first, but generally 

 become developed during the latter stages of growth of the bone. 



Three varieties of bone can be distinguished : the endochondral, 

 of which the development has just been noticed ; the sub-periosteal, 

 or perichondral ; and the membrane bone. To the first variety of 

 ossification is due very much less of the adult skeleton than was at 



