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Mr. F. A. Dixey. 



[Nov. 25, 



ossification in these phalanges, the peculiar features of which have been 

 found, with slight modifications, to remain constant throughout the 

 whole series of Vertebrata. 



It will be remembered that all long bones are at an early stage pre- 

 formed in cartilage, and that simultaneously with the calcification of 

 the cartilaginous shaft a deposit of true bone takes place in the invest- 

 ing subperiosteal tissue ; so that at a certain stage in development the 

 so-called "primary " or cartilage bone of the shaft is surrounded by a 

 hollow cylinder of "secondary" or true bone. An irruption of the 

 osteoblastic subperiosteal tissue into the cartilage then takes place, 

 which results eventually in the absorption of the primary bone and 

 its replacement by true osseous tissue, to be in its turn alternately 

 absorbed and renewed so long as the process of growth goes on. 



Cartilage that is about to undergo calcification presents certain 

 characteristic appearances ; the cells with their cell-spaces become 

 larger, flatten out, and usually show a tendency to arrange themselves 

 in parallel rows, between which, if the change has already been in 

 progress for some time, the lines of calcification may be seen advancing. 

 But whereas in the long bones as a whole,* including the first and 

 second phalanges of the digits, the alteration of the cartilage cells, 

 followed by calcification of the matrix, appears first in the centre of the 

 shaft and spreads thence pari passu towards the two extremities ; in the 

 ungual phalanx it is first seen to arise in the tip or distal extremity of 

 the cartilage, from which point it spreads gradually backwards towards 

 the base of the phalanx (figs. 2, 3). Similarly the subperiosteal 

 deposit of membrane bone in all other cases begins as a thin and 

 narrow ring surrounding the shaft and placed midway between the 

 two extremities, that is to say, in direct relation with the spot where 

 the cartilage first begins to calcify ; in the ungual phalanx, however, 

 still preserving its relation with the point of commencing calcification 

 in the cartilage, the deposit of subperiosteal bone first appears as a thin 

 layer closely applied to the tip of the cartilage and fitting over it like 

 a cap. The ring or hollow cylinder of bone formed in ordinary 

 cases enlarges in two ways, becoming thickened by the continual 

 deposition of new osseous tissue on its outer surface, and at the same 

 time growing at its edges towards the extremities of the shaft, thus 

 accompanying along the outer surface of the cartilaginous diaphysis 



* Dec. 9. — Further observations have shown that the received account as given 

 above requires some modification. In the ordinary phalanges, which differ from 

 most other long bones by having only one epiphysis, the changes accompanying the 

 calcification of the cartilage do not go on in precisely the same manner towards each 

 extremity. The well-known lines of advancing calcification which run between 

 ro ws of single cells parallel with the axis of the shaft, are in the ossifying phalanx 

 only visible towards the proximal end ; that is to say, the end which will be crowned 

 by the future epiphysis. As might be expected, there is no difference in this respect 

 between the proximal ends of the last and of the other phalanges. 



