AN EXOSTOSIS REMOVED BY MR. SYME 205 
the same as what occurs in the ossification of the epiphysis of a metatarsal bone. 
The only difference between the two cases appears to be that in the ossifying 
epiphysis, the calcified cartilage is a very thin layer, while in the exostosis it 
is present in considerable thickness ; but even in this particular there was not 
a constant difference between them ; for the calcified cartilage varied much in 
thickness at different parts of the exostosis, and at some spots was, to the naked 
eye, absent or nearly so, even where cartilage existed on the surface. 
The great thickness of the calcified cartilage probably results, in part at 
least, from a want of energy in the process by which it is converted into the 
cancellated tissue, and accordingly it was just at that part where the calcified 
cartilage was thickest (viz. at e in Fig. 2) that the osseous lining of the calcified 
cartilage was found almost universally present ; and we have seen that this 
condition implies an arrest in the process of conversion of calcified cartilage into 
cancellous structure. The languid condition of the ossific process at this part, 
was probably also the cause of the piece of cartilage c in Fig. 2 being left uncal- 
cified below the general level of calcification. 
The general conclusion to which the examination of this exostosis leads, 
is that it grew at the surface as cartilage, which became converted into can- 
cellated bone by an ordinary process of ossification, in which, however, the stage 
of calcification of cartilage occupied an unusually conspicuous position. 
