AN EXOSTOSIS REMOVED BY MR. SYME 203 
granular, and the cells were rounder, and often surrounded with a broad pellucid 
ring, but the cells of the immediately adjacent part of the uncalcified cartilage 
assumed more or less of the same characters. The boundary between the 
uncalcified and the calcified cartilage was rendered very distinct by the circum- 
stance that the matrix of the former was coloured brown by the action of the 
acid, while that of the latter generally remained colourless. 
At one spot, viz. at c in Fig. 2, a piece of cartilage existed at a considerable 
depth below the general surface of calcification, and surrounded on all sides 
by the dense pseudo-bone ; this cartilage was extremely soft, and its cells re- 
markably large; one of them of circular outline measured 1~—285th inch in 
diameter ; the nuclei also presented considerable variety of appearance, and 
complexity of structure. It might be supposed that these characters of the 
cells were connected with great rapidity of growth in this cartilage, and such 
may perhaps be the case; but the cells are not larger than exist in the central 
parts of costal cartilages. In the centre of a costal cartilage of a woman about 
sixty years old, I found cells very similar both in size and appearance, and it 
can hardly be supposed that any very rapid cell-development had been going 
on in that situation at such a period of life. 
The fact that the calcified cartilage was in some places looser in texture 
at its deep than at its superficial parts (e.g. looser at e than at ) in Fig. 2) seems 
to indicate a change going on in the calcified cartilage by which it becomes 
converted into the loose cancellous structure of the interior. Examination of 
the deepest parts of the calcified cartilage under a low magnifying power, after 
the earthy matter has been removed by acid, shows that it is invaded by pro- 
cesses of the medullary substance of the cancellated tissue beneath it, which 
advance for a variable distance into its substance, and give a very irregular 
eroded character to its outline. The excavations thus seen to occur in the 
deep border of the calcified cartilage, are often lined with a thin layer of true 
bone containing lacunae, which has, no doubt, been deposited subsequently 
to the formation of the excavations. Fig. 3 represents a small portion in this con- 
dition, where a is the deepest part of the calcified cartilage, b c is a layer of true 
bone lining the excavations e and /, which were formerly occupied by processes 
of the medullary substance ; and d is part of one of the spicula of the cancellous 
structure. The true bone is distinguished from the matrix of the calcified 
cartilage by having a higher refractive power than it. 
In the part where I first noticed this osseous lining of the calcified cartilage, 
it was almost universally present; very few of the excavations being even 
partially destitute of it. And the same appearance presented itself in the head 
of a metatarsal bone of a boy sixteen years of age, at the line of junction between 
