AN ESSAY ON CHRONIC PODOTROCHOLITIS. 
411 
Once established, they change into holes, and the whole bony sur- 
face appears as if it had been gnawed by worms (worm eaten, 
caries verminosa of the English). The exostoses also do mischief, 
and here and there may be perceived a slight depression or inden- 
ture in the cartilage of the trochlea. On the bone being macerated, 
it is found to be worn on the parts corresponding with these de- 
pressions : this latter presents irregular tooth-like edges, while the 
posterior surface of the navicular is uneven and rough throughout 
its whole extent, or on some points only. The trochlean cartilage 
covering the careous parts disappears, not as a consequence of pro- 
gressive absorption, but because it undergoes the same fate as the 
bone. It softens, becomes uneven, and insensibly the surface of 
the navicular is rendered bare ; when entirely deprived of the car- 
tilage which encrusted it, it becomes of a deep red colour. 
The caries irradiates from the centre towards the circumference ; 
it causes loss of substance, as well as holes of greater or less depth 
and extent, and of various forms ; these are filled with a red fun- 
gous mass ( caries fungosa ). It also happens that the caries invades 
a greater or less portion of the periphery of the bone, and that it 
progresses towards the centre. It does not destroy the whole of the 
bony matter which it encounters, but extends itself circularly 
round a portion of the compact layer of the bone. This portion, 
detached throughout its whole circumference, continues to adhere 
by its basis to the body of the navicular; the loss of substance is 
replaced by the fungous matter, so that an isolated piece of bone is 
seen surrounded on all sides with fungous tissue, giving it the 
aspect of an island or cone. The destruction of this piece of bone 
is gradually effected ; it yields at length to the invading progress 
of the fungus, which eventually replaces it; but, in general, be- 
fore the completion of the morbid action, the bone contracts an 
adhesion with the tendon. If, when in this state, the foot is dis- 
sected and the tendon lowered, after having been separated from 
its other adherences, the bony plate will remain suspended to it, 
and continue to maintain itself firmly there. The union of the 
sesamoid, when bare and altered by caries, with the tendon, is 
an almost invariable phenomenon ; the tendinous fibres first of all 
contract adherences with the fungous tissue, on which they exercise 
a constant compression. This morbid tissue finds itself, in a man- 
ner, driven back again into the holes from whence it springs, and 
the bone, the fungus, and the tendon become at last thoroughly 
united. This situation of things has led many observers into error, 
by causing them to believe in the formation of fresh tendinous 
fibres. 
The holes of which we have already made mention enlarge 
gradually as the caries increases in extent and depth ; sometimes 
