IN HERBIVOROUS ANIMALS. 
219 
which, perhaps, is as much owing to the putrefaction of the saliva 
in the excavation in the tooth as to the osseous decomposition. 
When this alteration has commenced, it progresses incessantly be- 
tween the folds of I he enamel, which is more resisting, on account 
of its greater density, to the dissolving action, but which, never- 
theless, takes on the blackest tint of the altered ivory, and be- 
comes sufficiently softened to allow of its being sliced by a cutting 
instrument. Sometimes even the planes of the enamel also dis- 
solve, and then the cubic mass of the tooth becomes so much 
altered that it resembles a deep cavity, blackish, of which the 
parietes are formed by the planes of enamel laid bare by the caries, 
and which disseminates a remarkably foetid odour sui generis. 
Most ordinarily the caries commences in the dental table be- 
tween two folds of the enamel. Sometimes it attacks the tooth 
on one of its four surfaces ; at others it commences at the root in 
the alveolar cavity ; but whatever may be its primitive seat, the 
blackish veins which discover its presence extend always into the 
substance of the ivory, thus isolating the plies of the enamel. 
Carious teeth rarely preserve their form and their volume. 
They become ordinarily at their roots the seat of an hypertrophy, 
or, to speak more clearly, of a veritable exostosis. This effect 
does not manifest itself until the caries, having undermined all the 
layers of the ivory substance which it has encountered, has pene- 
trated even to the root of the organ. Then the alveolar dental 
membrane, irritated by the continual contact of altered matters 
which penetrate down into the alveole, increases and aggravates 
its secretion, and deposes in the circumference of the altered tooth 
a thick layer of osseous matter, which concretes irregularly upon 
the normal layers anciently formed. The root, augmented in vo- 
lume by these new stratce , can no longer be contained in the in- 
terior of the cavity which did enclose it, and tends, by an incessant 
mechanical effort, to separate, after the manner of a wedge, the 
osseous parietes of this cavity : hence one cause of the intolerable 
pain and particular alterations in the osseous tissues and the ad- 
jacent parts, to which we shall return by and by. 
The deposition of new osseous layers does not always take 
place in the circumference of the root ; in some cases it is only 
at isolated places that this secretion of the alveolo-dental mem- 
brane occurs : then the root presents a succession of large osseous 
tubercles, which bar in the tooth, and render its evulsion very 
difficult. 
Finally, when the irritation caused by the caries upon the mem- 
brane of the alveole has been from the first onset sufficiently active 
to determine in it suppurative inflammation, the normal secretion 
