GENERALLY AFFECTS THE THIRD AND FOURTH MOLARS. 
43 
lines the alveolus, the inner layer the root of the teeth. At the point 
where the periosteum becomes continuous with the outer surface of the 
jaw-bone, the gum is attached, and its function is to prevent foreign 
bodies like food entering the alveolus. 
Depending on their character, two forms of alveolar periostitis may 
be distinguished. (1) Chronic ossifying, and (2) purulent alveolar 
periostitis. Those forms of alveolar disease resulting from local inva¬ 
sions of actinomyces or from tumour formation are described elsewhere. 
The chronic ossifying form is characterised by the formation of exostoses 
at the root of the tooth, which appears either roughened, or covered 
with flat, bony deposits, or with knobby masses of new bone; the latter 
rendering extraction of the tooth impossible. In the purulent foim the 
periosteum is usually thickened and extremely vascular ; at points it is 
covered with granulations, and separated from the wall of the alveolus or 
root of the tooth by a quantity of grey-coloured, offensive bone pus. 
Opposite these points the bone or root of the tooth is often eroded to the 
extent of one-sixteenth of an inch or more. In occasional cases, one 
sees extensive bone necrosis followed by suppurative osteomyelitis of the 
lower jaw. The periosteum usually becomes diseased in spots or on one 
side of the tooth, but in very old-standing cases disease may be so 
extensive that the tooth lies loosely in the alveolus and can be removed 
with the fingers. Frick has seen some cases where only a few plate¬ 
like fragments, the remains of the enamel, filled the alveolus. In all 
old-standing cases the smell is very offensive. 
The above two conditions are often associated, so that at one point the 
tooth shows discrete exostoses and at another limited areas of purulent 
alveolar periostitis. 
Causes. Alveolar periostitis almost always results from injury to the 
gum, or its separation from the tooth. Fissures of the tooth reaching to 
the root, and laying open the root-canal, occasionally produce it. Portions 
of hard food, especially hard chaff, may easily become insinuated between 
the tooth and the gum, and remain fast, movements of the tongue and 
cheeks not sufficing to remove them. Purulent periostitis, with separation 
of the periosteum from the tooth, supervenes. Masses of food penetrate 
into this enlarged space, widening the division between the alveolai wall 
and the tooth, and increasing inflammatory action. Separated from the 
periosteum, the tooth loses its firm seat in the alveolus, and, yielding to 
the pressure of its neighbour, is diverted from the common line, and may 
be so displaced that it can be removed with the fingers. Its root appears 
completely denuded of periosteum, and bathed in pus. Similai results 
are produced by the entrance of food between the teeth, as happens 
especially when the crowns are not in close contact. 
In dogs and horses, alveolar periostitis often results from epulis, the 
