DENTAL CARIES. 
39 
osteo-cement pulp, that is, in the tissue, which in the embryo foims the 
cement. This material, or at least the space left by its contraction, is 
seen when a tooth is cut through with the shears. It begins just below 
the grinding surface, and reaches downwards as far as the involution of 
the enamel. Sometimes it is even visible on the grinding surface, and 
food then penetrates, filling the cavity with a black powder. I his 
decomposes, micro-organisms develop in it, and the surrounding cement 
and dentine become carious; a hole thus gradually burrows in the tooth, 
and increases in size (fig. 17). The process, therefore, usually starts at 
the wearing surface, and, having destroyed the crown, extends to the 
base of the enamel cavity, and even to the pulp. The pulp cavity may 
F IGt i7 e —Central dry caries of a horse’s upper molar (Kitt). 
then be crammed with particles of food, and purulent alveolai peiiostitis 
result, with its attendant symptoms. The tooth itself takes a darker 
colour, and becomes loose. It may then split lengthways during 
mastication (spontaneous fracture), or pieces may break off. 
Symptoms. The disease is seldom recognised at first, the only sign 
being a scarcely appreciable darkening, generally on the wearing surface. 
At this point a cavity forms which gradually increases in size (fig. 17), 
its walls, formed of the remains of the dentine, being of a black-brown 
colour. This stage is rarely noticed, and the disease only receives 
attention when the tooth has split and mastication become difficult, or 
when alveolar periostitis has set in. On attempting to extract the tooth 
it is found to be soft and fragile. 
The process may continue for long periods, and finally lead to more 
or less complete destruction of the affected tooth, though the production 
of alveolar periostitis or splintering usually necessitates extraction. 
When removed, one or more deep cavities are found in the dentine. 
Stockfleth says the disease occurs even at six or seven years of age, 
