DENTAL CARIES. 



285 



and food then penetrates, filling the cavity with a black powder. 

 This decomposes, micro-organisms develop in it, and the surrounding 

 cement and dentine become carious ; a hole thus gradually forms 

 in the tooth, and increases in size (Fig. 285). 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 then be crammed with particles of 

 food, and suppurative alveolar periostitis result, with its attendant 

 symptoms. The tooth itself assumes 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 



Fig. 285. — Central dry caries of a horse's upper molar (Kitt) 



surface. At this point a cavity forms which gradually increases in 

 size (Fig. 285), 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 " quidding " occurs, or when 

 alveolar periostitis has set in. On attempting to extract the tooth 

 it is found to be very 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, a cavity is found in the dentine. Stock- 

 fleth says the disease occurs even at six or seven years of age, though 

 it is commonest at nine or ten ; but in older horses it is less frequent. 



Causes. In man caries, which consists in decalcification of the 



