170 THE PATHOLOC^Y OF THE TEETH. 



on the root, and at tlie expense of the perfectly formed 

 dentine lying in immediate contact with its inner sur- 

 face. That is to say, this layer of dentine is converted 

 into cement by the dentinal lacunae undergoing dila- 

 tation and becoming identical with the hollow spaces 

 or cells of the cement. The tooth now draws its nour- 

 ishment from the blood-vessels of the socket, and thus 

 continues, long after the obliteration of its pulp-cavity, 

 to perform its part in the living organism. 



"This is the natural condition of old teeth. But 

 when the pulp-cavity is obliterated at an early age, by 

 a too rapid formation of dentine, and consequent ob- 

 literation of the pulp when the cement is not yet suffi- 

 ciently developed to supply nourishment to the whole 

 tooth, caries must be the result. Many cases of caries 

 that have come under my observation have resulted 

 from this cause, and very often the disease is confined 

 to that part of the cement that dips with the enamel 

 into the interior of the tooth, splitting it into several 

 longitudinal fragments. 



"Caries of the neck of the tooth is seen in those 

 horses whose teeth are wide apart, and is caused by 

 the food remaining in the interspaces, and by decom- 

 position exciting inflammation in the periodontal 

 membrane. Caries of the neck is very commonly met 

 with in the teeth of dogs, sometimes causing abscesses 

 in the cheek. 



"Caries beginning at the crown is due to a portion 

 of the dentine losing vitality and the power of resist- 

 ing the chemical action of the fluids of the mouth. A 

 portion of the enamel of the crown may be fractured 

 by biting a stone or piece of metal contained in the 

 food. Mere fracture of the enamel, however, is insuffi- 

 cient of itself to lead to caries* of the teeth in the lower 



