THE TEETH. 495 



membrane, which is attacked with so much difficulty by chemical 

 reagents, is, very probably, still more impenetrable than the enamel 

 itself, and hence these two substances serve admirably to protect the 

 teeth. The sensibility of the teeth arises from the nerves of their pulp ; 

 they are affected by contact, heat, cold, and chemical agents. Slight 

 mechanical influence can only act by the vibrations which they may 

 communicate to the substance of the tooth and thence to the pulp ; it 

 is therefore the more remarkable that the teeth have a certain sense of 

 locality, so that it is possible to distinguish whether they are touched 

 internally or externally, above or below, on the right or on the left side. 

 The sensibility of the teeth is indeed tolerably delicate, especially on 

 the masticating surface, where the smallest foreign bodies, as hairs, 

 grains of sand, &c., are perceived when these surfaces are rubbed 

 against one another ; and as regards its acuteness, it is, in disease at 

 least, excessive, which is sufficiently explained by the considerable num- 

 ber of nerves in the pulp and the readiness with which they may be 

 compressed within their hard receptacle. 



With age the teeth become denser ; the pulp cavity is filled with a 

 kind of irregular dentine and may be totally obliterated, which is, per- 

 haps, the normal cause of their falling out. In certain cases observed 

 by Tomes, the fangs in old age were quite transparent, like horn. 



The following remarks may be made upon the pathology of the teeth. 

 Permanent teeth which have fallen out are sometimes replaced by a third 

 dentition ; but it must not be forgotten that the milk teeth occasionally 

 remain beyond their time, and care must be taken not to confound a 

 second tooth, late in its eruption, with a third. Teeth which have been 

 extracted may be replaced (in fifteen months a canine tooth which had 

 been extracted from the upper jaw was perfectly firm again). An abnor- 

 mal development of the teeth takes place particularly in the ovarium, 

 but also elsewhere. Fractures of the teeth may be reunited when they 

 occur within the alveoli, by imperfect dentine or cement. Regeneration 

 of the worn down parts takes place only in animals (Rodents, e.g.) in 

 which the teeth constantly grow. Hypertrophy of the cement (the 

 so-called exostosis), deposits of dentine in the walls of the pulp cavity 

 and ossification of the pulp itself, are exceedingly common, and result 

 from chronic inflammation of the periosteum and pulp.* A partial dis- 



* [Wedl in his recent work (Grundzttge der path. Histol.) has added some very inte- 

 resting observations to our knowledge of the structural changes occurring in the different 

 parts of teeth. In hypertrophy of the cement he observed the canaliculi dilated, so as to 

 form Haversian canals, and agrees with Kolliker (vid. 140 supra) as to the frequency of the 

 hypertrophy of the entire cement in old teeth. In partial hypertrophies of the cement he 

 detected numerous dentinal globules, bounded by irregular fissures, and on their external 

 border many bone-corpuscles. These latter were separated from each other by a yellowish 

 intercorpuscular substance, and in many instances by peculiar sinuous cavities, which traversed 

 the lamella; of the osseous substance. 



