THE MOUTH. 



345 



A portion of the tooth is buried and solidly implanted in one of the 

 alveolar cavities of the maxillary bones ; this is the root or imbedded portion, 

 (or fang). The other portion, circumscribed at its base by the gum, leaves 

 the alveolus to project into the interior of the mouth, forming the crown 

 or free portion. The narrow constriction between the crown and root is named 

 the neck. 



The fang is perforated at its inferior extremity by one or more excavations 

 (cavitas pulpce) which penetrate deeply into the substance of the tooth, and 

 admit into their interior the vasculo-nervous papilla, simple or ramified, 

 known by the name of the bulb or dental pulp. 



The crown, the portion submitted to friction during mastication and, 

 consequently, to wear, oilers the most varied forms : sometimes it is shaped 

 like a very acute cone ; at others, it is divided into several tubercles more 

 or less salient ; and sometimes, again, it carries at the extremity of the tooth 

 a wearing surface more or less plane and regular. 



STRUCTURE. Three essentially different substances enter into the structure 

 of all the teeth : the ivory, enamel, and cement ; to whicb ought to be added 

 the soft parts, the pulp, gum, and alveolo-dental periosteum. 



IVORY. The Ivory, or dentine, has the hardness of bone, is of a whitish- 

 yellow colour, and is rendered brilliant in places by its nacrous reflection. It 

 forms the principal mass of the tooth, enveloping everywhere the pulp cavity. 



Examined by aid of the microscope, this substance is found to be 

 channeled by a multitude of minute, undulating, and branching canals 

 (dental tubuli) imbedded in amorphous matter the fundamental substance. 



Fig. 153. 



SECTION THftOUGH THE FANG OF A MOLAR TOOTH. 



a, a, Dentine traversed by its tubuli ; 6, b, Interglobular, or nodular layer ; 

 c, c, Cementum. 



The tubuli, or canaliculi, extend from the dental cavity to the inner face 

 of the enamel ; single at their origin, they soon bifurcate, and again anasto- 

 mose several times during their slightly-undulating course. They terminate 

 in a cul-de-sac, or in irregular cavities situated beneath the enamel, and 

 named the interglobular spaces of Czermak (forming the interglobular or 

 nodular layer). These canals have a thin proper wall, and contain a dental 

 fibre, which very probably is a continuation of the pulp-cells. The 

 fundamental substance (or matrix) is amorphous, and not very abundant ; in 



