314 THE SOLIDS. 



ART. 16. THE TEETH. 



THE tissue of the teeth is to bepregarded rather as a modification 

 of the osseous than as a distinct type of structure : the truth 

 of this remark is especially apparent on an examination of 

 two of the three substances which enter into the formation of 

 each tooth, viz. the cementum and dentine ; the third con- 

 stituent, the enamel, is more nearly related in its organi- 

 sation to the epithelium, of which indeed it is a condition. 



Each tooth consists of two parts : the body or crown, and 

 the root or fang ; the limits of these are indicated by a slight 

 contraction called the neck ; the crown is either simple or 

 divided, and the same is the case with root also : those teeth 

 which have a simple crown are called incisors and canines, 

 those with a double crown bicuspids, and those with the crown 

 quadruply divided molars. Again, the substance of each 

 tooth is divisible into three portions, each of which presents 

 characteristic differences ; these have received the names of 

 dentine, cementum, and enamel. 



The dentine, also called the ivory, forms the chief bulk of 

 the tooth, occupies a central position, and its interior contains 

 the pulp cavity. 



The enamel forms a layer of compact substance which 

 immediately surrounds that portion of the dentine of which 

 the crown of the tooth is made up. 



The cementum, also called crusta petrosa, has a distribution 

 the very reverse of the enamel, and extends principally around 

 the fangs of the teeth, and terminates at the neck of the tooth, 

 In fact, just where the enamel commences. 



STRUCTURE OF THE TEETH. 



Having thus sketched the general position of the three 

 constituents of the teeth, the consideration of the intimate 

 structure of each may next be entered upon. 



