CHAPTER VIII. 



THE TEETH. 



FROM the standpoint of descriptive anatomy, every tooth is 

 composed of three parts : (1) the crown, that portion which 

 stands above the level of the mucous membrane of the gum ; 

 (2) the neck, a constricted part at the level of the gum ; and (3) 

 the root, which terminates in one or more fangs, and is firmly 

 embedded in the alveolar process of the jaw. Each fang also 

 is pierced from below by a canal, which extends up into the 

 crown, and is filled by a soft material rich in nerves and ves- 

 sels, called the pulp, which has the special province of sup- 

 plying nutriment to the dense tissue about it. 



From a histological point of view, every tooth may be di- 

 vided into : 1, enamel ; 2, dentine, or ivory ; 3, cement, or true 

 bone. The enamel forms the covering for the crown, the cement 

 for the root ; but they meet at the neck, and there- the cement 

 slightly overlaps. The ivory or dentine lies intermediate be- 

 tween the outer coatings and the pulp. 



The enamel. This substance, which is the hardest met with 

 in the body, consists of a series of long polyhedral columns 

 grouped in bundles and disposed mostly at right angles to the 

 surface of the dentine which lies beneath it. Each column or 

 pillar is a hexagonal prism, having a diameter varying between 

 Tvfans and -^^ inch. When viewed in cross-section these col- 

 umns look like a tesselated pavement. They are not, however, 

 closely applied to one another, but have interspaces which are 

 said to be filled with a homogeneous substance or fluid. 



All of the groups of columns do not stand vertical to the 

 dentine ; some are parallel to it, and thus are interwoven with 

 the vertical ones. This crossing of the fibres produces an 

 alternation of light and dark bands (Fig. 38, 1). But there are 

 other systems of markings. In the same figure are wavy 



