200 STRUCTURE OF A TOOTH. 



2. Structure of a tooth. 



Let eacli pupil prepare a longitudinal section of a tooth 

 as follows : Imbed a tooth in a little sealing-wax on the 

 end of a spool, cork, or block of wood. With a grindstone 

 grind away one half, showing the, pulp cavity to the tip of the 

 root as in Fig. 56. If human teeth cannot be obtained, almost 

 any kind will serve. Let each pupil keep his preparation. The 

 following parts should be clearly distinguished : 



1. The Pulp-cavity, communicating with a hole in the 

 tip of the root, through which the nerve and blood-tube 

 entered. 



2. The bulk of the tooth is made up of a substance 

 called Dentine (ivory). 



3. The crown of the tooth has a covering of Enamel, a 

 very hard substance. 



4. The root is covered with a bony substance called 

 Cement. 



Make a drawing of the surface thus exposed, naming the 

 parts. 



The Kinds of Teeth and their Arrangement. Begin- 

 ning at the middle of the front of the mouth, there are (in 

 the normal adult) eight teeth in each half-jaw : two Incisors, 

 one Canine, two Bicuspids (or Premolars), and three Molars. 



The kinds and arrangement of teeth are often expressed 

 by a Dental Formula, in which the numerators indicate the 

 upper jaw and the denominators the lower, thus : If, C}, 

 PMf, MJ (for one side of the head). 



The crown of an incisor is chisel-shaped ; but the root is 

 flattened in the opposite direction, i.e., at right angles to the 

 jaw, instead of parallel to it, as is the case with the crown. 

 If possible, have at hand also a skull from which the teeth 

 have been extracted, in order to see the cavities into which 

 the teeth fitted. 



