1 64 PHYSIOLOGY. 



Structure of a Tooth. i. The pulp cavity, communi- 

 cating 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. 



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 in- 

 cisors, one canine, two bicuspids (or premolars), and three 

 molars. 



Dental Formula. The kinds and arrangement of teeth 

 are often expressed by a dental formula, in which the nu- 

 merators indicate the upper jaw and the denominators the 

 lower, thus : If, C^, PMf, Mf (for one side of the head). 



Incisors. 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. Look at a skull from which the 

 teeth have been extracted in order to see the cavities into 

 which the teeth fitted. 



Canines. The canine tooth has a conical crown, and a 

 longer root than the incisor. 



Bicuspids. The bicuspid has two points. 



Molars. The molar has a cuboidal crown, and usually 

 two or three roots. 



The Milk Teeth. The thirty-two teeth of the perma- 

 nent set were preceded by a temporary set of twenty milk 



