158 Physiology. 



parts. If human teeth cannot be obtained, almost any kind will serve. 

 Let each pupil keep his preparation. 



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 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 Arrangement of the Teeth. Beginning 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 (see Fig. 68). 



Dental Formula. The kinds and arrangement of teeth 

 are expressed by a dental formula, in which the nume- 

 rators indicate the upper jaw and the denominators the 

 lower, thus : I|, C^, PMf, M| (for one side of the head). 



The Kinds of Teeth. 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 

 in the crown. The canine tooth has a conical crown, and 

 a longer root than the incisor. The bicuspid has two 

 points. The molar has a cube-shaped 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 

 teeth. Because the first set is temporary, it should not 

 therefore be neglected. Cavities in these should be filled 

 and the teeth kept clean. Before the temporary set has 



