STEUOTURE OF THE TEETH 



97 



animals. For example, in the rabbit the dental formula 

 is as follows : 



TJ UU-+J + !<■ 1 2 + 2,0 + 0,3+3.3+3 ^^ 



Rabbitdentalformula = ^ + {^q + 0^2 + 2,3 + 3 = ^^' 



Structure of the teeth. — (See Fig. 27. ) The part of a tooth 

 above the gum is called its crovm- When a tooth is pulled 

 we find that the crown is only a 

 small part of the entire tooth. 

 At the point where it enters the 

 gum it narrows into a part called 

 the neck and extends below in one 

 or more projections called the jangs 

 or roots which anchor it firmly in 

 its bony socket. The incisors and 

 canines have only one fang, the 

 bicuspids may have two, while 

 the molars may have as many as 

 five and as few as two. 



The internal structure of all 

 human teeth is practically the 

 same. If we split a tooth in two, lengthwise, examina- 

 tion shows that the crown is covered with an external layer 

 of hard matter called enamel. This thins down at the 

 neck and is succeeded by a fang covering called cement. 

 The main bulk of the tooth is formed of a bonelike sub- 

 stance called dentine which surrounds a small central cavity 

 called the pulp cavity. During life this cavity is filled 

 with nerve fiber and blood vessels interlaced in a network 

 of connective tissue, called the pulp. It is lined with a 

 layer of epithelial cells which secrete the mineral matter of 

 which the outer layers are formed. This cavity has a small 



ZDDY. PHYS. — 7 



Fig. 27 — Section of a tooth; 

 k, crown ; e, enamel ; », 

 neck ; f/, dentine ; /, fangs ; 

 e, cement ; p, puip cavity 

 inclosed l)y dentine. 



