DENTAL FORMUL.E. 89 



teetli of the kind symbolised, on each side of each jaw. we 



shall have the dental formula of a given animal. The 



dental formula of a child over two years of age is thus — • 



9.0 X 1 2'2 



di. — dc. dm. — = 20 : which means that the 



child should have two incisors, one canine, and two molars 

 on each side of each jaw. 



The neck of the sac of each deciduous tooth gives off a 

 diverticvdum, in which one of the permanent teeth is de- 

 veloped; as it grows, it causes the absorption of the fang 

 of the corresponding deciduous tooth, which thus becomes 

 shed, and is replaced from below by the permanent tooth. 

 The same letters, but without the prefix d, are used for the 

 permanent incisors and canines ; but the permanent teeth, 

 which replace the deciduous molars, are caUed premolars, 

 and have the symbol pm. Furthermore, three or, it may be, 

 four permanent grinding teeth, on each side of each jaw, are 

 developed altogether behind the milk molars, and thus come 

 into place without replacing any other tooth from below. 

 These are called molars, and have the symbol m. Thiis the 

 formida of the permanent dentition in Man is written : 



2-2 1 1 2'2 3'3 



i— c. ■ — Y p>m. --^ m. — - = 32 ; there being two incisors, 



one canine, two premolars, and three molars on each side 

 above and below. It is a iiile of very general applica- 

 tion among the Mainmalia, that the most anterior molar 

 comes into place and use before the deciduous molars are 

 shed. Hence, when the hindermost premolar, which imme- 

 diately precedes the first molar, comes into use by the 

 shedding of the last milk molar, the crown of the first molar 

 is already a little ground down ; and this excess of wear of 

 the first molar over the adjacent premolar long remains 

 obvious. The fact that, in the permanent dentition, the last 

 premolar is less worn than the first molar which imme- 

 diately follows it, is often a valuable aid in distinguishing 

 the premolar from the molar series. 



No veriebrate animal has teeth in any part of the ali- 

 mentary canal save the mouth and pharynx — except a 



