440 MAMMALIAN ANATOMY 



The deciduous teeth normally begin to appear between the sixth 

 and ninth months, and are all in place by the end of the second year. 

 They erupt in the following order : the medial incisors, the lateral 

 incisors, the first molars, the canines, and the second molars. As a 

 rule, the lower teeth appear before the corresponding upper teeth. 



The permanent teeth appear as follows : the first molars at six 

 years, the medial incisors at seven years, the lateral incisors at eight 

 years, the first premolars at nine years, the second premolars at ten 

 years, the canines at twelve years, the second molars at from twelve 

 to fifteen years, and the third molars at from seventeen to twenty-five 



years. 



EVOLUTION OF MAMMALIAN TEETH. 



We have now studied two distinct types of mammalian dentition, 

 namely, the secondont, 1 that of the cat, in which the crowns of the 

 cheek teeth are pointed and sharp for cutting, and the bunodont, 2 that 

 of man, in which the crowns of the cheek teeth are flattened and 

 tubercular for crushing. Other mammals, however, exhibit various 

 forms of tooth structure, according to the nature of the food, the 

 method of articulation of the lower jaw with the skull, and the de- 

 velopment and points of attachment of the muscles of mastication. 

 These various forms result principally from modifications in the 

 number and shape of the cusps. A few mammals have no teeth ; 

 in a few others every tooth consists of a single cusp ; from which 

 primitive condition every stage of advancement can be observed up to 

 the type in which some, at least, of the teeth are crowned by six 

 tubercles. Moreover, the tubercles on the crown may be united in 

 various ways by high crests, often V-shaped and crescentic, or the 

 entire crown may be divided crosswise into a number of distinct parts. 

 In the most complicated forms of teeth the enamel appears to be 

 thrown into folds, vertical, transverse, or oblique, and when these 

 wear through and expose the dentine, many intricate patterns are 

 produced on the surface of the crown. 



When the crown of a tubercular tooth is low, the neck above the 

 gum, and the fangs large, the tooth is said to be of the brachydont 3 

 type ; when, however, the crown is so high that the cusps appear to be 



1 From seco, to cut, and odous (Gr.), a tooth. 



2 From (Gr.) bounos, a hill, and odous. 



3 From (Gr.) brachys, short, and odous. 



