170 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



Here the function of the teeth is not only to seize 

 and bite the food, but also to masticate it and test 

 its quality. This change in function accounts for 

 the heterodont dentition, which must have arisen by 

 a modification of the simple homodont condition 

 in which the teeth are all small, conical, and of the 

 same size and shape. The primary arrangement of 

 the teeth is such that those of one jaw do not 

 usually correspond in position with those of the 

 other, but rather with the interspaces between them. 

 As a rule, the succession of teeth in man is nearly 

 always reduced to two functional sets, the deciduous 



Dental ridge. Enamel organs. Dental ridge. Enamel organs. Neck. Dental 



ridge. 



Fig. 123. Diagram illustrating the development of the enamel organ 

 of three teeth. 



teeth and the permanent teeth. Traces of an 

 earlier set have been found, which may be spoken of 

 as a "predeciduous" dentition, and occasionally one 

 or more teeth appear which replace corresponding 

 permanent teeth, and thus indicate the possibility of 

 an extra unrecorded set. An unlimited succession 

 of teeth takes place in nearly all vertebrates, except 

 with mammals. 



Development of Teeth. The enamel of the tooth 

 develops from the epithelium of the oral cavity. In 

 the seventh week of fetal life the mucous epithelium 

 covering the gums invaginates to form a dental 

 groove. The ridge or shelf thus invaginated is called 



