248 ORIGIN AND DEVELOPMENT OF THE TEETH. 



though the term is most usually restricted to misplacement 

 in position and number. 



The permanent teeth most frequently deviate from their 

 natural places and exhibit irt^egularity in their arrange- 

 ment. The incisors and cuspidati, it is said, present an 

 improper direction oftener than any of the other teeth. Some 

 teeth may be said to vary in improper direction when they 

 are out of the line of the proper arch, and project either too 

 far forwards or too far backwards ; or when, with this de- 

 viation, there is an additional irregularity, by which they 

 are turned about and have their anterior and posterior sur- 

 faces presenting very obliquely forwards and backwards. 

 Another irregularity in position is, where the teeth change 

 places ; thus the central incisor has been seen to take the 

 place of the lateral, and the lateral of the central. . Again, 

 irregularity of teeth may occur not only in consequence of 

 change of position, but different parts of the tooth itself- 

 may change, as in the upper jaw, for example, the crown 

 may be above and the roots below, looking towards the al- 

 veolar surface an instance of which is seen in the Museum 

 of the Baltimore College of Dental Surgery. 



The irregularity in number may be either a deficiency or 

 excess. Instances are cited in which the teeth have all been 

 wanting, or a great number have been absent ; or, as we 

 are told, they have been fused together so as to constitute 

 but a single piece. Those which exceed in number are 

 called supernumerary teeth. 



Irregularity in time of appearance is equally great. 

 Teeth have been seen at birth, and on the other hand, not 

 till ten years of age. These may be regarded as the ex- 

 tremes, between which we find every intermediate shade of 

 variety. 



SECTION III. 



ORIGIN AND DEVELOPMENT OF THE TEETH, AND FORMATION OF 

 THE ENAMEL. 



The teeth are appendages of the mucous system, and 

 have their origin from the mucous membrane of the mouth. 



