Ixxiv INTRODUCTION. 



Teeth further agree with the extravascular appendages of the 

 skin in being shed and reproduced sometimes once, sometimes 

 frequently, during the life-time of an individual : the latter may 

 be termed * interrupted' reproduction. In some cases again, as 

 with certain epidermal appendages, the reproduction of the tooth 

 is uninterrupted, and goes on during the life-time of the indi- 

 vidual ; new matter being added to the base as the old is worn 

 away from the apex, or working surface of the tooth. A tooth, 

 when fully formed, is subject to decay, but has no inherent power 

 of reparation. A tooth of limited growth can only increase in size 

 after its formation is completed by abnormal growth of its most 

 highly organised constituent, the cement. Thus, then, it appears 

 that, the analogy of the dental organs to those of the corneous 

 system holds only in their mode of development (1), in their 

 shedding and reproduction, and in their exposure to external 

 influences and to the contact of extraneous bodies : but the 

 antlers of Deer are similarly exposed, and are likewise shed and 

 reproduced annually, and also contemporaneously with the fall 

 of the hair; but antlers are not, therefore, classed with the 

 corneous tissues, any more than is the bony core of the horns of 

 the cavicorn Ruminants. 



(1) The cells and fibres of the horny tissues are formed in and not excreted from the surface 

 of their formative pulps. 



