134* Preternatural Growth of the Incisor Teeth, 



Art. IV. Observations on a preternatural Grotvth of the Incisor 

 Teeth, occasionally observed in certain of the Mammctlia rodentia. 

 By the Rev. Leonard Jenyns, F.L.S. 



Instances of the wild rabbit have occasionally been met 

 with in Cambridgeshire, in which the fore-teeth had grown to 

 so great a length, as to be rendered wholly unfit for the pur- 

 poses they are intended to serve. 



This disease, as, in truth, it may be strictly called, has been 

 often observed before *, both in these, and, I believe, also in 

 other animals of the order Rodentia (les Rongeurs Cuv.), whose 

 incisors are all constructed on the same, or nearly the same, 

 plan. Nor is it for the novelty of the occurrence that the 

 following remarks are offered: my present object is merely 

 to draw attention to the circumstances of at least two cases, 

 that were not attended by that accident which is usually 

 supposed to produce the singularity in question. 



It appears to be the statement of some authors, that, in 

 order to occasion this anomaly, it is necessary that one pair 

 of incisors, or one single incisor, be either broken or fallen 

 outf; and that it is for want of the accustomed attrition 

 against the teeth which are deficient, that the opposite pair 

 grow to an unusual extent. This is so far true, that, I believe, 

 in all cases, it is in immediate consequence of the cutting edge 

 not being worn away, or at least to that degree that it is in 

 healthy individuals from constant use, and to supply which 

 loss these teeth are provided with the power of growth J, that 

 this irregularity shows itself: but I would observe that the 



* See Plott's Natural History of Staffordshire, p. 252. tab. 22. fig. 6.; 

 also Morton's Natural History of Norttiamptonshire, p. 445. 



f Cuvier, speaking of the incisors of the Roddntia, observes, " Leur 

 forme prismatique fait qu'elles croissent de la racine a mesure qu'elles 

 s*usent du tranchant, et cette disposition a croitre est si forte, que si Vune 

 ffelles se perd ou se casse, celle qui lui etait opposee n'ayant plus rien qui 

 la comminue, se d^veloppe au point de devenir monstrueuse." — Regne 

 Animal, tom.i. p. 187. 



J It is well known to comparative anatomists, that the incisors of the 

 Rodentia, like the tusks of the elephant and hippopotamus, are in a con- 

 stant state of growth, and that they are furnished with roots which in 

 length nearly equal the jaw itself, curving back underneath the grinders, 

 and extending in some cases as far as the coronoid processes. In con- 

 sequence of this singular provision of nature, so admirably adapted to the 

 habits and economy of this tribe, there is a constant, yet gradual, advance- 

 ment of the interior part of the tooth, to supply the place of the portion 

 worn down in feeding, &c. ; and, under ordinary circumstances, this in- 

 crease is so nicely regulated, that the cutting edges of the two pairs of 

 incisors always preserve the same relative situation with respect to each 

 other. A good description and representation of this contrivance may be 

 seen in Blake* s Essay on Teeth. {Disputatio med, inaug. de Dentium For- 

 matione, &c., p. 88 — 91., tab. 5. fig. 9.) 



