PROBLEMS OF HUMAN DENTITION 141 



cases, and especially in the canine, the persistence of a milk 

 tooth may not be caused in this manner, but this explanation 

 is undoubtedly an incoi-i-ect one regarding the second milk molar. 

 I have been able to jirove this in a decisive manner by radio- 

 graphically examining thirty jaws with a persisting milk molar. 

 A radiographical photograph was made of each of these objects. 

 What appeared from this examination? In only two cases was 

 the second premolar present in the jaw, lodged in a horizontal 

 direction beneath the roots of the second milk molar and first 

 molar. Only these cases are really pathological, as the others, 

 showing not the least trace of a second premolar, cannot be con- 

 sidered as such. My material included eight jaws in which the 

 persisting milk molar occurred on both sides. I have examined 

 thirty-eight cases of persisting milk molars. From this investi- 

 gation it is proved that such an occurrence cannot be due to an 

 irregularity in the topography of the second premolar, because 

 this tooth is absent. 



Figures 24 and 25 represent two of my preparations. Both 

 cases show jaws of fairly aged individuals, as indicated by the 

 worn occlusal surfaces of the molars. 



These figures show, in the first place, that the persisting 

 milk molars participate in an entirely regular manner in the 

 construction of the set, as well from the functional as the morpho- 

 logical aspect. It is an important fact that the occlusal surface 

 of the milk molar is on the same level as that of the preceding 

 premolar and the succeeding molar, for there is a perfectly 

 regular occlusion of the upper and lower set. Furthermore fig- 

 ures 24 and 25 establish the fact, that the persisting of the milk 

 molar is attended with an absolute absence of the second pre- 

 molar. Notwithstanding the most careful examination of both 

 radiograms it is impossible to discover even the smallest trace 

 of this tooth, and the same is true of all other cases. This is a 

 fact of great importance, for it proves that there is an inborn 

 functional correlation between the second milk molar and the 

 second premolar. The loss of the latter tooth and its substitu- 

 tion by the first, are not two independent events, but one indi- 

 visible process. Because the individual, in consequence of heredi- 



