上顎第 3 大臼歯の退化的形質変異に関する研究
スポンサーリンク
概要
- 論文の詳細を見る
Measurements were undertaken on a total of 429 samples of maxillary third molars, each of which was obtained from individual subjects of definite sexes and ages. These teeth were observed from morphological viewpoint and their forms were discussed with reference to possible regressive traits. Results were summarised as follows. 1. A more marked reduction in size was observed in the present samples of the third molars as compared with those observable with the 1st and 2nd molars. The rate of reduction was especially greater in the mesio-distal breadth of crown. As to sex difference in size, it was shown that male teeth exceeded the female teeth in each of the length, breadth and thickness of crown. There existed no appreciable difference with sexes in the length of root part nor in the length of whole teeth. 2. An arbitrary criterion was set to define dwarfism of this kind of teeth by preliminary measurements, and under this definition occurrence of dwarfism was calculated to be 4.7% in the male teeth and 7.7% in the female teeth. 3. When the contour of the occlusal surface was classified into 4 categories consisting of triangular, square, round and other nondescript forms, occurrence of triangular forms was most frequent with approximately 47% of total. 4. With regard to the number of occlusal tips, occurrence of 4 tips was rare and those teeth having less than 3 tips occupied 80% of total. 5. There occurred in about 15.8% those teeth with their crown extremely compressed in mesio-distal direction. This did not seem to have any correlation with diminishing number of occlusal tips. 6 The teeth showed very weak development of mesio-distal marginal ridge and diagonal ridge in their occlusal surface, and in 74.6% of total teeth, the occlusal surface showed wrinkled appearance with grooves and small ridges running irregularly. 7. Roots of the teeth tended to be fused, and monoradicular teeth occupied 67.7% of total. 8. When the retrogrssive traits of the present teeth were expressed in terms of 'reductive proportion', the values for the 3rd molars of male and female subjects were respectively 8.78 and 9.34 as compared to corresponding figures for the 1st molars and 2nd molars, 0.86 and 0.99, and 4.03 and 4.25. There were observed some particular traits of form occurring to some extent, which apparently did not belong to the retrogressive forms.