アカゲザル (Macaca mulatta) の乳歯形態について

Abstract

Investigations of the teeth of the primates are invaluable for the phylogenetic study of the teeth of the mammals. Furthermore, the fact that the milk teeth are more primitive compared with the permanent teeth makes the morphological study of the milk teeth of the primates essential for the phylogenetic study. Because of the foregoing viewpoint, the teeth in position of forty-eight skulls of rhesus monkeys, in possession of the department, were measured and examined morphologically. Rhesus monkeys belong to the cercopithecids of the primates and, their tooth formura is the same as that of the humans in both milk and permanent teeth. Morphologically, their teeth are closely allied to the humans. The results were as follows : 1. The development of the cingulum in the anterior teeth region was extremely little labially but rather advanced lingually. The development was prominent particularly at the central milk incisor and milk canine of the maxilla and at the milk canine of the mandible. 2. The development of the cingulum of the milk molar was not recognized lingually but was apparent buccally. Equivalents of the human molar tubercles were observed particularly at the first milk molar in the maxilla and mandible, not only at the mesial tooth neck but also at the distal tooth neck. 3. The occlusal forms of the milk molars in the maxilla and mandible showed bilophodonty as the molars of other cercopithecids. 4. The number of cusp of the milk molars was four in all cases, but the second milk molars of the mandible showed a trend for a slight degree of differentiation in the hypoconulid. 5. The form of the occlusal surface grooves was Type + for the first milk molar and mostly Type X for the second milk molar in the maxilla, and, in the mandible, Type + for the first milk molar and either Type + or X for the most of the second milk molars. Typc Y was extremely few. This differed greatly from the humans whose milk molars are mostly Type Y. 6. Supernumerary tubercles of the milk molar were extremely few. Occurrence of Carabelli's tubercles was 9.4% at the maxillary second milk molar, protostylid 2.1% and the sixth cusp 5.3% at the mandibular second milk molar. 7. According to the measurements of the crowns, the maxillary second milk molar was the largest of the milk teeth and the mandibular lateral milk incisor the smallest. Comparison by crown index of the rates of the mesiodistal diameter to buccolingual diameter of the crowns with those of the human milk teeth revealed little difference in the rates between them in the anterior portion. In the molar portion, however, the rate of the mesiodistal diameter of rhesus monkeys to the buccolingual diameter was extremcly greater characteristically in comparison with the humans'. 8. In the comparison between the mesial and distal buccolingual diameter of the milk molar crowns, the mesial buccolingual diameter was greater in the maxilla and the distal buccolingual diameter was greater in the mandible. Of the mandibular first milk molar in particular, the distal buccolingual diameter was significantly longer compared with the mesial buccolingual diameter. 9. In the relation of the size between the milk central incisor and milk lateral incisor, the milk lateral incisor was smaller in the maxilla, as in the humans, and the mesiodistal diameter of the milk central incisor was shorter in the mandible and the labiolingual diameter of the milk lateral incisor was shorter. 10. The second milk molar was larger in size than the first milk molar in both the maxilla and mandible in all cases.