Bite raising is indispensable for the orthodontic correction of anterior protrusion and deep overbite. If the elevator muscle does not adapt to the new situation in bite raising, the mandible would not be able to keep the new position. The author used seventeen mature dogs as the experimental subjects. To know the adaptation of the elevator muscle, the changes for several months of the intermaxillary distance were measured. And at the same time, the elevator muscle, the zygomatic arc and the tooth supporting tissue were studied histologically. The results obtained were as follows : 1. When the elevator muscle was made not to act on the tooth supporting tissue by setting a suitable bite plate, the intermaxillary distance was gradually increased by the elongation of the molar teeth. Then the elevator muscle was made to act again on it by taking off the bite plate, and the increment of the intermaxillary distance was diminished gradually to be put out entirely in about the same time interval with that required for the elongation. 2. After three months' bite raising, the microscopic appearance of the elevator muscle was changed considerably. Namely, its myofibrils became not to be found, and the muscle fibers became swelled and round. And the latter were partly disintegrated to show a moth-eaten appearance which dis appeared for some reason in two months' retention following three months' bite raising, though the muscle fibers remained to be swelled and round even after the six months' retention following the bite raising. 3. After three months' bite raising, the zygomatic arc atrophied all-over the surface and several large Howship's lacunae were observed in some parts, while the formation of new bone and osteoid tissue was observed on the original zygomatic bone surface. After two months' retention following three months' bite raising, the formation of new bone became vigorous so as the surface of the original zygomatic bone to be covered with a thin layer of new bone. And after six months' retention the newly formed bone became an outer basic layer which is characteristic for the normal zygomatic arc. 4. The tooth supporting tissue became stable after four months' retention and the zygomatic arc after six months' retention. But at least six months' retention the elevator muscle could not be recovered to its normal feature. 5. The fact that the attaching portion was recovered histollogically after six months' retention may mean any internal stress was no longer acting on the elevator muscle itself. The deformation of the elevator muscle still remained at that time stage would not mean this portion to have any function to pull back the maxillary distance. 6. The adaptation of the elevator muscle was slower than that of the tooth supporting tissue, for the former requiring six months and for the latter four months. From the observations above mentioned the author may safely conclude that the elevator muscle would play a very important role for the orthodontic treatment of anterior protrusion and deep overbite.
