In the practice of clinical orthodontics, the principal goal of treatment lies not so much in the recovery of morphological and esthetic appearance of human dentition as in its functional recovery as masticatory organ. This would imply that more emphasis should be laid on the study of biting force of the mouth. Hitherto, the so-called biting force has been measured with a variety of methods by many investigators and expressed as a combined force as produced between pair of antagonizing upper and lower teeth. The value of force thus measured, however, has failed to distinguish the two important factors involved in the act of biting jaws, that is the maximum force of biting to which the antagonizing teeth can stand without aching and/or the total closing force of masticatory muscles exerting on the teeth under the control of processus mandibularis as fulcrum. In the study of biting force, therefore, it is essential to distinguish these two factors and measure them respectively for their relative magnitude. Present study was undertaken to measure the maximum force which individual tooth can tolerate in biting and the total closing force of muscles, because no distinction was so far made between the two. The total closing force of muscles was also studied with relation to ages of subjects examined. Results were summarized as follows : 1. Significant differences were shown to exist between the biting force as measured on the dental arch and the one measured at the middle of sagittal plane, so that one would not be able to predict anything about the difference of biting force possibly existing between right and left dental arches on the basis of data from measurement at the middle of sagittal plane. This would lead to the conclusion that measurement of biting force as needed in the orthodontic practice should be conducted on the dental arches. 2. The biting force of masticatory muscles as exerted on those part of dental arch occupied by the premolar and molar teeth was essentially a kind of momental leverage action with processus mandibularis as its fulcrum. 3. When the total closing force exerted on individual tooth and the maximum force of tolerance of each tooth was compared, it was shown that the latter force measured in the anterior teeth was considerably smaller than the former, while the same force measured in the molar region was approximately equal to the total closing force. 4. In those subjects who showed greater closing force also showed greater value of tolerance of their teeth. 5. The closing force tended to increase with advancing age in subjects from 9 to 14 years of age and in the male subjects the value of the closing force was higher than that in the female. 6. When the closing forces of right and left sides of dental arch were measured respectively on two groups of subjects whose habitual side of chewing was either right only or both sides by subjective judgement, the data obtained agreed with the subjective judgement in more than half of the subjects examined. However, the data obtained from those subjects who claimed that their habitual side of chewing was left side of jaw did agree to the extent of only one third of the total.

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Growth phases of Meckel's cartilage and subsequent appearance and development of the mandible were studied with reference to the patterns of ossification using frogs as materias- 1. Appearance and Disappearance of Meckel's Cartilage The lower lip cartilage and Meckel's cartilage made their appearance as respective anlage when the head-tail length of frog reached 8-mm. At about the time when the length was 38-mm, both organs showed hyaline ossification and at the same time lost clearness of boundary separating them. At 42-mm stage of the animal a part of the lower lip cartilage disappeared, but the remaining part continued to grow and took a horizontal posi. tion. Meckel's cartilage, on the other hand, extended itself toward outer-posterior direction in company with the backward turn of palatoquadratum, and the tapering end of the cartilage became fused with the rear end of the lower lip cartilage. When the frog lost its tail and the body was 20-mm long, the cartilage had completed its general shape with cylindrical, slender U-curve. Although the cartilage continued to elongate and expand thereafter, the foremost part became almost lost in the adulthood of the frog and even the posterior part showed the trend of degeneration. 2. Generation and Development of Mandible At 42-mm stage of head-tail length, there started the formation of mentale in the region where the lower lip cartilage and Meckel's cartilage were fused. At 40-mm stage of body length when the frog started losing its tail, anlages of both dentale and angulare made their appearance and these organs showed ossification with progressive development and body enlargement. With 20-mm stage of body length, vigorous growth of mentale, dentale and angulare was taking place around Meckel's cartilage as central axis of the mandible, the first near the tapering end, the second in the posterior lateral side and the third in the posterior inner side, and at this stage the tapering end of dentale was already fused with mentale by means of its outer posterior branch. With 45-mm stage of body length, ossification of respective bone proceeded further and altogether gave an approximate appearance of adulthood mandible. The mandible of frog, therefore, is constituted from the bony components of mentale, dentale and angulare. 3. Pattern of Ossification Dentale and angulare are formed by the desmoid ossification, while mentale is completed by the metaplastic ossification where Meckel's cartilage is directly converted into bone tissue.

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A total of 247 of lower 3rd molar obtained from the Japanese people was subjected to measurement with reference to possible variations in their physical traits and findings were discussed in the light of anthropology, and special attention was directed to the problem of their retrogressive trend and discussed from phylogenetical viewpoint. Some of the interesting results were summarized as follows. 1. Measurement of the size of teeth revealed that almost similar reduction was observed in every direction from M_1 to M_2 and M_3 in order, but the rate of reduction was not so conspicuous as in the corresponding teeth of upper jaw. In the teeth of female, no difference in size was observed between M_2 and M_3. 2. Those teeth characterized by the dryopithecus pattern occupied only 4.1 % of the total and conversely type +_5 and type X_5 which are recognized to represent evolutive trend showed more frequent occurrence. In general, variable forms of cuspal groove were observed more frequently than retrogressive trend of cusps of the teeth. 3. The physical traits of tooth crown such as protostylid, the 6th and 7th cusps that are considered significant manifestations in anthropological viewpoint could be observed relatively frequently, accounting for 15.4%, 20.2% and 2.8% of the total respectively. This suggested that these traits are prevalent in the Japanese more than in other races. 4. Monoradicular tooth was found in 44.9%, of which 12.2% had gutter-shaped root. However, such trend of single-rootedness in the lower 3rd molar was not so conspicuous as in the corresponding molar in upper jaw. 5. When the degree of retrogression was evaluated by means of reduction proportion analysis, the lower 3rd molar exhibited the strongest trend of reduction, followed by M_2 and M_1, although was much weaker as compared with that for the corresponding tooth in upper jaw.

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The status of dental health care in terms of F-T rate was examined in the population of primary and middle school pupils in Bungotakada city, Usa city and in the suburb of Bungotakada city and following results were obtained. 1. F-T rates in subjects of both sexes ranged in age from 6 to 15 The percentages of F-T in subjects of each district were given bellow : Takada city : male 13.9 to 48.5(%), female 14.9 to 52.2(%) Suburb of Bungotakada city : male 6.7 to 30.8(%), female 8.7 to 39.4(%) Usa city : male 10.2 to 33.2(%), female 8.0 to 25.9(%) The percentages for all subjects increased with advancing age. 2. Sex difference in F-T rate For all districts, the female population showed higher rates of F-T than the male. 3. Regional difference in F-T rate With both sexes, Takada city showed the highest rate, followed by Takada city suburb and Usa city. 4. When the rate for Takada city subjects was compared with the average figure in F-T rate of Ohita prefecture, the former was lower than the latter with both sexes. 5. When the rate for Takada city subjects was compared with the rates for rural districts subjects of Ohita prefecture, the former was lower than the latter with both sexes. 6. The rate for Takada city subjects was higher than that for Kitakyushu Moji district subjects with both sexes, and the latter rate was nearly equal to that for Usa city subjects. 7. The rate for the male middle school pupils of Takada city was lower than that for comparable subjects of Kitakyushu city Yawata district, while the rate for the female populations of both districts was nearly equal. The rate for Takada city male subjects was nearly equal to that for male in Kitakyushu rural districts, while the same for female subjects of Takada city conversely higher than that for comparable subjects in the latter districts. 8. With respect to the kinds of filling materials, amalgam filling was most frequent amounting to 65.4 to 83.0 %. followed by inlay filling amounting to 11.9 to 27.8 %, and other kinds of filling were very rarely encountered. This was the case with all the population groups examined in this study.

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We investigated the tooth, gingiva and its environment in June to August, 1972. The subject of the study consisted of 73 students aged 22 to 28 years, of Kyushu Dental College, and the data obtained were compared with comparable student groups of Yahata University and Dental Hygiene School. The result were summarized as follows, 1. Average number of DMF-T In the Dental College students the mean value of DMF-T was 8.2, in Yahata University the mean was 5.2 and in Dental Hygiene School the mean was 6.2. Average number of F-T Average number of F-T for Dental College students was almost equal to that for Dental Hygiene School and both were greater than that for Yahata University students. 2. Bodecker's Modified Dental Caries Index was 16.6 in the mean. The amount of tooth surface destruction was slightly more than that of Bodecker's report. 3. Clune's Actual Dental Health Capacity was 83.6 % and DMF Index of first molar was 16.2 in the mean. 4. Oral Hygiene Index was 2.85 in the mean. (DI=2.08, CI=0.95) 5. Anterior P-M-A Index was 3.0 in the mean. (P=1.8, M=1.1, A=0.2). 6. Snyder Test was negative in 24.7%, questionable in 45.2%, and positive in 30.1%. 7. Snyder Test had significantly high degree of correlation with the number of DMF-T and D-T.

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