In 1959 a group of 333 infants in Tokyo, off first-or second-born, were subject to collective dental examination, and the first-born and second-born groups were compared as to the eruption of deciduous teeth and decayed teeth (def). The results were as shown below:
As for the eruption of deciduous teeth, the firstborn group had less teeth appearing than the secondborn group at each age, particularly at one and five. In other words, the first-born are slower than the second-born in having deciduous teeth and more-liable to lose them latter.
Generally, decayed teeth were found more of ten among the second-born proximal surface and distall proximal surface of the lower lateral milk incisor, and the labial and buccal surface infour-year-olds.

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A total of 215 premature infants in Tokyo-97 male and 118 female-who had been nine months or more in the womb and weighed 2, 500g or less at birth, were subjected to clinical dental examination. The results were as follows:
1. The above infants were divided by weight at birth into two groups, A and B. Group A had a little more teeth erupting than Group B at each age (months), but in the range of two years to two years and five months, the two groups were close to each other. Comparison with the standard value for ordinary infants found Group A within the range of-16 and Group B, -26, but at 2 years or more both groups showed values to the average.
2. As for the ralation between the individual growth type and the number of erupting teeth the “B-to-A” group had the largest number of teeth, followed by the “A-to-A”, “B-to-B” and “A-to-B” groups in the order mentioned. At two years to two years and five months, however, all the groups showed much the same values.
3. Examination of central milk incisors, lateral milk incisors and first milk molars showed that, of each kind, the “B-to-A” group had more teeth than any other group, rollowed by “A-to-better”, “B-to-B”and “A-to-B” groups, the number of teeth increasing with the increased body weight.
4. As we have seen, the eruption of teeth and the weight of the body are in close relation to each other. In evaluating the physical growth of an infant, therefore, we believe it is necessary to consider its oral cavity region, particularly the number of teeth it has.

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I examined 272 pit workers in the metal mine for dental diseases, especially for gingiva disea ses, which I investigated thoroughly on changes of gingiva and deposit conditions of each individual person year by year. I also examined external wounds in the oral region arising out of duty and furthermore I researched particularily for the causes of stain in the oral cavity as well as its conditions, and then I worked out the preventive measures oral hygienically for the aboves.
I sumarize the results of this study as follows:
1. Any occukational differences are not seen on the conditions of pit workers suffered from dental caries.
2. Dental attrition is found at 77.5%, that seems due to occlusion pressure continusly owing to muscular labor together with crushing powered dusts unconsciously.
3. The cases of gingiva disease are at 72.43%, rather as a high percentage. Observing the above cases year by year, one having healthy gingiva, over 35 years old and working more than 16 years becomes gingivitis at 100% after 5 years, and among which the 50% changes from gingivitis to pyorrhea alveolaris after 5 years.
4. Among pit workers of gingiva disease, pyorrhoea alveolaris is exceedingly increased on the cases more or less forty years old and worked more than 16 years.
5. Occupational accidents in the oral region among the pit workers are at 1.35-1.68% yearly.
6. Resulting from the observation made year by year for the relation between the deposit conditions of dental calculus and gingiva disease in pit workers, one having healthy gingiva suffers from dental calculus comparatively a little, and even though the conditions of gingivitis continue to some degree, the gingivitis shows to change into pyorrhoea alveolaris depended upon an increase of deposited quantity of dental calcus.
7. Deposit of melanin pigment of gingiva is compartively less (27.04%). Even the cases of pyrrhoea alveolaris are at 37.05%. Considering from the aboves, the pit workers suffer much from a' malignant type of pyorrhoea alveolaris, which is a rapid progressive and serious case type, and its recovery seems to be very difficult.
8. Deposit of dental calculus is found in vitry many pit workers at average 99.25%.
9. The deposit cause of dental calculus seems due to powdered dustst and dringking water in, the. pit.
The aboves to be proved with the following:
i. Resulting from a spectroscopic analysis of dental calculus, Pb. Zn. and other metallic elements as the chief composition of mine are detected, which. are not found in the control.
ii. I observed some differences in quantity of Pb. Zn. etc. such as metallic elements in gargled water made at the beginning of work from one made after-finished work.
iii. Comparing gargled water made by unhanled dust worker before entering the pit with one made after the spot inspection, some differences in quantity of Pb. Zn. and other metallic elements are found.
10. Dental calculus in pit workers has Ca.: 31.43% and P.: 13.21%. The dental calculus seems to be adhered firmly with the teeth. Upon the histological view, it is stratified, and the space of its layers becomes narrower according to come nearer to the teeth. The inside of shows layers a retiicuture.
11. An artificial experimental dental calculus made with the mixture of gargled water from a pit worker and granules of powdered dusts has a great difference in metals compared with an experimental dental calculus made with gargled water from an unhandled dust worker, as the latter is not found any Pb. Zn. and other metallic elements. From the above, it can be assumed as powdered dusts get mixed in dental calculus.
12. As to a countermeasure for preventing dusts in pit workers, the preventive for dusting at the scene of labor as well as preventive for inhaling dusts on the part of the workers are important.

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Choosing Sapporo as representative of the cool northern part of the country, Nagoya as representative of central Japan, Kumamoto as representative of the warm south and Nagano as representative highlands, this author studied a total of 6, 655 children, 3, 429 male and 3, 226 female, at sixteen primaryand junior high schools in urban and rural communities in these areas with respect to the growth of their teeth, and at the same time measured their stature, body height, circumference of the chest and weight. For the junior high school girls, he also checked whether they are having the menses or not. Thus, he considered regional variations in dental growth in relation to body growth and phycical maturity. Neither in dental growth nor body growth nor physical maturity, and tendency toward quicker progress in the south than in the north was noted, or in lowlands than in highlands. But comparison of urban and rural areas showed that, in each of these respects, and in each region surveyed, progress is quicker in urban areas than in rural communities. This means that climatic or geographical conditions cause no great regional variations in dental growth or physical growth in this country. Much more conspicuous are variations in growth due to civilizational differences (presumably differences in dietary habits).
Body growth (especially, height) and physical maturity always showed fairly parallel progress both in inter-regional comparison and in urbanrural comparison. Dental growth is generally in parallel with these, but in a lesser degree than body growth and physical maturity are with each other.

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Conducting a survey on dental caries affecting primary and junior high school children in the cities of Sapporo, Nagoya, Kumamoto and Nagano and farm communities in their neighborhoods, this author studied regional variations in the rate of caries occurrence in permanent teeth and the rate of treated caries-affected permanent teeth.
Comparison of the rates of caries occurrence in permanent teeth in the four regions showed much similarity between urban and rural areas and between the sexes, with Nagano showing an apparently higher rate but little difference noted among the other three regions.
These regional differences can hardly be considered to derive from climatic or geographical conditions but appear to come from the local cultural environment characteristics of the schools chosen for the survey.
As for urban-rural differences in dental caries occurrence, it was found that, in Sapporo and Nagoya, in both sexes, higher rates were seen in urban communities than in rural areas, whereas in Kumamoto and Nagano, in both sexes, the latter showed higher rates than the former.
Differences in the rate of treated dental caries were most conspicuous. In each region, in both sexes, higher rates were always seen in urban areas than in rural communities.

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Choosing a total of 861 primary and junior high school children in Nagoya, 435 male and 426 female, the authors divided them by family occupation and studied the number of permanent teeth each child had, his height and, in the case of a girl, whether she is having the menses or not, in an attempt to find occupational characteristics, if any, in these respects, and also to learn if there is any correlation among these three factors-dental growth height and physical maturity. They also studied the rate of dental caries occurrence in each occupational group.
The survey, however, produced no results suggesting that the nature of a child's family occupation has any particular effect on his dental growth, height, physical maturity or vulnerability to dental caries.

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Using repeated acid decalcification, the solubilit of treated powdered enamel in acetic acid was compared between stannous fluoride and sodium fluoride.
100 mg of 100 or 150 mesh powdered enamel was treated with 10 ml of 2% sodium fluoride solution (NaF) and 4% or 8% stannous fluoride solution (SnF₂) for 20 minutes at 20°C. After washing with distilled water, 10 ml of N/5 acetate buffer was added to the protected enamel sample, and then the decalcification was carried on as follows. The decalcification procedure was repeated ten runs, stirring 5 seconds per minute (600 rpm) for 20 minutes at 37°C in each run. Amounts of calcium and phosphate disolved from a sample were determined in each run. The control group was treated by the same procedure except fluoride treatment.
Following conclusions were obtained:
1. The stirring method in the decalcification procedure was very important for evaluating effects of different reagents on powdered enamel.
2. SnF₂ group had the lowest solubility of enamel when tested by author's decalcification method, and 8% SnF₂ group appeared to the less soluble than 4% SnF₂ group. (Fig. 1, 2 and 3.)
3. As for Ca/P ratio, they were 2.3 or 2.4 in NaF group, 2.5 or 2.7 in SnF₂ group and 2.1 or 2.2 in control group in the earlier runs of 10 successive decalcification. There were statistically significant difference between these three group. (Fig. 5, Table. 1.)
And the variation coefficient of SnF₂ group in the earlier decalcification runs was shown to be larger than that of NaF group. (Fig. 4.). It was suggested that the made of action on enamel structure might be different between NaF and SnF₂.

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100 mg of 100-150 mesh powdered enamel was treated with 10ml. of 2% sodium fluoride solution (2% NaF group) or 0.2, 2, 4, 8 and 16% stannous fluoride solution (0.2-16% SnF₂ group) respectively 5 minutes at 20°C. The treated sample was decalcificated by 200 ml. of N/5 acetate buffer (pH 4.0) for 2 hours at 37°C, stirring with a magnetic stirrer. Amounts of calcium and phosphate disolved out of the sample were determined after each run in 7 successive decalcification runs. For obtaining the initial content of calcium and phosphate in the sample, 100mg of powdered enamel of each group was disolved in 2N nitric acid and then total acidsoluble-calcium and phosphate were determined. Calcium was determined with EDTA-2Na titration method, and phosphate with molybdium-vanado phosphate method. The decalcification rate was also investigated.
Conclusions as follows
1) The enamel solubility in NaF and SnF₂ group were less than that of control, and the difference were more marked during the earlier runs of decalcification.
2) 2%, 4% and 8% SnF₂ group seemed to demonstrate the lower acid-solubility when compared with 2% NaF group. 0.2% SnF₂ group was inferior in resistance to acid than to other higher concentration groups, and there was no definite difference between this group and NaF group.
In 16% SnF₂ group, the disoved Ca and P from samples was rather higher than other lower concentration (2 or 4%) group during the earlier runs of decalcification.

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