The directions of the enamel rods of all kinds of human permanent teeth were observed in the central perpendicular sections by using the so-called Schreger's bands as the marks of directions of rods bundles. Results were summarized on a schematic illustration of Fig. 3, which showed the directions of rods in bucco-lingual and mesio-distal sections for each tooth kind. This will make a good practical reference for cavity preparation.
The biting forces of the lower first molars and the grasping forces were measured using they newly designed spring type gnathodynamometer and grasping force measuring appartus on 519 subjects. Subjects were in age of 9, 10 and 15 years in male and female, 12, 13, 14 and 16 years in female and 16, 17, 21, 22 and 23 years in male. Following results were obtained: 1. The biting forces in both sexes increase with age gradually till 15 years, but in older than them are nearly constant. The grasping forces increase also till 17 years, but in older than them are nearly constant. 2. The biting forces are nearly equal in both sides of the jowl however, the grasping force of the right hand is considerably greater than that of left hand. These differences are approximately 10 per cent in male and 25 per cent in female. 3. Histogram of biting force are skew, because the limit of measuring powers of the apparatus is 52 kg. The scatter of grasping force about the mean increases with age and male's is greater than female's. 4. The regression lines and correlation coefficients between biting-and grasping force are analysed on 10 years boys and girls. The regression lines for girls are significant, but are not for boys.