Measurement of the mandibular movement in the free directions has been one of the major problems in the field of dentistry, to which various approaches were attempted by many investigators. To date, however, no adequate method of measurement in this line is yet available. This is presumably because of the inconveniences encountered in the procedure that the mandible takes not only three dimensional, but also apparently irregular directions in its movement, and further that any tooth to serve as target of measurement is normally concealed from the view inside the mouth not to permit direct observation. The present author has devised a new method of measurement in which the movement of the mandible was taken on the same movie films from two right angled directions, utilizing Dr. Yokota's cephalometric photo-apparatus. By this method, most of the inconveniences hitherto experienced are believed to be successfully eliminated. The outline and main features of the method are summarized as follows. 1) The basis for measurement of the mandibular movement was laid on the maxillar bone in the present method for the purpose of obtaining true value of the movement. This is in contrast with all methods heretofore adopted, except for ths simple tracing method. Since the basis for measurement was placed outside of the subject in those methods, there was always danger of counting any incidental sway of the head into that of the mandible. This was proved true in one of the preliminary experiments where the maxilla of a subject as measured on the suborbital point was dragged upward reflexly in vertical distance by 9.0mm, when he was ordered to open his mouth as wide as possible, even though his head was seemingly kept in a fixed position. 2) High accuracy of measurement could be expected with this method as far as the enlarging capacity of the film could afford. In the present experiment, the pictures obtained were enlarged by two-fold to secure the accuracy within 1/10mm, but the higher accuracy may readily available by increasing the rate of enlarging. 3) Another feature of this method is that direct observation of the mandibular movement can be realized from outside of the mouth, regardless of whether the portion to be measured lies hidden in the mouth or in tissues. This was rendered possible in the present method, by placing a marker outside the mouth which is directly connected to the portion concerned, and by tracing its movement successively with the movie films. 4) There was attached neither a marker nor any cumbersome appliance on the target point to hinder the natural movement of the mandible during the experiment, and this also ensured true observation. 5) The progressive movement of the mandible can be directly recorded in the films three-dimensionally at every desired moments of time, and the pictures simultaneously projected on both frontal and sagittal planes allow to be analyzed three-dimensionally. Using this apparatus, a series of preliminary measurement was conducted on the movement of the mandible, taking the distal angle of lower left central incisor and the mesial buccal cusp tip of lower left first molar as the targets. Since main object of the present study was to make sure whether or not the apparatus could work sufficiently well to yield desired efficacy, gross directions of the mandibular movement such as those occurring in the maximum opening of the mouth and the bilateral movement of it were studied for the sake of clarity. These studies have clearly shown that the pictures obtained from tracing the marker of the apparatus accurately represented the three-dimensional movement of the target on the mandible. Although the loci produced in the maximum opening of the mouth were similar with both reciprocal pathways in the general pattern, they were not identical in minute detail, and showed the largest divergence at the middle portion of the stroke. Thus the

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