Some investigations have been made on the morphology of the mandibular fossa from radiological as well as anatomical viewpoints, but suffered from the lack of appropriate means of analysis.In the course of radiological study of the growth of the mandibular fossa, we therefore attempted to express the shape of the fossa as revealed by lateral radiograph as a single function using Fourier Progression.
It turned out that this method had sufficient reproducibility;i.e., the function derived was capable of reproducing a curve indistinguishable from the original one in the radiograph.Since such functions are convenient for mathematical analysis, this method will find a wide use in radiology.

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The response of a dental film to X-rays is represented by the characteristic curve (the H-D curve) which was originally introduced to study the response of a film to light.In this paper, another method of expressing the relationship between X-ray exposure and the resulting density in non-screen system is used, in which density is plotted against exposure on a linear scale.
Fuji dental film and Kodak Morlite film were directly exposed to X-rays generated from a dental X-ray apparatus (Tokyo Engine Co.Type R-802) operated at tube voltage 60kVp, HVL 1.5 mm Al.
Exposure-density relationship was approximated by the following equation:
D=a (1-e^-bx) (1)
where D is density, x is exposure and a, b are constants.This equation became
D=abx (2)
in a low exposure range.Eq. (2) has the linear relationship between exposure and density.
Eq. (1) was fitted to the curve for Fuji film and the value of a and b were obtained a=2.82, b 3.30.Eq. (2) was fitted to the curve for Kodak Morlite film, resulting in a=9.06, b=1.44.When eq. (1) was used for a single emulsion layer of Fuji film, a or b was 1.37 or 3.37 respectively.Only the value of b was significantly different for 90kVp, HVL 2.3mm Al;a=2.85, b=3.62.Therefore, a was proportional to the number of silver halide grains per unit area of a film and b, the probability of a X-ray photon absorption.
By differentiating the eq. (1) and (2), exposure-photographic contrast relationships were derived.
dD/d log₁₀X=2.30abxe^-bx (3)
and dD/d log₁₀X=2.30abx=2.30D (4)
When these equations were graphically represented, the curve for Fuji film expressed by eq. (3) was bending and the curve for Kodak film eq. (4) was straight.
Thus both the sensitivity and the photographic contrast of a film are the larger for Kodak film, which has the larger value of the product ab.In general, the larger the sensitivity, the larger the photographic contrast when exposure-density relationship can be approximated by eq. (2).

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The results of the fundamental studies on the mechanism of image formation and distortion of image associated with rotatary motion of the focus and film were reported.
The materials used in the study were as follows: Orthopantomogrpah^®, phantom similar to the dental arch in cross section on which lead wires were placed in a regular intervals, ichnograph in which the main parts of apparatus, position of the phantom, centers of rotation of x-ray beam and the orbit of secondary slit were laid down and a simulator as the illustrative apparatus of geometrical relations among the focus, object and film in movement.
The principle of the image formation in Orthopantomography was well comprehended by the relativity in velocity of rotation of the film and of the beam of x-rays at passing through the object (lead wire) positiond at a given distance from the center of rotation of the beam.
In the Orthopantomograph^® (Palomex) the velocity of the film was regulated by the velocity of x-ray beam on the orbit of the slit and the ratio in them was kept constant, if the rotative center is fixed for an elementary unit of time.
Therefore, it was the necessary condition for the image formation that the object must be positioned at the definite distance from the center by which the velocity of beam on the object or the length of pass way of beam on the object per unit time was kept equal to the velocity of film or the length of film shifted per unit time through the slit.
The discrepancy between the theoretical and actual size or shape in image might be found, if that necessary condition was not be satisfied because of the wrong position of object and this discrepancy might be enlarged by the width of the slit.
From these, the blur of orthopantomographic image was considered proportional to the width of slit S and (R/F×A-1), where R is the velocity (sec/cm) of x-ray beam on the orbit of slit, F is the velocity (sec/cm) of film in its own movement and A is a variable concerning the length of pass way of x-ray beam on the object when it is radiographed.
Values of R and F and change in A according to the position of lead wire were estimated by measuring on the ichnogram, by the simulator and under the drived condition of apparatus for so-called N and S position.
The results showed that the blur of the image of lead wire at the position corresponded to molar and premolar regions was small and was large in incisor region.Using the calcula-ted values of which put (R/F-1) equal to zero, the adequate position of phantom was obtained at which the objects (lead wires) would be radiographed without blur.
Then, the examinations were made on the changes of blur by shifting the phantom forwards and backwards from that position and the effects of width of the secondary slit on the blur were studied by the experiments and calculation.
From these results, the image formation and distortion of it were illustrated by the relativity in the velocity of the film and x-ray beam and the width of the secondary slit.
Furthermore, the reason of the appearance of the cervical vertebra on either side of the orthopantomogram was examined in the geometrical relation among the focus, center of rotation and object by using the phantom and simulator.
The differences in film density which was relatively low at the marginal portion of orthopantomogram and high at the middle portion were observed.The film inserted horiz-ontaly in Phantom (KSS, WPEH-I) was exposed experimentally by Or thopantomograph^® and the spatial distribution of film density was obtained.The differences in film density were caused by the variation in exposure associated with the changed position of the center of rotation of x-ray beam and the change in velocity of beam of x-rays.

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The authors reported a case of accessory parotid gland.A 19 year-old girl had been suffering from the recurrent swelling on her left cheek and repeated pain since one and a half years.
The sialograms revealed the presence of accessory parotid gland of the thumb-head size solitary, which was connected to Stenon's duct.Its feature showed triangular shape with particlely scattered contrast medium, which appeared its irregular pattern.Three days after taking sialography, orthopantomographic survey could detect remaining contrast medium in this accessory parotid gland, but its corresponding view disappeared.
The extirpated specimen was of the size, 18×14×10mm, yellowish-white, elastic hard, and its hemisected surface disclosed lobulated appearance.
Histopathological diagnosis rendered it as chronic inflammation of accessory salivary gland.

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