Department of Anatomy, Nippon Medical Schoo
Measurements were taken from three directions to determine the sites of the prominent points of the semicircular canals in 26 half-crania of 13 individuals using methods directly applicable clinically to gain exposure. The four standard planes used in the measurements were the horizontal Frankfort (O. A. E.), coronal porion (P. F. E.), porion sagittal (P. S. E.) and median sagittal plane (M. S. E.). The results are as follows.
1. The prominent points of the lateral and superior semicircular canals lie on almost the same coronal plane passing through the point suprameatal spine, although the plane on which the prominent point of the lateral semicircular canal lies, is on an average a little more than 1mm before that of the superior semicircular canal.
2. The prominent point of the lateral semicircular canal is approximately 15mm medial to the suprameatal spine and a little over 3mm the horizontal plane through the suprameatal spine.
3. The prominent point of the superior semicircular canal lies approximately 19mm medial to the suprameatal spine and about 10mm above the horizontal plane through that point
4. The prominent point of the posterior semicircular canal lies on the coronal plane 4mm behind the suprameatal spine and is 17mm medial, 2mm above the horizontal plane through that point.
5. Regarding the position of the suprameatal spine to the plane O. A. E., the three prominent points of the semicircular canals are situated rather superiorly when the position of the suprameatal spine is above the plane O. A. E. The sites of the prominent points do not appear to bear any relation to the anterior or medial position of the supremeatal spine.
6. In the triangular plane formed by connecting the three prominent points of the semicircular canals, the distance between the points of the posterior and superior semicircular canals is the greatest.
7. The triangular plane formed by connecting the three prominent points transects the horizontal plane in a lateral and slightly posterior direction and forms an outer angle of 114°45' on the average.
8. For determining the three prominent points of the semicircular canals clinically, the line oonnecting the more stable prominent points of lateral and posterior semicircular canals may be taken as the base line and the prominent point of the superior semicircular canal can be fixed at a position about 9mm above the plane O. A. E. and 1mm posterior, 10mm superior and 19mm medial to the suprameatal spine at an angle of 114° medial and slightly anterior to the base line.
9. The prominent point of the superior semicircular canal is found to be higher than the lowest part of the bulge of the tegmen tympani in specimens when the suprameatal spine is above the plane O. A. E., and the distance from the operative site to the anterior wall of the sigmoid sinus is less than 5mm. Consequently, the temporal bone in such instances would be considered as dangerous for operative procedures.
10. The floor of the internal auditory meatus and the internal auditory foramen lie approximately on the horizontal plane passing through the prominent points of the lateral and posterior semicircular canals. They are situated also in a relatively higher position when the suprameatal spine is above the plane O. A. E.
11. The anterior small portion of the jugular foramen lies on the coronal plane that passes through the prominents of the lateral and superior semicircular canals and the posterior large portion on the coronal plane that passes through that of the posterior semicircular canal. Those portions also situated somewhat higher when the suprameatal spine is above the plane O. A. E. and the diameter of the foramen is greater.
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