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Objective: To propose another two new standard lines for the external base of the skull which pass across almost all significant foramens, for easier observation and to remember the sites of the foramen.
Methods: 50 Thai dry skulls 24 males and 26 females were observed from the external base of skull. All of them were photographed and the imaginary oblique lines were drawn by the Photoshop program. 2 lines were drawn, firstly the medial oblique line and secondly the lateral oblique line.
Results: The medial and lateral oblique lines, 50 right and 50 left sides of Thai skulls were passed along the same canals and foramens. There was no significant difference between the 2 sexes. These could be proposed to be standard lines. Firstly, the medial oblique line extends from the tip of the mastoid process medialward to the alar of vomer. This imaginary line passes across the stylomastoid foramen, the lateral border of the jugular foramen, the carotid canal, the foramen lacerum, the pterygoid canal and the alar of vomer. Secondly, the lateral oblique line extends from the anterior margin of the base of mastoid process medialward to the medial pterygoid plate. This imaginary line passes across the external auditory meatus, the mandibular fossa, the spine of the sphenoid bone, the foramen spinosum and the foramen ovale.
Conclusion: We proposed another 2 imaginary lines at the external base of the skull for better alternative landmarks in finding the nerves and blood vessels that pass through the foramens in and out of the skull. The previous imaginary planes, the anterior and posterior transverse lines do not pass across the carotid canal and foramen spinosum. Therefore, we can alternatively use the medial and lateral oblique lines as landmarks to find almost all the important structures such as the facial nerve, the common carotid artery etcetera.
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