This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
In operating on the skull for middle-ear disease, it is desirable to know how to reach and how to avoid the brain and lateral sinus. The lower level of the brain in the region of the ear corresponds to a prolongation directly backward in a straight line of the posterior root of the zygoma. If one keeps below this line, he is not likely to open the brain case. If it is desired to explore the under surface of the brain or dura directly over the middle-ear cavity, then one trephines above this line or suprameatal crest, the lower edge of the trephine opening being .5 cm. above it. This will lead to the middle fossa of the skull, occupied by the temporosphenoidal lobe. The sharp upper and posterior edge of the petrous portion of the temporal bone gives attachment to the tentorium and separates the middle cerebral fossa in front from the posterior fossa, containing the cerebellum, behind. The point at which this ridge and tentorium reach the side of the skull is indicated by the point of crossing of a line drawn up from the tip of the mastoid process, midway between its anterior and posterior borders, and the line of the posterior root of the zygoma. The course of the lateral sinus is indicated by a curved line from above and to the right (about .5 to 1 cm.) of the external occipital protuberance to the upper posterior portion of the mastoid process and thence to its tip. The anterior edge of the lateral sinus reaches as far forward as a line drawn from the tip of the mastoid upward, midway between its anterior and posterior borders. The point at which it turns is where this mastoid line intersects the line of the zygoma. Its upper edge rises above this line approximately 1 cm. The sinus is I cm. in width. The distance of the sinus from the surface varies from .5 cm., or even less, at the top of the mastoid process to 1.5 cm. at its tip. So uncertain is this that the only safe way to expose the sinus is to cut the bone off with a mallet and gouge in thin chips parallel to the surface. The use of a trephine or other boring instrument is not to be advised. If the infection of the lateral sinus has extended to the jugular vein this latter must be reached by means of a separate incision in the neck.
Fig. 111. - Transverse section of the right side of the skull just behind the mastoid process; looking forward.