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310

Cunningham Memoirs.

this point it runs forwards at rather less than a right angle to the plane
of the orbital roof (i. e. in other words, the plane of the inferior surface of
the frontal lobe), to form the outer border of the tip of the temporo-
sphenoidal lobe where it rests against the posterior portion of the inner
surface of the great wing of the sphenoid. The margin of the hemisphere
here bends outwards, separating the lateral surface of the temporal lobe
from the front aspect of its tip. The line taken by the margin next passes
downwards and backwards immediately in front of the course of the middle
meningeal artery, and finally runs just outside and above the same where it
is entering the skull through the meningeal foramen. From this foramen,
which lies at the deepest part of the middle cranial fossa, the line of the
margin of the hemisphere ascends over the petrous bone where this latter
forms the squamous bone. At the highest level of the petrous bone in this
function (i. e. the roof of the tympanum), the margin of the hemisphere
presents the commencement of the groove which is so obvious on the under
surface of the temporo-sphenoidal lobe, and which is simply due to the
moulding of the hemisphere over the prominence caused by the long labyrinth
. This notch or groove is not noticed in text-books of anatomy,
but in consequence of the roof of the tympanum being not infrequently
perforated by disease opposite this spot, it assumes a certain importance.
I may add, in passing, that it is marked in the new-born infant, the
anthropoids, and can be detected in the lower apes, Macacus rhesus and
M. sinicus. The margin of the hemisphere then sinks gently as it passes
backwards, and descends to the level of the anterior border of the
tentorium; from this spot it shortly rises again, and here the lateral
margin is notably indented by a fold of dura mater, raised, apparently,
by the temporal veins entering the lateral sinus. This indentation forms
a most constant and important notch in the border (and partly the inferior
surface) of the temporo-sphenoidal lobe, and, indeed, naturally marks the
separation of this lobe from the temporo-sphenoidal. It has received the
name of the prse-occipital or Schwalbe's notch, and, besides being
produced by the tension (Cunningham) of the veins referred to, is caused,
in part, by the growth of the skull at the asterion (which is just a little
lower, but in a corresponding line), drawing the dura mater into salient