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Mr. Victor Horsley—Cranio-Cerebral Topography.

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folds (Cunningham). It may also be easily shown that this region is
a distinct acme in the course of the lateral sinus where it arrives about
opposite the middle of the cerebellar hemisphere.

From this point the margin of the hemisphere trends downwards over
the cerebellar hemisphere, and then curves round upwards to the Torcular
Herophili. On the right side where the mesial margin meets the lateral
margin just traced, there is a well-marked groove (Bastian), which cuts
deeply into the hemisphere, and is produced into the lateral margin a
certain distance, being caused by the prominence of the lateral sinus.

To trace, now, the lateral margin of the hemisphere on the exterior of
the skull, as we have just followed it on the inner surface, is not so easy.
The point of chief difficulty is to determine the lowest point at which the
lateral margin crosses the middle fossa. Professor Cunningham has long
devoted much attention to this question, and has prepared nineteen preparations
from which I have obtained the following facts.

The preparations referred to were made as follows:—After simple
removal of the hemispheres the cranial cavity was rilled with plaster. Pins
were then driven through the skull in the lines of the principal sutures and
along the upper margin of the zygoma. On removal of the cast from the
interior of the skull the above-mentioned lines were distinctly pricked out
upon it. Now, since the point at issue is the amount of the temporal lobe
which lies so deeply as to be below the level of the upper border of the
zygoma, I made the following observations from the above-mentioned
specimens :—

The plane of the under surface of the temporal lobe being easily taken
by a plate of glass laid across both lobes, the distance from the ptero-
squamal junction to this plane was first measured with compasses, styled
Pp., and represented, of course, the vertical depth of the temporal lobe at
the commencement of the posterior limb of the Sylvian fissure. From the
same sutural point the distance between it and the zygoma (upper border
line) was measured and termed Pz. The difference between Pp and Pz
gives the total depth (vertical) of the temporal lobe, which is below the
upper border of the zygomatic arch. The measurements are stated in the
accompanying Table.