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328

Cunningham Memoirs.

The general direction and the topography of the lower end of the Fissure.—-It
will be advisable to deal with the above-named points before commencing
to discuss the more important, but more difficult, special localization of the
points in the course of the fissure, notably the inferior genu.

It will, of course, be obvious that whereas the upper end of the fissure
may be reasonably easily fixed, it is quite otherwise with the lower end.
To gain the topography of this point is a sine qua non. It can be accomplished
with accuracy in one of three ways—either (1) the angle which
the fissure of Rolando makes with the middle plane of the body having been
measured, and the site of the upper end known, the true general direction
of the fissure is found, and having been drawn on the surface of the head,
and having been terminated just above (2—5 mm.) the fissure of Sylvius,
the terminal point thus obtained is the position of the lower end; (2) the
lower end of the fissure can be estimated by determination of the relative
indices which express the distances between it and the neighbouring sutures;
these calculations are then confirmed by making absolute measurement of
the distances in question; (3) by Zernoff's encephalometer.

(1) The determination of the lower end of the fissure of Rolando by finding the
general direction of the fissure.—Hare, having previously shown that the
average angle made by the fissure of Rolando with the middle line was
67°, it occurred to me that a plan based on this fact would secure the
localization of the lower ends of the fissure, that of the fissure of Sylvius
being well known. For this purpose I had constructed a simple brass
instrument, consisting of two narrow strips of metal fixed at an angle of
67°, and marked in inches. It was then easy, by placing this on the head
with the j ancture of the parts corresponding to the previously determined
locality of the upper end of the fissure, to draw the line of the fissure with
considerable accuracy, and so denote the lower end. Wilson, at the instigation
of Chiene, improved very greatly the form of my instrument, but
substituted for the exact measurement of the distances required in the
mesial plane, Hare's proportional scale, the graduation of which was
indicated, not by figures, but by letters, of such character that if the position
of the inion corresponded to a capital letter, the upper end of the Rolando
was opposite to a small letter of the same name. This plan, although