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322

Cunningham Memoirs.

any given case makes it absolutely necessary to study the relation of both
to some known point. That usually chosen is the parietal eminence. The
centre of this can readily enough be ascertained as the extremity of a line
joining the maximal transverse diameter through the eminences of each
side. Under these circumstances it is found that the ending termed (a) lies
just in front of the centre of the parietal eminence, whereas the ending
termed (£) lies in a line drawn at right angles to the mid-line through the
parietal eminence, and at a variable distance below the latter, often in
merely the line of direction of the posterior limb.

For the present nothing more definite can be stated of the topographical
localization of the termination of the limb.

(B) Fissuee of Rolando.—Of all the fissures most attention in topography
has been paid to that of Rolando. In accordance with what has been stated
in the introductory section of this chapter it is obvious why this should
have been the case. Before directly examining the facts of anatomical
and topographical relationship of the fissure to surrounding structures
it is essential to dwell for a moment on certain features already partly
described by Prof. Cunningham. Of these the first are the characteristics
of the course taken by the fissure.

The upper end of the fissure cuts the mesial margin of the hemisphere,
and turns backwards and downwards, in part forming the upper limit of,
in part subdividing, the paracentral lobule. For topographical purposes
the point at which the fissure cuts the margin of the hemisphere is taken to
be the " upper end " of the fissure, and is the very important point on which
so many measurements have been taken.

The course of the fissure, namely, its sinuous line of direction is
according to most observers, marked by two bends, spoken of respectively
as the superior and inferior genu. Of these the latter should be topographically
regarded as of immense importance. This is dwelt upon in the
introductory remarks (also on p. 334), and need not be repeated ; but in
special confirmation of what is therein stated are the observations of Prof.
Cunningham and of Eberstaller, given on pages 173-4, respecting its
developmental history. It will therefore be specially necessary to consider,
later on, the topographical relations of the inferior genu.