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54

Cunningham Memoirs.

and a "stem" or anterior calcarine fissure. Further we have seen that
occasionally in the adult brain the cuneate gyrus is absent and the " stem "
and the parieto-occipital are directly continuous. Are we to regard the
"stern" in these exceptional cases as being formed by the parieto-occipital?
Strong as the evidence is in favour of our adopting this view, I find it
difficult to do so. I am inclined to believe that the stem is formed by the
anterior part of the calcarine in every case, and that a very complete union
of the precursors of the two fissures with an obliteration of the calcarine
precursor up to the very point of junction is to be regarded as giving the
true explanation of the exceptional cases I have quoted.*

Certainly upon phylogenetic grounds we might infer that the stem
belongs solely to the calcarine fissure. In the anthropoid apes, as we have
noted, the gyrus cunei is usually on the surface.

IX. Topography of the Parieto-occipital fissure.—I have endeavoured
to determine the topography of the parieto-occipital fissure from two points
of view—(1) in so far as this relates to its relative position on the mesial
border of the cerebral hemisphere; and (2) in so far as it relates to its
position with reference to the cranial wall.

In determining the position of the fissure on the upper border of the
hemisphere, I have taken as the starting-point of my measurements the point
at which it reaches this margin, and have therefore disregarded altogether
the inclination of the external parieto-occipital fissure. Further, in those
cases (and they are not few) in which the parieto-occipital fissure bifurcates
before it reaches the cerebral margin, I have chosen neither of the terminal
off-sets as the point to which I should measure, but have drawn an
arbitrary line upward in the direction of the lower part of the fissure, and
inserted a pin into the mantle-border at the point at which tins line reaches
it. This point I have taken as indicating the anterior limit of the occipital
lobe on the upper margin of the hemisphere. The points between which I
determine the mesial length of the upper border of the cerebrum I have
already sufficiently defined in the introductory chapter of this Memoir.
The different measurements which I have taken I have reduced to indices,

* I am aware that this statement is not altogether in keeping with the views I have
previously expressed on this suhject, hut a more extensive study of the question, and a con-
siderahle increase in the number of specimen is at my disposal, have caused me to alter my
original opinion.—Journal Anat. Phys., vol. xxiv.