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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 33

later on, when we come to deal with the development of these three parts.
In order to pave the way for this, as well as to render certain obscure
points clear, it is necessary to examine closely the condition which is
present in the adult brain.

When we open up the entire extent of the >- -shaped fissure in the adult
brain, so as to obtain a continuous and complete view of its bottom
(PI. iv., fig. 5), we observe that the "stem" is much the deepest part.
Further, we note that it is not directly continuous with the parieto-occipital
branch. A high, deep annectant gyrus (gyrus cunei of Ecker, or inferior
internal u pli de passage" of Gratiolet) extends from the apex of the cuneus
across the bottom of the lower portion of the parieto-occipital fissure to
the opposite bank, which at this point is formed by the gyrus fornicatus.
Frequently this deep gyrus cunei as it runs forward divides into two smaller
gyri. Of these, the lower goes to the gyrus fornicatus and the upper to
the lower part of the precuneus. The deep gyrus cunei forms then a barrier
between the " stem " and the parieto-occipital limb (PI. iv., fig. 5, c. 1, c.2).

As a general rule, the "stem" runs backwards for a short distance in
the calcarine branch—forming about half an inch of its fore-part—but soon
it is arrested by a deep annectant gyrus, which extends from the apex of
the cuneus, across the bottom of the calcarine fissure, to the opposite bank
formed by the gyrus lingualis. This annectant may be termed the anterior
cuneo-lingual gyrus (PI. iv., fig. 5, a. I.).

The " stem " varies in length, but as a rule it will be found to be about
40 mm. long. The facts stated above in regard to it may be put briefly
thus: the apex of the cuneus gives off two deep annectant gyri, viz. the
gyrus cunei and the anterior cuneo-lingual gyrus. The "stem" by its
hinder end is carried into the cuneate lobe between these gyri, whilst
by its anterior end it cuts into the gyrus fornicatus. The gyrus cune-
forms a barrier between the parieto-occipital fissure and the '* stem,"
whilst the gyrus cuneo-lingualis anterior is interposed between the " stem"
and the posterior part of the calcarine fissure.

For reasons which will be afterwards evident we may term the " stem "
the fissura calcarina anterior, and the portion of the calcarine fissure
behind this the fissura calcarina posterior.

ROYAL IRISH ACADEMY.-CUNIsLNGHAM MEMOIRS, NO. VII. [5]