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34

Cunningham Memoirs.

The posterior calcarine fissure cannot properly be regarded as extending
further forward than the anterior cuneo-lingual gyrus. Not only is this
seen to be the case in the adult brain when the fissure is freely opened up,
and the bottom exposed along its whole length (PL iv., fig. 5, c. C"); but
it is still more forcibly brought under our notice when we study the
development of the fissure. Its length varies with that of the " stem,"
or anterior calcarine fissure, because the anterior cuneo-lingual gyrus is
not by any means constant in its position, and as it slides forwards or
backwards it affects the length of the two portions of the fissure in question.
As a rule the posterior calcarine fissure is shorter than the " stem" and
we may give its average length as being about 35 mm. Further the
posterior calcarine fissure is much shallower than the "stem" or anterior
part; and it is divided into two parts by a deep annectant, which
traverses its bottom, and connects the cuneus with the posterior part of
trie gyrus lingualis. This annectant may be termed the posterior cuneo-
lingual gyrus (PI. iv., fig. 5, p. I.). The part of the calcarine which it cuts
off comprises little more than its hinder forked extremity.

The parieto-occipital fissure is deeper than the calcarine, but it is, as a
rule, not so deep as the " stem." In what may be considered the normal
arrangement, a feeble deep annectant crosses it at the junction of its upper
and middle thirds, and connects the cuneus with the precuneus (fig. 12, i.e.).
In addition to this its opposing walls frequently present interlocking gyri.

In the parieto-occipital and posterior calcarine fissures the opposing
walls are vertical; the fissures, therefore, incise the cerebrum at right
angles to the surface. This is not the case, however, with the " stem." It
cuts obliquely into the hemisphere in such a manner that it undermines
the gyrus lingualis, which therefore comes to overlie the gyrus cunei
and the apex of the cuneus.

The arrangement (fig. 12) which. I have described above may be
regarded as being that most commonly present. It is by no means,
however, the only arrangement that may be met with. In 127 hemispheres
(including ten negro brains, a considerable number of children's brains,
and fourteen brains from foetuses about the eighth month of development
) it was present 43 times or in 33*8 per cent.