Freiburger historische Bestände - digital

36

Cunningham Memoirs.

But occasionally one or other of these annectant gyri come altogether
to the surface, so as to constitute a complete break in the continuity of the
fissure. It is the posterior cuneo-lingual gyrus that most frequently shows
this condition ; and. the natural effect is that the posterior forked extremity
of the posterior calcarine fissure is cut off from the fore portion, and forms
an isolated short vertical sulcus on the occipital pole, and at the bottom
of the venous groove in the case of the right hemisphere. This occurred
in no less than 31 "5 per cent, of the hemispheres examined. This variety
was particularly common in the negro brain, and in the foetus.* It was
noticed in 50 per cent, of the former and 60 per cent, of the latter. We
shall have occasion to allude to this again when we come to discuss
the development of the calcarine fissure.

In five cases only, or in other words in 3'9 per cent., did the gyrus cunei
reach the surface, and completely cut off the parieto-occipital fissure from
the stem (PI. iv., figs. 6 and 7, c.); and it should be noted that in each of
these, other peculiarities were present. In three cases (2*3 per cent.), and
no more, did I find the anterior cuneo-lingual gyrus on the surface, and
the posterior calcarine fissure quite separate from the " stem."

I have only once seen the gyrus inter-cuneatus reach the surface in
the same way as the others. Sernoff, of Moscow,t has also figured such a
case.

Of course many different combinations of the various conditions of
this fissural system which we have described above are possible. That
which I have noted as normal, or in other words, that which is most frequently
seen, is the arrangement in which all the annectant gyri are present,
but concealed at the bottom of the fissure. The combination which comes
next to this in point of frequency is that indicated in the accompanying

* In connexion with this it is interesting to note that in the two Fuegian brains described
by Seitz, the hinder part of the posterior calcarine fissure is represented as being separate and
distinct in three out of the four hemispheres. " Zwei Feuerlander-Gehirne" ; Zeitschrift fur
Ethnologie, 1886. See his figures 3, 7, and 15.

f " Individual Types of the Convolutions of the Human Brain" : Moscow, 1877, p. 71, fig. 69.
The"sanie author has also figured a brain in which the posterior cuneo-lingual gyrus is on the
surface (p. 71, fig. 70); and another in which the anterior cuneo-lingual gyrus is on the surface
(p. 72, fig. 72).