Freiburger historische Bestände – digital

42

Cunningham Memoirs.

bottom at any point so as to connect its opposite banks with each other.
Further, when we study the relation which this fissure presents to the
calcar avis, we observe that the entire length of this ventricular eminence
corresponds with that portion of the sulcus which lies opposite to it. Such
being the case are we to conclude that the calcarine fissure in the ape
represents both " the stem " and the posterior calcarine fissure of man; and
if not, of which part is it the equivalent ? I am of opinion that it only
represents the "stem;" and I believe that I shall be able to prove this
assertion when we come to deal with the development of the fissure in the
human brain. The entire length of the precursor of the calcarine fissure in
the early human foetus is the equivalent of the calcarine fissure of the ape;
but in man, the posterior portion of this becomes obliterated, whilst the
anterior part is retained as the " stem." In the place of the portion which
is abolished, and at a much later date, the posterior calcarine sulcus is
formed.

The remarkable depth of the calcarine fissure in the apes—a depth
which appears to be relatively greater in low apes than in the high apes—■
is thus easily accounted for when we consider that it is formed by a
retention of the whole length of its precursor, or, in other words, that it is
developed as a complete fissure from one end to the other.

In the orang,* chimpanzee,"]* and gibbon,J and probably also in the
gorilla, there appears to be a tendency towards the assumption of a condition
which approaches somewhat towards the human arrangement. In
many cases the calcarine fissure is shortened. It is then confined entirely
to the mesial and tentorial surfaces of the hemisphere, and does not
extend beyond the upper border to the outer surface. Its hinder extremity
turns upward, and ends in a forked extremity (fig. 21). This condition
presents a striking similarity to that which we have figured in page 39
(fig. 18), and which occurred in four human brains. In these the " stem"

* Bischoff—" Ueber das Gehirn ernes Orang-outan," Sitzung der Math.-Phys. Classe vom 17.
Juni, 1876. K. bayer. Akad. Vide PL it., fig. 6.

f Bischofi:—Ueber das Gehirn eines Chinrpanse," Ibid., 4 Pebruar, 1871. Vide fig. 4.

X Bischoff—" Beitrage zur Anatomie des Hylobates leuciscus." Aus den Abhandlungen der
k. bayer Akad. der "W. ir. CI. x. Band. in. Abtb. Vide PI. n., fig. 4.