Freiburger historische Bestände - digital

50

Cunningham Memoirs.

little younger, in which the "stem" alone is present, and there are no
signs of the posterior calcarine sulcus.

The union of the intermediate piece of the posterior calcarine sulcus
with the " stem " or anterior calcarine fissure is represented in figs. 27 and
28, PI. in. It usually occurs in the early part of the seventh month of
intra-uterine life. In some cases, however, as we have noticed (2'3 per
cent.), the connexion is never established, and the gyrus cuneo-lingualis
anterior remains on the surface. The union of the two pieces of the posterior
calcarine sulcus takes place at a period somewhat later. The hinder
part forms little more than the forked extremity of the sulcus; and this is
not as a rule joined on until the eighth or ninth month of intra-uterine life
is reached. We have already noticed how extremely common it is in the
last month of foetal life to find the hinder end of the sulcus still separate,
and the posterior cuneo-lingual gyrus on the surface. Indeed it is no
rare thing to meet with this as a permanent condition. More especially
is this the case in the negro brain. In half of the negro hemispheres which
were examined it was present.

This interrupted mode of development of the posterior calcarine sulcus,
viz. by isolated portions which ultimately run into each other, is very frequently
observed in connexion with other permanent cortical furrows.
Thus, the intra-parietal, prsecentral, calloso-marginal sulci, the fissure of
Rolando, and many others, afford examples. The posterior calcarine
sulcus must therefore be included within the same category; and this
relationship between the posterior calcarine sulcus and the fissures we
have named is rendered all the more evident by the fact that in early
stages of its development sections through the occipital lobe show that the
depressions which represent the sulcus produce no corresponding elevations
on the ventricular aspect of the hemisphere wall. The posterior calcarine
sulcus, therefore, is not a complete fissure, and from a developmental point of view it
is quite distinct from the anterior calcarine fissure or u stem" which is a complete
fissure, and, as a rule, shows an unbroken continuity of existence with the
fore-part of the early calcarine precursor.

In the ape, as we have already seen, the secondary posterior calcarine
sulcus is apparently absolutely unrepresented. The calcarine fissure of the