Freiburger historische Bestände - digital

52

Cunningham Memoirs.

ventricular wall is gradually obliterated as the hemisphere-wall thickens.
In the adult brain the fissure reaches down to the ventricular cavity, and
no further. The portion of the hemisphere in the adult with which it is
in relation is solid, and not hollow.

The connexion which exists between the parietooccipital and calcarine
fissures in early times is a matter of importance. Many of the cases
which have been described as instances of the presence of the parietooccipital
fissure in the early hemisphere and of the absence of the calcarine
fissure are upon closer study seen to be capable of receiving a totally
different interpretation. There are several hemispheres of this kind
figured in PI. in. (vide figs. 9, 12, 16, 26, &c). Certainly on superficial
inspection, the elongated fissure on the inner surface of the posterior part
of the hemisphere appears in these cases to be the representative of the
parieto-occipital, and of it alone. In most cases, however, a slight backwards
projection placed a short distance below the mid-point of this fissure
indicates that above this point we are dealing with parieto-occipital, and
below this point with the anterior calcarine fissure, or the " stem." If the
lips of the fissure be now opened up, a feeble interruption is seen at the
bottom of the furrow immediately above the slight backwards projection.
This is the primitive gyrus cunei, marking off thus early the parietooccipital
fissure from the " stem."

As a rule, however, there is no difficulty in recognizing the parietooccipital
fissure as a distinct furrow, and as one more or less separate from
the " stem " or anterior calcarine fissure. In figs. 7, 8, 10, 13, 17, 21, 23, 29,
&c, PI. in., this is evident; and the distinction is more especially manifest
in those cases in which the permanent parieto-occipital fissure is developed
secondarily after the abolition of its precursor. Of course, in the event of
the secondarily formed parieto-occipital fissure failing to unite with the
"stem" or anterior calcarine fissure the gyrus cunei remains on the surface,
as is usually the case in the anthropoid apes. In the adult human brain,
however, we have seen that this is a very rare occurrence (3*9 per cent.).

The parieto-occipital fissure, in its upper part, at first assumes the form
of a wide open furrow which extends upwards towards the mesial border
of the hemisphere, and finally forms a broad notch upon it (PL in., figs. 28