Freiburger historische Bestände - digital

58

Cunningham Memoirs.

In regard to the relative length of the parietal lobe, measured along the
mesial border, there is little difference between the high and low apes.
Indeed in this respect it is not the anthropoid apes that stand nearest to
man. The baboon and the mangaby show the highest parietal index, or,
in other words, the greatest antero-posterior length of the parietal lobe.
It is curious to note that it is the latter which exhibits the closest
approximation to man.

But the study of the occipital indices will show that the reduction of
the parietal indices is due to very different causes in the high and low
apes. In the chimpanzee and orang the parietal lobe is reduced in mesial
length, not so much by an increase in the occipital region as by a considerable
increase in the mesial frontal region. The occipital length in the
chimpanzee and orang is certainly greater than we find it in the adult
human brain, but not much greater than in the child of four or five
years old.

The low apes stand out in marked contrast with the high apes in this
respect. The parietal length is reduced entirely by the greatly increased
occipital length. The frontal length is also reduced as compared with
that in man and the anthropoids. The mesial length of the occipital lobe is
indeed remarkable, the index varying, as it does, from 29*5 to 33*1. It
is interesting to observe that it reaches its maximum in the cebus.

As compared with the human brain, then, we learn the following points
when we study the relative proportion of the different sections of the
mesial upper border of the cerebral hemisphere:—

1. In both high and low apes there is a great reduction of the parietal
portion of this border.

2. In the low apes there is an enormous increase in the length of the
occipital portion of the border; whereas in the high apes, although there is
likewise an increase of the corresponding part, this is of small amount.

3. The shortness of the parietal portion of the mesial border of the
hemisphere in the high apes is due to the relative increase in the length
of both the frontal and occipital sections of the border.

4 In the low apes the reduction of the parietal portion of the border
is due entirely to the great size of the occipital lobe.