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184

Cunningham Memoirs.

This Table brings ont the curious fact that the upper end of the fissure of
Rolando in the chimpanzee and orang is placed relatively further back than
in the adult human cerebrum. In its upper part, therefore, the relative
antero-posterior length of the frontal lobe in the anthropoid ape exceeds
that of man; in its lower part, however, it is relatively much shorter. In
the orang and the chimpanzee the lower fronto-Rolandic index is 39*2,
whilst in man it is 43"3. It is a matter of some interest to observe that the
permanent position of the upper end of the fissure of Rolando in the
anthropoid ape (index 55-9 in the chimpanzee, and 55*5 in the orang)
closely corresponds with that of the human foetus of 7\ to 8^ months
(index 55 "2).

In the lower apes of the old world the fissure of Rolando, in its whole
length, is placed relatively further forwards than in the cerebral hemisphere
of man. The upper fronto-Rolandic index varies from 45*4 to 50, whilst
the lower index varies from 37*9 to 42'1. In Cebus, owing to the course
pursued by the fissure of Rolando the lower end of the sulcus is placed very
far back, and the inferior fronto-Rolandic index is higher than in man
(viz. 43-8).

As we have seen, one of the leading peculiarities of the human brain is
the relatively great antero-posterior extent of the parietal lobe, and this is
attained by a corresponding decrease in the length of the occipital lobe.
In the anthropoid the encroachment of the parietal lobe upon the occipital
territory has not proceeded to such an extent as in man, and further, in its
upper part, the length of the parietal lobe is reduced by the extension backwards
of the superior part of the frontal lobe. In the low apes the gain to
the parietal area, by the shortness of the frontal lobe, is more than compensated
for by the great size of the occipital lobe.

The fronto-Rolandic indices which I have obtained for the human brain
at different periods of growth (p. 176), and also for the ape brain, render
it impossible for me to adopt the conclusions which have been arrived at
by Mingazzini.* In a very exhaustive memoir upon the cerebral surface
this author remarks: " From these observations it results that in the

* "Intorno ai solchi e le circonvoluzioni cerebrali dei Primati e del feto umano," Estratto
dagli Atti della K. Acad. Med. di Boma. Anno xv., vol. iv., serie n. 1888.