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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 241

IX. Length of the Sagittal part of the Intraparietal Sulcus.—While

engaged in the determination of the intraparietal angle, and the relative
size of the arcus parieto-occipitalis, I also made a series of measurements
of the length of the sagittal part of the sulcus. A pin being introduced
into the hemisphere at either end of the furrow, the distance between these
was ascertained by means of a tape measure, without any attempt being
made to follow the nexuosities of the fissure. These measurements have
been reduced in the following Table to relative results or indices by
assuming the mesial length of the hemisphere to be 100.

Relative Length of the Sagittal Part of the Intraparietal Sulcus.

Mesial Length of the Hemisphere = 100.



Male.

Female.

Number
of Hemispheres
.

Average
Length-
Index.

Number
of Hemispheres
.

Average
Length-
Index.

Adult human brain, ......

15

22-2

13

227

Foetal human brain (from eighth month to full-time),

10

24-2

14

24-6

Chimpanzee, .......





4

20-4

Orang,........

4

25-7





X. Summary.—1. The intraparietal sulcus, single and continuous in
some of the lower apes {e.g. Cebus), becomes broken up in the human
brain into a group of furrows which present different relations to each
other in different cases.

2. Three of the elements of the sulcus in the human brain, viz. the
sulcus postcentralis inferior, the ramus horizontalis, and the ramus occipitalis
, arc disrupted portions of the original fissure; one, the sulcus postcentralis
superior, is a superadded element.

3. In the development of the sulcus in the human foetal brain all the
four segments of the sulcus have as a rule an independent origin, although

ROYAL IRISH ACADEMY.-CUNNINGHAM MEMOIRS, NO. VU. [31]