FLOWER ON THE BRAIN OF THE SIAMANO. 
283 
development of tlie cerebellum. In the Chimpanzee this angle is 
55°, in the Siamang the same, in the Entellus monkey 50°, in the 
Chacma baboon 43°. Diminution of the cerebellum in this region 
would have no influence upon its backward prominence as related to 
that of the cerebral lobes, so I conclude that the size of the cere¬ 
bellum, although it may have some effect, is not largely concerned in 
the varying extent to which it is covered in the posterior direction by 
the cerebrum, in the different forms of Primates. 
A comparison of the cast, Pig. 1., with that of the brain of either 
a higher or lower form of ape, will show that the second proposition 
is well founded. In the brain of most Primates, the line bounding 
interiorly the temporal and occipital lobes, commencing at the apex 
of the former, passes backwards, ascends rapidly, then sweeps back¬ 
wards again, making a wide curve with the concavity downwards, 
before terminating at the hinder end of the occipital lobe. In the 
Siamang this line (marking the tentorial plane) is almost straight, 
rising at an angle of 40° to the long axis of the brain, and cutting 
off, as it were, abruptly, that portion of the hemisphere so extensively 
developed in the Cijnocephcdi and CJirysothrix (Pigs. 2 and 3). And it 
will be seen by referring to the figures, that it is not alone to the 
obliquity of the inferior surface that the smallness of the occipital 
lobe is due; there is also a marked falling away of the upper con¬ 
tour of the brain in the occipital region. 
Not being able to deduce anything further from the cast as to 
the relative proportions of the several parts of the Siamang’s brain, 
and having no actual specimen available for examination, I have had 
recourse to the recent brain of a young Gibbon of a different 
species, dissected last summer (and which is now under the care of 
my friend Dr. Polleston, in the Museum of the University of Oxford), 
for some further illustrative facts. This brain has a more globular, 
and less depressed form than that of the Siamang; this may be 
partly due to younger age, and partly to specific difference. The 
cerebellum is very large, and barely covered by the cerebral lobes. 
The convolutions present the same general characters as those 
described in the brain of H. leuciscus by Gratiolet, but I am able to 
add that on the inner face of the hemisphere (not figured or 
described by Gratiolet) it exhibits the anthropic character of an 
internal perpendicular fissure joining the calcarine, observed also in 
some flemnopitheci, but not in any lower form of Old World apes. 
The convolutions and sulci of the external surface exhibit a similar 
high grade of development, as shown by, 1. Their multiplicity, taking 
into account the small size of the brain. 2. Their want of bilateral 
symmetry. 3. The superficial position of the superior annectent 
gyrus, which completely intercepts the external perpendicular fissure 
on the right side, and partially on the left. The limit between the 
parietal and occipital lobes is indicated by this fissure, and its- back¬ 
ward position shows the reduction of the size of the last. 
