FLOWER ON THE BRAIN OP THE SIAMANG)-. 
285 
Fig. 3. Cast of the interior of the skull of the Saimiri or Squirrel Monkey 
( Chrysothrix sciureus ), adult. 
The figures are all from specimens in the Museum of the Royal College of 
Surgeons, and are of the natural size. 
In order to practise the mode of estimating the length of the 
posterior as compared with the antero-median portions of the cere¬ 
brum, which I have described elsewhere, # a horizontal section was 
made through the right hemisphere, when it was ascertained that 
the portion anterior to the convex posterior border of the hippocampus 
major, including the corpus striatum and optic thalamus, measured 
1*7 in., while the portion behind this point, including the posterior 
cornu and hippocampus minor, was but •7 in,; the proportion being 
as 100 to 41. On comparing this figure with those in the table given 
in the appendix to the paper just referred to, it will be seen that it 
is inferior to that of any true ape yet measured; the figures in 
other genera ranging from 47 (Semnopithecus) , to 62 ( Hapale ), the 
proportion in the Human brain averaging about 53. I have no 
doubt, from the external form of the lobe, that in the Siamang it 
would be even less than in the Gibbon. 
The above mentioned external characters are all corroborated in 
a brain of Hylobates lar , in the Museum of the Royal College of 
Surgeons, and an inspection of the skulls of several members of the 
genus points to the same conclusion, namely, that a very great reduc¬ 
tion of the occipital lobe of the cerebrum is one of the most marked 
characters of the brain of Hylobates , although it is not in any other 
species at present examined carried to so great an extent as in the 
Siamang. As the Gibbons take rank, by virtue of all their struc¬ 
tural affinities, among the higher members of the order to which they 
belong, we see in this fact a conclusive illustration of Gratiolet’s 
remark, to the effect that the large development of a particular 
character which distinguishes an elevated group, canno b always be 
selected as a sign of elevation among the individual members of the 
group, f 
* “ On the posterior lobe of the Cerebrum of the Quadrumana.”—Phil. Trans. 1862 . 
f The genus Colobus is not referred to anywhere above, as the brain is at present 
unknown, but I should infer, from the form of the skull of C- vellerosus , that it is 
closely allied to Semnopithecus , though in the shortness of the posterior lobes 
approaching even nearer to Hylobates . 
