282 
ORIGINAL ARTICLES. 
feral lobes stand out in an unmistakeable manner, the latter to the 
extent of nearly 2-10ths of an inch. In the cast, the impression of 
the cavities, in which the lateral sinuses are lodged, give an apparent 
increase to this projection, but this is readily distinguished from that 
of the lobes themselves. The median portion or vermis is lodged in a 
slight vallecula as in the human cerebellum, and although not quite 
extending to the level of the lateral lobes, is more prominent than 
in the higher anthropoid apes. 
This peculiarity of the Siamang’s brain is due to two causes— 
firstly, the large development of the cerebellum; secondly, and I 
shall afterwards show, mainly, to the actual shortness of the posterior 
or occipital lobe of the cerebrum. 
It is difficult to find a method of easy application for estimating 
the proportionate size of the cerebellum to the cerebrum, but I 
think that the relative breadth of the two portions of the encephalon 
(which may be ascertained from the skulls alone) affords a measure, 
approximatively, at least, accurate. The following table, embracing 
examples of the principal genera of the Old World apes, drawn up 
upon this plan, shows that, as we descend from the anthropoid 
forms, the proportionate size of the cerebellum decreases:— 
Greatest 
Greatest 
Proportion 
Breadth of 
Breadth of 
of 
Cerebrum. 
Cerebellum. 
Breadth. 
Inches. 
Inches. 
Troglodytes gorilla . . . 
3-75 
3*4 
100 to 91 
T. niger ....... 
365 
3*25 
100 to 89 
Pithecus satyrus .... 
3-75 
33 
100 to 88 
Hylobates syndactylus . . 
25 
2*2 
100 to 88 
Semnopithecus entellus . . 
2*6 
1*9 
100 to 73 
Cynocephalus porcarius . . 
2*9 
2-0 
100 to 69 
Cercopithecus delelandii 
2*2 
1*45 
100 to 66 
Macacus rhesus .... 
21 
1*3 
100 to 62 
It will be seen by this, that the Siamang is grouped with the 
three large anthropoid apes, which have all very well developed 
cerebella, but it does not exceed any of them in this respect. 
On examining and measuring a large series of skulls and brains 
of various apes, it appears to me that the diminution in the size of 
the cerebellum affects its length far less than its breadth, and is 
chiefly evident at the antero-lateral portions of each hemisphere. So 
that the horizontal angle formed by the superior margin of the 
petrous bone (which marks the antero-lateral attachment of the 
tentorium, and consequently the margin of the cerebellum) with the 
median line of the skull, affords a fair index of the proportionate 
