THE CHIMPANZEE AND ORANG UTAN. 379 
tution, where the transverse diameter is 13 lines and a half, this space measures only 
2 lines, or less than one sixth; while in the Pongo, where the same diameter is 17 lines 
and a half, it is no less than 7 lines and a half, or nearly equal to one half the breadth 
of the orbit.” 
The cause of the proportional differences in the size of the orbits, and extent of the 
inter-orbital space, is explained in the text, p.358. And the Figures in this plate show 
that they operate equally in the Simia Troglodytes as in the Simia Satyrus, and that the 
proportionally greater breadth of the inter-orbital space which distinguishes the young 
Chimpanzee from the young Orang is equally characteristic of the adults of the two 
species. 
A comparison of Fig. 5 with Fig. 6, and of Fig. 7 with Fig. 8, will show that in each 
species a gradual change takes place in the plane of the occipital foramen: this is occa- 
sioned by the downward and lateral growth of the upper jaw, and is accompanied with 
a corresponding enlargement of the glenoid cavities, and of the whole base of the skull, 
which, as the cerebral portion remains stationary in its development, throws the occi- 
pital foramen more backwards. This change is of course proportionally greater in the 
Simia Satyrus than in the Simia Troglodytes, corresponding to the greater difference 
which subsists between the young and adult states of their jaws and dentition ; but the 
relation that is preserved equally proves the identity of the young with the adult Orang, 
as between the young and the adult Chimpanzee. 
PLATE LVI. 
Side view of the skull of a human idiot, natural size. 
PLATE LVIII. 
Base of the same skull, natural size. 
This figure and the preceding were taken, by the liberal permission of Edward Stan- 
ley, Esq., F.R.S., from the specimen in his Museum of Human and Pathological Ana- 
tomy at St. Bartholomew’s Hospital. 
