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way of feeding, for the influence on this ratio of the difference in 
body weight (as 4:3), can be but slight. When the two median- 
sagittal cross-sections of the skull are divided by straight lines GB into 
an upper (cerebral) and a lower (facial) part, it is found, on determination 
of the area of these parts that per unit of area of cerebral part 
Hylobates syndactylus has exactly twice as much facial part as 
Hylobates agilis. The ratio between the areas of the cerebral and 
the facial parts of the skull, which is thus changed, must certainly 
cause the former to extend more in length than in height, directly 
by geometric adaptation, but at the same time it must modify indirectly 
the form of cranium as it imposes higher demands on the muscles 
that support the head. 
_ The second remarkable fact met with when the Anthropoid Apes 
are compared, is the great difference of the shape of the skull 
between the Orang utan and the two other large Anthropoids, the 
Chimpanzee and the Gorilla. The skull of the Orang utan is vaulted 
more highly, comparatively short and round, with high forehead 
and round occiput, and without prominent supra-orbital ridges. 
Now it is also found that in both sexes of the Orang utan the 
heavy head is supported in front by a huge air throat pouch, or 
laryngeal sac (Fig. 3), which (like the small laryngeal sacs found in 
many other Monkeys) can be filled and emptied at will from the 
larynx, and serve as buffer or shock reducer and air-brake. The 
Gorilla, the Chimpanzee, and the Siamang — again in both sexes — 
have only comparatively small laryngeal air sacs; they are entirely 
absent in the small Gibbon species. The superior parts of the laryngeal 
sac of the Gorilla are no more than small pouches stretched be- 
tween the hyoid bone and the larynx towards the right and the 
left (Fig. 4), while a median continuation, passing before the wind- 
pipe downwards, forms the channel to the subclavian and axillary 
pouches as in the Chimpanzee. The Chimpanzee has no pouches worth 
mentioning above, whereas the air pouch of the Siamang is confined 
to the space between the body of the mandible and the lower side 
of the larynx, and a continuation towards the chest is wanting. 
The significance of the very large laryngeal sac of the Orang 
utan cannot be ascribed to strengthening of the sound, for its voice 
is much seldomer heard and is much less loud than that of its 
African cousins, which even scare away elephants out of their 
neighbourhood by their terrible roar. Besides the small gibbon species 
have, without throat pouches, as loud voices as the Siamang. 
The opinion pronounced for the first time by Duniker and Bourarr, 
that the laryngeal sac of the Orang utan has the significance of an 
pike 
