1046 
no longer on the air-cushion, the muscles of the neck pull at the 
occiput in as favourable a direction, thus raising the front of the 
head as in the Chimpanzee. But in the Orang-utan these muscles 
then pull in the direction of the orbital arch of the frontal bone, 
while through its elevation the front part of the head hangs much 
less heavily at the occipital part of the cranium, which is besides 
shorter; they draw in the Chimpanzee in the direction of the crown 
of the cranium, the frontal part of the head still hanging with its full 
weight at the occipital part of the cranium, which is besides longer. 
When, as occurs frequently during locomotion, the body is moved 
up with great velocity, or is checked in its speed obtained by 
gravity, the heavy head will fall forward with great force through 
inertia, unless it is stopped. This takes place in front in the Orang- 
utan, by means of the elastic laryngeal air sac, in the other 
Anthropoids and most lower Apes only by means of the muscles 
of the neck, which acting behind the transverse axis of rotation, 
pull the skull backward. The stretching strain thus arising between | 
the front part and the back part of the calvaria, is comparatively 
small in the Orang-utan, great in the other Anthropoids, whose 
cranial vault would certainly run a risk of breaking, if there were 
no mechanism to strengthen it, through transference and dispersion 
of the excited strain. In Man of the present type the head turning 
about the condylar axis, never hangs over forward so heavily, 
because in the ordinary erect attitude it balances on the vertebral 
column, the planum nuchale lies very flat, and the muscles of the 
neck, which thus act almost straight downward, pull the head 
backward, which causes the strain excited between the occiput and 
the front to be much less great in all positions. However, also in 
Man and in the Orang-utan, the cranial vault might possibly not 
always be able to resist it, without the mechanism in question, now 
to be described, which is however less strong here *). 
Apparently the strain is borne certainly not entirely, probably 
only for a very small part by the brittle bony substance, but for 
the greater part by the very elastic apparatus of the musculus 
epicranius or occipito-frontalis, the two-bellied flat muscle, whose 
uniting tendon, the strong epicranial aponeurosis or galea aponeu- 
rotica, which chiefly consists of longitudinal fibers, and is loosely 
') The principal functional meaning of the air pouches, found in so many Monkeys, 
most probably consists, in general, in this that they help the muscles of the neck 
to prevent sudden stretching of the encephalon and the medulla spinalis of which 
there might be a danger from the generally heavy front part of the head, and 
the situation of the foramen occipitale under the back part of the cranium. 
