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different degrees of development. This muscle, which consists of the 
flat right and left frontal muscles and the right and left occipital 
af | 
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ee | mal | p 
GORILLA 
CHIMPANSEE 
Fig. 4. Upper part of the laryngeal sac of a female gorilla. 
Laryngeal sac of a female chimpanzee; in this genus the 
upper part is not developed. (Both reproductions taken from 
EHLERS). The median air-cushion and the subclavian and 
axillary cushions protect windpipe and bloodvessels, and 
also to a certain extent the contents of the cranium against 
concussions and shocks caused by the heavy head which 
is thrown forward. H hyoid bone, which is placed on a line 
with that of the Gorilla, C coracoid process. !/1 of nat. size. 
muscles with the epicranial aponeurosis, the galea aponeurotica, as 
intermediate tendon, is stretched out over the upper surface of the 
cranium. The epicranial aponeurosis is attached loosely and movably with 
the skull bone, but firmly bound to the hairy skin of the head, which is 
thus pressed firmly, but elastically against the bone, in every position of 
the head, by the whole apparatus, forming a mechanical whole with 
it, and playing a similar part as paper stuck to glass or the iron 
rods in reinforced concrete, i.e increasing the shearing and tensile 
strength of the cranium, and at the same time deadening the shocks 
that might injure the contents of the cranium. The action of the 
frontal muscle in mimicry can certainly not be the only function, 
nor the principal significance of this muscle apparatus. This follows 
already from the fact that the frontal muscle in Man, where it 
arises for the greater part at the skin under the brows, and is 
generally only little slightly attached to the bone, draws up the 
brow and wrinkles the forehead transversally, expressing in this way 
attention and astonishment, whereas in monkeys, where it is more 
attached to the bone of the supra-orbital ridge, 2 smoothens the 
forehead. In Man just as in the Monkeys, the occipital muscles are 
