STORY OF THE CHIN—ROBINSON. - 605 
genio-glossus muscle is capable—and appears to be the only structure 
capable—of exercising a quick and exact control. The same applies 
to the vowels, as is well shown in the accompanying diagrams after 
Von Meyer’s drawings. Von Meyer, however, has not shown the 
genio-glossus muscle in action as it is shown here, and indeed, strangely 
enough, does not give it a word of mention as a factor in articulate 
speech. 
It is worth while to take note of the fact that practically all the 
speech movements of the tongue take place in the neighborhood of 
its central line, and that the sides play a very subordinate part. 
Hence the other extrinsic muscles, such as the hypo-glossus and 
stylo-glossus can have little or no part in articulation. (See fig. 65.) 
Now let us return to our inferior maxilla and examine the attach- 
ments and relations of the genio-glossus. It is obvious that for quick, 
precise movements, such as those demanded by articulate speech, 
it must be unhampered and have plenty of room to act. An exami- 
nation of the arrangements for the play of the muscles in different 
animals is exceedingly instructive. In the dog, and indeed the 
majority of the mammalia, the tongue lies flat upon the lower jaw- 
bone, leaving practically no room for any muscular machinery. If, 
however, a photograph of a plaster cast of the inner surface of the 
wolf’s jaw (see fig. 48) is compared with that of the baboon (see 
figs. 50, 51), which outwardly resembles it, a remarkable difference 
of shape is evident. 
In all the monkeys—and even lower down the scale among the 
lemurs—we find that nature has made provision for working room 
for the genio-glossus muscle by excavating a kind of pit on the inner 
surface of the mandible beneath the tongue. This pit has been 
noticed by various comparative anatomists, but I had never seen any 
explanation of the reason why it exists, nor was I aware of its func- 
tion, until a series of dissections of monkeys’ jaws showed in every 
case the tiny tendon of the genio-glossus coming from the lower 
surface of the deepest part of the pit (see figs. 10, 63). The more 
doglike the jaw is, as in the baboons—the more, in fact, it corre- 
sponds in general outline with the prevalent type of the mandible 
among the lower vertebrates—the deeper is this pit. As soon, how- 
ever, as the mandible begins in some degree to resemble our own, ‘as 
in some chimpanzees and gibbons, and the whole lower surface 
becomes tilted forward, the pit seems to be no longer needed, and 
becomes shallower. One may as well rémark in passing that it is of 
course obvious that originally the genio-glossus muscle had nothing 
whatever to do with articulate speech. The need it met in the 
economy of lemurs and apes was probably that of giving increased 
mobility to the tongue for sorting food already in the mouth. This 
is plainly seen when we give a monkey a nut and see him crack it 
