EOANTHROPUS DAWSONI 
3^-3 
downwards and backwards to a chinless lower border. 
On the hinder surface of the symphyseal region — the 
surface directed towards the tongue — there is seen a deep 
pit, almost large enough to take the tip of the little 
finger. From the interior of the genial pit arise the 
two chief muscles of the tongue — the genio-glossus 
muscles. The lower margin of the pit is formed by a 
plate of bone — the simian plate — which unites the lower 
borders of the right and left halves of the jaw (fig. 105). 
It is a strengthening plate. From the posterior margin 
of the plate two paifs of muscles take their origin — the 
MAN. 
Fig. 104. — Section of the human 
tongue, chin, lower jaw, and lip 
made along the middle line, to 
show the origin of muscles from 
the region of the chin or symphysis. 
CHIMPANZEE. 
Fig. 105. — A corresponding section of 
the same region of a young chim- 
panzee. 
genio-hyoid muscle, which draws the larynx forward dur- 
ing the act of swallowing, and the digastric muscle, which 
depresses the front part of the jaw, and thus assists in 
opening the mouth. Such is the conformation of the 
symphyseal or chin region of the lower jaw in apes 
(see fig. 105). 
When a corresponding section is made of the symphyseal 
region of a human lower jaw, a very different conformation 
is seen (fig. 104). There are the same two parts exposed in 
the section — the upper dental part, which carries the incisor 
teeth, and a lower part for the attachment of muscles. The 
dental part, compared with that of an ape, is small and 
