1039 
muscles taken into consideration by Warknorr and of the facial 
muscles, becomes negligible *). . 
In order to be able to resist the stretching strains described above, 
which are caused by the action of the masticatory muscles, the 
lower jaw bad to be reinforced at the symphysis. This has happened: 
first, by a general uniform heightening or thickening, both in the 
mandibular type with stretching strains on the outside and in that 
with stretching strains on the inside; secondly, through locally 
restricted strengthening, and then: in the type of Homo sapiens 
(and Homo wadjakensis) with stretching strains on the outside, 
through: (a)a protuberantia mentalis, (6) the lower border bent 
outward (torus marginalis), which is not found in the Wadjak- 
Man; in the type of Homo neandertalensis (and Homo heidelbergensis) 
and most Apes, with siretching strains on the inside, through: (a) a 
torus mentalis internus, (ba lamina submentalis (Kutn’s 
“simian plate, shelf or ledge”), which latter is only met with in 
Monkeys, not in the Neandertal-Heidelberg Man *). 
The existence of these strengthenings of the mandible need not 
only be accepted as mechanically efficient and necessary, such a 
growth of bony substance may also be considered as a definite 
consequence of muscular action, which — as ArcHEL*) has 
demonstrated — causes directly or indirectly stretching strain (“Zug”), 
and with it physiological stimulation of the periost. 
What then explains further the difference in direction of the 
muscles of mastication, which is the cause of the two mandibular 
types? Why is the direction of the musculus masseter slanting from 
above and outside towards below and inside in the type of Homo 
neandertalensis-heidelbergensis and the Monkeys, and on the contrary, 
at least the resulting direction of contraction of the muscles of 
mastication in the type Homo sapiens-wadjakensis from above and 
inside downward and outside? 
The explanation is to be found in the special function of the 
masseter and the other muscles of mastication. A different direction 
1) In the large Anthropoids (Orang-utan) the strength of the muscles of masti- 
cation, estimated by their weight, is three times as great as in Europeans. 
2) This torus mentalis internus is another than the torus mandibularis 
met with by C. M. Fürsr (Verhandlungen der Anatomischen Gesellschaft 22. 
Versammlung, p. 295. Jena 1908) in about 80°) of the lower jaws of Eskimos 
examined by him, on the inside of the premolars. 
3) O. Aicuet, Vorläufige Mitteilung über Entstehung und Bedeutung der Augen- 
brauenwülste, zugleich ein Beitrag zur Abänderung der Knochenform durch plhysio- 
logische Reizung des Periostes. Anatomischer Anzeiger, Band 49 (1916), p. 497. 
