1000 
Journal of Agricultural Research 
Vol. XXVIII, No. 10 
Fig. 15.—Individual variation in the 
lateral development of the zygo¬ 
matic arches of adult mice. No. 
3848 on left; No. 3864 on right 
As can be seen, the variation, both individual and with age, of this measurement 
is great, and the variability in the exact configuration of the zygomata is even 
greater. 
The development of the zygomatic region is considerably involved, and this, 
together with the temporal and lambdoidal ridges, reflects the influences of the 
muscular growth more decidedly than any other portion of the skull. Excessive 
development, at these points is a result almost if not quite entirely of a similar 
excessive development of the major muscles of the head. The latter state of 
affairs is, in turn, brought about largely by the character of the food and feeding 
habits of the individual, although, as previously 
mentioned, there are probably inherent trends, 
evolutional, specific, etc., partially influencing the 
muscular development and complicating the results. 
A digression fiere may be permitted to indicate 
that it may occur to the reader that if the above 
statement be true a captive animal that was fed 
exclusively on pap of some sort would grow to full 
size still having a skull that exhibited all the weak¬ 
ness of the juvenile cranium. Such would not 
necessarily be the case, however, for other investiga¬ 
tions have indicated that the unworn molar pattern 
of nestling meadow mice is worn down a surprising amount during a space of 
two days, apparently merely by a gritting together of the teeth. Hence, an 
individual, even though fed on soft food, might be stimulated thereby to an 
unusual amount of teeth-gritting, gnawing wood, or even a piece of loose wire, as 
the writer has seen captive specimens do by the half hour at a time, apparently 
for the sole purpose of exercising the jaws and the muscles. The inherent desires 
of such an individual, therefore, might nullify the effects 
of any deficiency in toughness of the diet. 
Scrutiny of the series shows that there is only slight 
correlation (aside from ridging) between the cranium proper 
and the anterior half of the skull. In other words, the 
squarest, most angular skull with greatest spread of zygo¬ 
mata may have a cranium no larger than examples showing 
marked weakness in these respects. In addition, although 
skulls with great zygomatic width are usually the most 
heavily ridged and angular, these are not necessarily the 
largest skulls, and there is considerable variation in this 
respect, as mentioned in the last paragraph. The distance between the glenoid 
cavities varies practically not at all individually, showing that there is corre¬ 
sponding uniformity in the distance between the condyles of the mandible. There 
is much variation in the degree of lateral projection of the zygomatic processes 
of the squamosals, this conforming to the amount of angularity exhibited by the 
remainder of the zygomata. It is in the region of the zygomatic processes of 
the maxillae, from which arise the masseter muscles, that any difference in the 
strength and angularity of the skull is most pronounced, for it is there that 
variation in the development of this great muscle is directly manifested. 
In specimens showing the greatest angularity and width of the zygomata the 
septum formed by the maxillary root is correspondingly extensive, the antero- 
superior indentation more accentuated, and the superior border often heavier. 
As the process extends from the long axis of the skull at more nearly a right 
angle, the anterior angle of the zygomatic arch is better defined in the examples 
with greatest zygomatic width. Correlated with the above points is correspond¬ 
ing angularity of the lower jaw, as discussed under the proper heading. 
Fig . 16—Individual vari¬ 
ation in depth of the zy¬ 
gomatic arch of adults. 
Right lateral aspects of 
No. 3835, a; No. 3845, 6 
