990 
Journal of Agricultural Research 
Vol. XXVIII, No. 10 
The juvenile skull is the product of heredity, and there are undoubtedly present 
developmental factors of one sort or another which contribute toward general 
trends in certain directions; but the muscles exert powerful strains which can 
not fail to have the most profound influences in shaping the bones of the young 
animal. For this reason it is deemed impossible to gain a correct comprehension 
of the interrelationships and correlations of the different bones of the skull 
without some understanding of its major muscles, their attachments, and func- 
Fig. 3 —Head, showing position of the digastricus muscle 
tions (1 ,4, 6,8j 9 ). The interrelationship of these muscles is so complicated, espe¬ 
cially in rodents, that only the main function of each muscle is touched upon 
here. Although this is not the place for a detailed treatment of the myology of 
the head, a brief description of the chief muscles used in mastication is deemed 
Fig. 4—Head, showing (a) superior attachment of the deep part of the temporalis, the superficial part 
having been removed; and (6) the masseter superficialis 
advisable. A discussion of the specific development of these as compared with 
the musculature of other species and genera of the Microtinae would be out of 
place in the present paper (7, 8 ). 
Digastricus. —This arises from the posterior portion of the symphysis of the 
rami of the mandible, as well as for a millimeter or two upon the inferomedial 
part of the bone immediately posterior thereto, and extends as a slender band 
upon either side to insertions upon the paroccipital processes. Its functions 
are to draw the mandible backward and to open the mouth forcibly, although 
the latter action ordinarily requires very slight muscular force. 
