June 7,1924 
Variation in Microtus Montanus Yosemite 
991 
Masseter. —The chief muscle of the skull and the one most used by the ma¬ 
jority of mammals during mastication is the masseter. It is a compound muscle 
conveniently divided into three parts as follows: 
Masseter superficialis. —This arises from the slightly defined, small process 
directly below the anteorbital fossa (see fig. 4, b) , in a stout aponeurosis, which 
rapidly broadens to form a thin band, developing muscle fibers and extending 
obliquely rearward to an insertion upon the inferomedial edge of the angular 
process of the mandible and the portion of the ramus immediately adjacent. 
The fibers superficial to this insertion are in turn inserted after the usual fashion 
in the intersection between this muscle and the pterygoideus internus. It is 
used in drawing the mandible forward. 
Masseter zygomatica. —This arises (see 6, fig. 5) by a superficial fascia and 
deeper muscle fibers upon the inferior portion of the zygomatic arch from the 
anterior to the posterior angle, but additional strength is secured in this species 
by the extension of the aponeurotic sheet a couple of millimeters still farther for¬ 
ward along the zygoma, and to this extent it directly overlies a portion of the 
superficial division of the masseter major. It then descends with a posterior 
a 
Fig. 5.—Head, showing (a) superior portion of the superficial part of the temporalis; and (b) the masseter 
zygomatica (masseter superficialis removed) 
trend to an insertion upon the lateral face of the angular process, while a small 
portion is inserted upon the superior part of the medial face of the same. This 
muscle is used in closing the jaws, and probably to some extent in pulling the 
mandible forward. 
Masseter major. —This is properly divisible into two portions, the pars 
superficialis and the pars profundus. The former (see fig. 6) arises from the 
superior edge of the maxillary root of the zygomatic arch, covering the whole 
anterior face as far as the anterior angle of the zygoma. Its forward bulge covers 
the anteorbital fossa. It descends with a slight posterior inclination to an in¬ 
sertion upon the masseteric ridge of the mandible. The deep part (see fig. 7), 
considerably less in mass, arises within the orbit from the posterior edge of the 
maxillary root of the zygomatic arch, extending backward all along the supero- 
medial edge of the zygoma. There is also a sfnall division, ostensibly of this 
muscle (the exact status of which, in myomorphine rodents, is a matter of con¬ 
troversy), that arises from the anteorbital fossa and extends through the adjacent 
foramen to join the main muscle. This deep part is thinly inserted upon the 
mandible in a line extending from a point upon the lateral face of the ramus 
directly anterior to the prominence formed by the root of the incisor to the an¬ 
terior termination of the masseteric ridge. The anterior third of the insertion 
is purely aponeurotic. The chief use of the masseter major is in closing the jaws, 
both while gnawing with the incisors and champing the cheek teeth; in addition, 
