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AREOLAR TISSUE, TENDONS, AND MUSCLES. 179 ;
The Masseter is a stout, thick, short, quadrilateral muscle extendiug
from the zygomatic arch to the inferior maxillary bone. It is composed
of two portions, superficial and deep, which differ in size and direction.
The /Superficial Portion is the largest and strongest, and arises by a
thick tenclinons aponeurosis (which passes downward into the muscular
fasciculi) from the lower border of the anterior two-thirds of the zygo-
matic arch and the lower margin of the malar bone : extending down-
ward and backward, it is inserted into the lower and outer half of the
angle of the jaw.
The Deep Portion is of triangular form ; it is smaller and its muscu-
lar fibres are shorter than those of the superficial jiortion. It arises
from the posterior third of the lower border and from all the internal
surface of the zygomatic arch passing downward and slightly forward,
;
it joins some of the superficial portion, and is inserted by a tendinous
aponeurosis into the upper half of the ramus and outer surface of the
coronoid process of the lower jaw ; only the upper and back part of the
nniscle is lefb uncovered by the superficial portion.
Relations.—It is princijially covered by the skin and the fascia or
platysma myoides and its own fascia, which latter adheres intimately to
the tendon at its origin ; at the back portion the parotid gland, the duct,
which lies across the muscle, and at the anterior border turns inward,
pierces the buccinator muscle and opens into the mouth through a little
teat-like projection of the mucous membrane : the upper portion of the
masseter muscle is overlaid by the orbicularis palpebrarum and zygo-
matic. A few branches of the facial nerve (seventh) and the transverse
facial vessels pass over it.
The muscle lies in contact below with the ramus of the jaw and the
buccinator muscle. Between the two muscles there is a large quantity of
delicate fat covering a nerve and vessels which enter the muscle through
the sigmoid notch.
Artery.— The Masseteric Artery, a branch from the second division of
the internal maxillary, conveys the blood-supply.
Nerve.—The masseteric branch of the inferior maxillary (third divis-
ion of the fifth).
The ^Temporal Fascia is a dense glistening layer of fibres firming
an aponeurosis covering the temporal muscle above the zygoma, and
giving origin to its superficial portion. The fascia is attached supe-
riorly to the temporal crest of the frontal bone and to the upper of the
two curved lines on the parietal bone, extending as far back as the
parieto-occipito-temporal junction. It is thin and weak at its origin,
becoming thicker and stronger as it approaches the zygomatic arch,
near which it divides into two layers, these being separated by a quan-
tity of compact adipose tissue ; these layers are attached respectively to
the inner and outer margins of the sujierior border of the zygomatic
arch. The fascia is separated from the skin by a thin membrane which
descends from the epicranial aponeurosis, and by the auricular muscles
also by some adipose tissue at the lower portion. If an abscess should
form beneath this fascia or within the nmscle, the pus would be directed
to the coronoid process of the inferior maxilla, and thence into the mouth
along the adipose tissue of this region.