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PTERYGOIDiEUS EXTERNUS. '25
The thick and ordinary portion of it arises tendinous and
fleshy, from almost the whole external surface of the external
Ala of the Pterygoid Process of the Sphenoid Bone, excepting
a little bit of the root at the posterior edge ; and towards the
lower part, it arises a little from the inner surface of that Ala.
The thin portion arises from a ridge of the Sphenoid, that is
continued from the process towards the Temple, just behind the
Foramen Lacerum inferius, which terminates in a little pro-
tuberance. This origin is sometimes wanting ; and in that case,
the Temporal Muscle arises from that protuberance; and very
often this origin is common to both. These two origins of this
Muscle are sometimes so much separated, as to make it a Biceps.
From these origins the Muscle passes outwards, and a little
backwards, converging ; that is, the superior fibres passing out-
wards and backwards, and a little downwards ; while the inferior,
or larger portion of it, passes a little upward.
It is inserted tendinous and fleshy into a depression on the
anterior part of the Condyle and Neck of the Lower-Jaw, upon
the inside of that ridge, which is continued from the Coronoide
Process. A little portion is likewise inserted into the anterior
part of the moveable cartilage of the joint.
When this Muscle acts singly it is a rotator ; for it brings
the Condyle of the Jaw forwards, and likewise the moveable
cartilage, which throws the Chin to the opposite side ; but if it
acts in conjunction with its fellow of the opposite side, instead
of being turned to one side, the whole Jaw is brought forwards,
and thus these counteract the Temporal, &c.
These two Muscles generally act alternately ; and when they
do so, one acts at the time of depression, the other at the time

" whilst the zygomatic arches are longer and span across a wider tem-
poral fossa than in the ordinary Ruminants," (1) in which the upper in-
cisors are absent, being replaced by a callous pad, the upper canine is
in many instances wanting, whilst the lower canine resembles and ibrrns
one of the same series with the lower incisors.]

(1) Osteoloft Catalogue R. Col. Surgeons, vol. ii., p.