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178 ANATOMY.

co-ordination of movement that the muscles of the orbit cause the eye-
ball to move in the desired direction.
Fascia of the Orbit.—The orbital space that is not occupied by the
eyeball, muscles, vessels, nerves, ganglia, and glands is tilled up with
a soft cushion of fat and delicate yielding connective tissue. The
Capsule or Fascia of Tenon is formed from this connective tissue. It
is a thin membrane surrounding the greater part of the eyeball, and
forms a socket for the globe to turn in. It arises from the boi'ders of
the orbit, passing behind the conjunctiva and giving it support, thence
backward over the eyeball to the entrance of the optic nerve. The
capsule is pierced behind by the optic nerve and the ciliary vessels and
nerves. The tendons of the muscles of the eyeball also perforate it
near their insertions, and it sends tubular prolongations over each
muscle, these extensions gradually taking the appearance of simple
areolar investment, except in the case of the superior oblique, to M-hich
it forms a sheath as far as the pulley of that muscle.
The sheaths of the recti muscles send prolongations from their outer
surfaces to be attached to the outer margins of the orbits, which pre-
vent too great contractions of the muscles. The prolongations from
the inner and outer recti are stronger than those from the others, this
being especially so with the external recti, which are attached to the
malar bone and external tarsal ligament ; the inner expansion is fixed
to the crest of the lachrymal bone, and the npper one connected with
the tendon of the levator palpebrie, thus enabling the superior rectus to
have, an influence in the movement of the eyelid.
The inner surface of the capsule is connected with the eye by delicate
bundles of yielding connective tissue, allowing a large lymph-space to
exist between the capside and the eye, which appears to act as a synovial
membrane 'in the movements of the globe.
The movements of the eye and its lids are to a certain extent governed
by the sympathetic nerves supplying the involuntary (non-striated)
muscular fibres Avhich are found interspersed among the voluntary
muscles of this region.
Nerves.—The levator palpebrfe, inferior oblique, and all the recti
muscles are supplied by the third nerve (motor oculi), the superior
oblique by the fourth, and the external rectus by the sixth nerve.

MUSCLES OF MASTICATION.
The Masseter, the Temporal, the Internal Pterygoid, and the External
Pterygoid are generally classed as the muscles of mastication, leading
tlie student to infer that they are the only ones brought into action in
the ]ir()ccss. This is not correct ; the first three act in closing the jaws
together, while the fi)urth protrudes the lower jaw l)eyond the ujiper,
none of them having jiower to oj^en the mouth, although with the head
erect the relaxation of the masseter, temporal, and internal ptervgoid per-
mits the lower jaw to drop by gravitation. The muscles of the neck
open the mouth Avhen the head is thrown backward. The mouth is
rigidly closed during the tonic spasm of the first-named muscles, as in
locked jaw or trismus.