Download The trochlear nerve.

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The orbital cavity
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The orbits are two bony cavities occupied by the eyes and associated muscles,
nerves, blood vessels, fat, and much of the lacrimal apparatus. Each orbit is shaped
like a pear or a four-sided pyramid, with its apex situated posteriorly and its base
anteriorly. The orbit is related (1) on its superior side to the anterior cranial fossa
and usually to the frontal sinus, (2) laterally to the temporal fossa in (anterior) and
to the middle cranial fossa (posterior), (3) on its inferior side to the maxillary sinus,
and (4) medially to the ethmoidal and the anterior extent of the sphenoidal sinuses.
The margin of the orbit, readily palpable, is formed by the frontal, zygomatic,
and maxillary bones .It may be considered in four parts: superior, lateral, inferior,
and medial.
The superior margin, formed by the frontal bone, presents near its medial end
either a supraorbital notch or a supraorbital foramen, which transmits the nerve and
vessels of the same name.
The lateral margin is formed by the zygomatic process of the frontal bone and
the frontal process of the zygomatic bone.
The inferior margin is formed by the zygomatic and maxillary bones. The
infraorbital foramen, for the nerve and artery of the same name, is less than 1 cm
inferior to the inferior margin.
The medial margin, formed by the maxilla as well as by the lacrimal and
frontal bones, is expanded as the fossa for the lacrimal sac. The fossa passes
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inferiorly through the floor of the orbit as the nasolacrimal canal, which transmits
the nasolacrimal duct from the lacrimal sac to the inferior meatus of the nose .
Walls of the orbit.
The orbit possesses four walls a roof, lateral wall, floor, and medial wall.
The roof (frontal and sphenoid bones) presents the fossa for the lacrimal gland
anterolaterally and the trochlear pit for the cartilaginous or bony pulley of the
superior oblique muscle anteromedially. The optic canal lies in the posterior part of
the roof, between the roots of the lesser wing of the sphenoid bone. It transmits the
optic nerve and ophthalmic artery from the middle cranial fossa.
The posterior aspect of the lateral wall (zygomatic and sphenoid bones) is
demarcated by the superior and inferior orbital fissures. The superior orbital fissure
lies between the greater and lesser wings of the sphenoid bone. It communicates
with the middle cranial fossa and transmits cranial nerves III, IV, and VI, the three
branches of the ophthalmic nerve, and the ophthalmic veins .The inferior orbital
fissure communicates with the infratemporal and pterygopalatine fossae and
transmits the zygomatic nerve. The lateral walls of the two orbits are set at
approximately a right angle from one another, whereas the medial walls are nearly
parallel to each other .
The floor (maxilla, zygomatic, and palatine bones) presents the infraorbital
groove and canal for the nerve and artery of the same name. The inferior oblique
muscle arises anteromedially, immediately lateral to the nasolacrimal canal.
The medial wall (ethmoid, lacrimal, and frontal bones) is very thin. Its main
component (the orbital plate of the ethmoid) is papyraceous (paper-thin). At the
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junction of the medial wall with the roof, the anterior and posterior ethmoidal
foramina transmit the nerves and arteries of the same name.
In summary, the orbit communicates with the middle cranial fossa (via the
optic canal and superior orbital fissure), the infratemporal and pterygopalatine
fossae ( via the inferior orbital fissure), the inferior meatus of the nose (via the
nasolacrimal canal), the nasal cavity (via the anterior ethmoidal foramen), and the
face ( via supraorbital and infraorbital foramina).
Orbital Muscles Muscles of eyeball (extraocular muscles)
The orbit contains seven muscles, the first being the levator palpebrae superior
muscle and the other six controlling the eye movements: four rectus muscles
(superior, inferior, lateral and medial) and two oblique muscles (superior and
inferior). The levator palpebrae superior is a fine, triangular muscle, which
originates above and in front of the optic canal at which point it is fine and
tendinous .It runs along the upper wall of the orbit just above the superior rectus
muscle (covering its medial edge). It terminates in an anterior tendon that spreads
out in the form of a large fascia, which extends out to the eyelid.
Rectus muscles: these four muscles form a conical space that is closed in front by
the eyeball. They arise in the common annular tendon common tendinous ring; this
tendon is located on the body of sphenoid near the infraoptic tubercle, and it
surrounds the superior, medial and inferior edges of the optic canal.
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The Arteries of the Orbit
The arteries of the orbit correspond essentially to the ophthalmic artery and its
branches, although the orbit is also supplied by the infraorbital artery, a branch
of the maxillary artery which is itself the terminal branch of the external carotid
artery. The ophthalmic artery, with a diameter of 1.5 mm, is a branch of the
internal carotid artery, arising anteriorly where it emerges from the cavernous sinus
medial to the anterior clinoid process. This artery makes its way through the
subarachnoid space below the optic nerve and then continues on into the optic
canal, carrying on laterally to perforate the sheath at the exit of the canal.
The ciliary arteries can be divided into three groups: The posterior long ciliary
arteries, commonly two in number, arise in the ophthalmic artery at the point at
which it crosses over the optic nerve. These enter the sclera not far from where the
optic nerve enters it. The posterior short ciliary arteries, seven in number, pass in a
forward direction around the optic nerve. The anterior ciliary arteries arise from
muscular branches and pass in front, over the tendons of the rectus muscles
Veins of the Orbit There is a very dense venous network in the orbit, organized
around the two ophthalmic veins that drain into the cavernous sinus. These veins
are valve-less. Periorbital drainage also occurs towards the facial system via the
angular vein. Thus, for the venous system, as with the arterial system, the orbit is a
site of anastomosis between the endocranial and exocranial systems.
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Nerves of the Orbit
The orbit contains a huge number of nervous structures of various types. There
include: – a component of the central nervous system: the optic nerve; – three
motor nerves: the third, fourth and sixth cranial nerves(Oculomotor, trochlear
and abducent nerves (III, IV, VI) – a sensitive nerve: the ophthalmic nerve, a
branch of the fifth cranial nerve – an autonomic center: the ciliary ganglion.
The oculomotor nerve.
The oculomotor (third cranial) nerve supplies all the muscles of the eyeball except
the superior oblique and the lateral rectus muscles. It emerges from the brain stem,
passes lateral to the posterior clinoid process, traverses the lateral wall of the
cavernous sinus, and divides into superior and inferior divisions, which pass
through the superior orbital fissure within the common tendinous ring The
oculomotor nerve supplies the levator palpebrae superioris and superior rectus (by
its superior division) and the medial rectus, inferior rectus, and inferior oblique
muscle (by its inferior division). The oculomotor nerve "opens" the eye, whereas
the facial nerve" closes" it (by the orbicularis oculi). The oculomotor nerve also
conveys
preganglionic
parasympathetic
nerve
fibers
to
the
orbit. A
parasympathetic communication arises from the nerve branch to the inferior
oblique muscle. This nerve enters the ciliary ganglion and synapses there.
Postganglionic parasympathetic nerve fibers pass from the ganglion to the eyeball
through the short cilliary nerves, innervating the sphincter pupillae and ciliary
muscle .In the act of focusing the eyes on a near object the oculomotor nerves are
involved in adduction (medial recti), accommodation (ciliary muscle), and miosis
(sphincter pupillae). Paralysis of the oculomotor nerve results in ptosis (paralysis
of the levator), abduction (unopposed lateral rectus), and other signs.
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The trochlear nerve.
The trochlear (fourth cranial) nerve supplies only the superior oblique muscle of
the eyeball. It emerges from the dorsum of the brain stem, winds around the
cerebral peduncle, traverses the lateral wall of the cavernous sinus, and passes
through the superior orbital fissure just superior to the tendinous ring It then enters
the superior oblique muscle. The trochlear nerve is tested by asking the subject to
look downward when the eye is in adduction
The abducent nerve.
The abducent (sixth cranial) nerve supplies only the lateral rectus muscle of the
eyeball. It emerges from the brain stem at the junction of the pons and the medulla.
It enters the dura matter over the dorsum sellae and follows the slope of this bone
supeior and anterior, bending sharply anteriorward across the superior border of
the apical portion of the petrous part of the temporal bone. This perhaps accounts
for abducent involvement in almost any cerebral lesion that is accompanied by
increased intracranial pressure. The nerve next traverses the cavernous sinus,
passes through the superior orbital fissure within the common tendinous ring and
enters the medial side of the lateral rectus muscle. Paralysis of the lateral rectus
results in inability to abduct the eye and horizontal diplopia (double vision) which
is worst when attempting to look toward the side of the nerve injury.
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The ciliary ganglion, is a reddish-gray, pinhead-sized (2 mm 1 mm) structure
which is somewhat flattened out along its major horizontal axis. It is located near
the apex of the orbit (at the junction of the anterior 2/3 and the posterior 1/3) in
loose fatty tissue between the optic nerve and the lateral rectus muscle, usually
lateral to the ophthalmic artery. There are three roots: – the motor or
parasympathetic root which comes from the inferior branch of the third cranial
nerve (by the inferior oblique branch). Its fibers mainly innervate the ciliary
muscle and, to a lesser extent, the sphincter of the iris; – the sympathetic root, a
branch of the carotid plexus which enters the orbit via the common tendinous ring;
– the sensitive root, a long and fine fiber which rejoins the nasociliary nerve where
it enters the orbit. This supplies the eye and the cornea. The 8–10 filamentous
branches of the ganglion are called the short ciliary nerves and these make their
way in a forward direction around the optic nerve, together with ciliary arteries and
the long ciliary nerves.
Lacrimal Gland
This comprises two di¤erent, laterally continuous parts: the upper, broader orbital
part, and the lower, smaller palpebral part. The orbital part is Anatomy of the Orbit
and its Surgical Approach 55 accommodated in the lacrimal fossa of the zygomatic
process of the frontal bone. It is located above the levator muscle (and laterally,
above the lateral rectus muscle). Its lower surface is attached to the sheath of the
levator muscle and its upper surface to the orbital periosteum; its anterior edge is in
contact with the orbital septum and its posterior edge is attached to the orbital fatty
tissue. The palpebral part, which is in the form of two or three lobules, extends out
below the fascia of the levator muscle in the lateral part of the upper eyelid. It is
attached to the superior conjunctival fornix.
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