Orbit
Anatomy
The orbit is a
pyramidal cavity, the apex of which is directed posteriorly and base anteriorly
(Fig. 7.83). Its bony walls separate it from the anterior cranial fossa above, the
ethmoidal air cells and nasal cavity medially, the maxillary air sinus
inferiorly and the lateral surface of the face and temporal fossa laterally (Fig. 7.84). Anteriorly, the orbit presents a roughly rectangular aperture which
is closed by the eyelids. Within the orbit are the eyeball, the
extraocular muscles, cranial nerves II, III, IV, V (ophthalmic
and maxillary divisions) and VI, and blood vessels, lymphatics and fat.
Bony walls
The roof of the orbit (Fig. 7.85) comprises the
frontal bone, which anteriorly contains the frontal air sinuses. The lateral
wall is formed anteriorly by the zygomatic bone and posteriorly by the greater
wing of the sphenoid bone. The floor consists of the maxilla anteriorly and the
greater wing of the sphenoid posteriorly.
From anterior to posterior the medial wall comprises the maxilla, lacrimal,
ethmoid and sphenoid bones. The medial walls of the two orbits lie parallel to
the median plane while their lateral walls are directed forwards and laterally
so that they lie at right angles to one another.
Several foramina and the superior and inferior orbital
fissures (Fig. 7.85) allow various
structures to enter and leave the orbit. On the medial wall, close to the
orbital margin, is a depression called the lacrimal groove, bounded by the
maxilla and the lacrimal bone. The nasolacrimal canal descends from this groove
and opens into the inferior meatus of the nasal cavity. Anterior and posterior
ethmoidal foramina perforate the medial wall and allow communication with the
lateral wall of the nasal cavity. At the apex of the orbit are the optic canal
and the superior and inferior orbital fissures, through which the orbit is in
continuity with the middle cranial and pterygopalatine fossae.
Movement of the eyeball is facilitated by its fascial
sheath, the vagina bulbi (fascia bulbi) (Fig.
7.90), which invests it but does not adhere to the sclera and is incomplete
anteriorly. The vagina bulbi is attached to the eyeball around the margin of
the cornea. Thick-enings of the inferior part of the fascia (the suspensory
ligament) extend laterally and medially to attach to the orbital walls (check
ligaments). The fascia is pierced by the six muscles that move the eyeball.
Anteriorly the orbit is closed by the orbital septum, which forms the fibrous
layer of the eyelids.
Within each eyelid, the orbital septum is thickened to
form a tarsal plate (Figs 7.86 & 7.87) and is perforated between the
eyelids by the palpebral fissure. Anterior to the septum lies orbicularis oculi
and skin. Levator palpebrae superioris is attached to the upper edge of the
superior tarsal plate while a few fibres of inferior rectus are attached to the
lower edge of the inferior tarsal plate. Posteriorly each plate has tarsal
(meibomian) glands and is covered by conjunctiva. The conjunctival epithelium
is reflected onto the surface of the eyeball, where it
blends with the margin of the cornea. Each eyelid carries a double row of
eyelashes together with associated sebaceous glands (which when inflamed form a
stye). The lashes on each eyelid extend medially as far as a small elevation
containing a central aperture, the lacrimal punctum, leading into the lacrimal
canaliculus. The canaliculi carry tear fluid to the lacrimal sac in the
lacrimal groove, and the sac in turn drains via the nasolacrimal duct, in the
nasolac- rimal canal, into the nasal cavity beneath the inferior concha (Fig.
7.51).
The lacrimal gland (Figs 7.86, 7.87 & 7.89) lies
in the superolateral angle of the orbit behind the upper eyelid and is deeply
indented by the lateral border of the tendon of levator palpebrae superioris.
Small ducts open from the deep surface of the gland into the conjunctival sac.
Fluid produced by the gland passes medially towards the lacrimal puncta across
the surface of the cornea, assisted by blinking of the eyelid. Reflex blinking
is initi- ated if the cornea is touched or becomes dry. Evaporation of the
fluid is retarded by the oily secretion of the tarsal glands.
Extraocular muscles
Within the orbit most muscles comprise only striated
fibres, but those that move the eyelids also contain smooth fibres under
sympathetic control. Damage to this supply results in a drooping upper eyelid,
ptosis.
The extraocular muscles are the four recti, the two
obliques and one muscle which attaches to the upper eyelids, levator palpebrae
superioris (Figs 7.86–7.88). This is the uppermost muscle in the orbit, and from
its attachment to the lesser wing of the sphenoid it passes forwards to form a
wide tendon, which enters the upper eyelid and blends with the superior tarsal
plate.
The medial, lateral, superior and inferior recti (Figs
7.87, 7.88, 7.90 & 7.91) attach posteriorly to the
common tendinous ring that surrounds the optic canal and part of the superior
orbital fissure. Passing forwards, these four muscles attach to the eyeball
immediately behind the corneoscleral junction in positions corresponding to
their names. Collectively, they form a cone with its apex at the optic canal and its base around the equator of the eyeball. Nerves
and vessels entering the orbit run their course either within or outside this
cone of muscle.
Superior oblique (Fig. 7.89) has a posterior
attachment to the lesser wing of the sphenoid adjacent to the common tendinous
ring. The muscle passes anteriorly along the junction between the medial wall
and the roof of the orbit and forms a tendon which the optic canal and its base around the equator of the eyeball. Nerves
and vessels entering the orbit run their course either within or outside this
cone of muscle.
Superior oblique (Fig. 7.89) has a posterior
attachment to the lesser wing of the sphenoid adjacent to the common tendinous
ring. The muscle passes anteriorly along the junction between the medial wall
and the roof of the orbit and forms a tendon which traverses a loop of fibrous tissue, the trochlea, which lies at the
superomedial angle of the orbital margin and allows the tendon of superior
oblique to turn backwards across the upper surface of the eyeball. The tendon
lies beneath superior rectus and attaches to the superolateral part of the
eyeball behind the equator.
Inferior oblique (Figs 7.86 & 7.87), lying entirely in the anterior part of the orbit, attaches to the
floor of the orbit just lateral to the nasolacrimal
canal. The muscle passes posterolaterally below the inferior rectus to attach
to the inferolateral part of the eyeball behind the equator.
Three cranial nerves supply these muscles. The
abducens (VI) nerve (Figs 7.90 & 7.91) innervates
lateral rectus, while the trochlear (IV) nerve (Fig. 7.88) supplies superior
oblique. All the remaining muscles receive motor branches from the oculomotor (III) nerve (Figs 7.89 & 7.91).
In defining the actions of individual extraocular
muscles, it is assumed that the eyeball is positioned so that the gaze is
directed forwards into the distance. Within its fascial sheath, the eyeball is
rotated by the extraocular muscles, which displace the gaze upwards
(elevation), downwards (depression), medially (adduction) and laterally
(abduction). Rotation about an anteroposterior axis (torsion) may also occur.
Collectively, the extraocular muscles also contribute to the stability of the
eyeball, the recti tending to pull the globe backwards and the obliques tending
to pull it forwards.
The only actions of the medial and lateral recti are
adduction and abduction, respectively. Superior rectus elevates and adducts
while inferior rectus depresses and adducts. Both oblique muscles produce
abduction, the inferior oblique elevating the gaze and the superior oblique
depressing it. Eye movements in general involve the coordinated contraction and
relaxation of several individual muscles, and
elevation and depression are accompanied by movement of the eyelids. Levator
palpebrae superioris raises the upper eyelid (opposed by the orbicularis
oculi), while inferior rectus depresses the lower eyelid.
Nerves
Several nerves reach the orbit from the middle cranial
and pterygopalatine fossae. The optic (II) nerve (Fig. 7.89), which conveys visual sensation, traverses the optic canal with the
ophthalmic artery. Enveloped by meninges and cerebrospinal fluid, the nerve
passes forwards and laterally within the cone of rectus muscles and enters the
eyeball just medial to its posterior pole. Increased intracranial pressure is
transmitted through the cerebrospinal fluid to the eye, giving rise to the
clinical sign called papilloedema. Other nerves reach the orbit through the
orbital fissures.
The oculomotor (III) nerve (Figs 7.89 & 7.91) enters the cone of muscles via the superior orbital fissure. It has
superior and inferior divisions, which are often formed before entering the
orbit. The superior division supplies the superior rectus and levator palpebrae
superioris while the inferior division gives branches to inferior rectus,
inferior oblique, medial rectus and the ciliary ganglion. Sympathetic fibres to
the smooth muscle in levator palpebrae superioris and inferior rectus enter the
oculomotor (III) nerve in the cavernous sinus and travel with its branches to
these muscles. Damage to this pathway results in Horner’s syndrome, including ptosis.
The trochlear (IV) nerve (Fig. 7.89) enters the orbit
via the superior orbital fissure, passing above the muscle cone to supply
superior oblique.
The abducens (VI) nerve (Figs 7.90 & 7.91) gains the orbit via the superior orbital fissure and passes forwards
on the inner surface of lateral rectus, which it supplies.
The ophthalmic (V1) division of the trigeminal nerve
divides into lacrimal, frontal and nasociliary nerves, each of which enters the
orbit through the superior orbital fissure. The lacrimal nerve (Fig. 7.88)
passes forwards, outside the muscle cone, along the angle between the roof and lateral wall of the orbit. It is joined by
parasympathetic secretomotor fibres from the zygomatic nerve (p. 354), which
are destined for the lacrimal gland. In addition, the lacrimal nerve conveys
sensation from the lacrimal gland and the lateral part of the upper eyelid.
The frontal nerve (Fig. 7.88) lies on the upper
surface of levator palpebrae superioris and divides into supraorbital and
supratrochlear nerves. The supraorbital nerve (Figs 7.86 & 7.88) curves
around the upper part of the orbital margin, occupying the supraorbital notch,
and conveys sensation from the upper eyelid, forehead,
scalp and frontal air sinus. The supratrochlear nerve (Figs 7.86 & 7.88)
lies more medially, leaving the orbit just above the trochlea to supply sensory
fibres to the medial part of the upper eyelid, forehead and scalp.
The nasociliary nerve (Fig. 7.89), lying within the
muscle cone, crosses above the optic (II) nerve and continues forwards along
the medial wall of the orbit to terminate below the trochlea. Its branches
include one to the ciliary ganglion, two long ciliary nerves, posterior and
anterior ethmoidal nerves and the infratrochlear nerve. The long ciliary
nerves, which carry sympathetic vasoconstrictor fibres that join the
nasociliary nerve in the cavernous sinus, pass forwards to
supply vessels within the eyeball. The posterior and anterior ethmoidal nerves
leave through their respective foramina, supplying ethmoidal air cells. The
anterior ethmoidal nerve passes between the frontal and ethmoid bones and
emerges on the upper surface of the cribriform plate. Leaving the anterior
fossa, the nerve penetrates the plate to run on the inner surface of the nasal
bone as the external nasal nerve and eventually reaches the tip of the nose. It
conveys sensation from a strip of nasal skin close to the midline and from the
nasal septum. The infratrochlear nerve supplies the medial part of the upper
eyelid (Fig. 7.86).
The ciliary ganglion lies just behind the eyeball,
lateral to the optic (II) nerve. It
receives sensory fibres from the nasociliary nerve, sympathetic fibres from the
internal carotid plexus in the cavernous sinus and parasympathetic fibres from
the oculomotor (III) nerve. Only the last synapse in the ganglion. From the
anterior part of the ganglion, short ciliary nerves (Fig. 7.89) pass forwards
into the eyeball, conveying general sensory fibres from the eyeball (including the cornea), parasympathetic fibres to the ciliary muscle and sphincter pupillae and sympathetic fibres to dilator
pupillae.
Vasoconstrictor sympathetic fibres reach the eyeball
in the long ciliary branches of the nasociliary nerve. Other intraorbital
sympathetic fibres travel in the oculomotor (III) nerve to the smooth muscle
component of levator palpebrae superioris and inferior rectus. Parasympathetic
fibres reach the lacrimal gland via the lacrimal nerve, which communicates with
the zygomatic branch of the maxillary (V2) division of the trigeminal nerve.
The cell bodies of these fibres lie in the pterygopalatine ganglion (p. 354).
Vessels
The ophthalmic artery (Figs 7.89 & 7.90), a branch
of the internal carotid artery in the middle cranial fossa, enters the orbit
through the optic canal inferior to the optic (II) nerve. The artery then
accompanies the nasociliary nerve, passing above the optic nerve and continuing
forwards along the medial wall of the orbit. Its first branch, the central
retinal artery, enters the optic nerve and passes into the eyeball to supply
the retina. Occlusion of this artery results in blindness. Other branches of
the ophthalmic artery accompany those of the ophthalmic (V1) division of the
trigeminal nerve. In addition, there are posterior and anterior ciliary
branches to the eyeball and branches to the extraocular muscles.
Venous blood from the eyeball and adjacent structures
drains into inferior and superior ophthalmic veins. The superior ophthalmic
vein terminates posteriorly in the cavernous venous sinus while the inferior
vein passes through the inferior orbital fissure into the pterygoid venous
plexus in the pterygopalatine fossa. Both superior and inferior ophthalmic
veins communicate with veins on the
face.
