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.
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.
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.
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).
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.