The soft tissues of the orbit include the extraocular muscles, the orbital fascia and the orbital fat that fills the spaces between these.
Extraocular muscles
There are seven striated extraocular muscles, as well as two muscles containing smooth muscle fibres. The four rectus muscles attach to the apex of the orbit via the common tendinuous ring. This is a circular band of tissue the overlies the optic canal and part of the superior orbital fissure.
Structure
Superior rectus
The superior rectus arises from the common tendinuous ring, and runs anteriorly to attach to the sclera of the eyeball anterior to the equator. It causes the eyeball to elevate, adduct and rotates the eyeball medially. It is innervated by the superior branch of the oculomotor nerve (III).
Inferior rectus
The inferior rectus mimics the superior but attaches to the inferior sclera. It is innervated by the inferior branch of the oculomotor nerve (III). When active, it depresses, adducts and rotates the eyeball laterally.
Lateral rectus
The lateral rectus arises from the common tendinuous ring, running laterally and anteriorly to insert into the sclera of the eyball. It is supplied by the abducens nerve (VI). This muscle abducts the eyeball.
Medial rectus
The medial rectus runs anteriorly to the sclera. It is supplied inferior branch of the oculomotor nerve (III). It adducts the eyeball.
Superior oblique
The superior oblique arises medial and superior of the tendinuous ring at the apex of the eyeball. It passes anteriorly and medially to the trochlear, where its tendon passes around the small cartilagenous 'pulley'. The muscle then passes beneath the superior rectus and inserts into the sclera, posterior to the equator. The superior oblique is supplied by the trochlear nerve (IV). It causes the eyeball to abduct, depress and medially rotate.
Inferior oblique
The inferior rectus is unique in that it does not arise at the apex of the orbit, instead arising from its medial wall. It curves under the eyeball and passes above the inferior rectus. It inserts beneath the lateral rectus, posterior to the equator of the eyeball. It is innervated by the inferior branch of the oculomotor nerve (III).
Levator palpebrae superioris
The levator palpebrae arises at the apex of the orbit, just above the superior rectus. It follows the roof of the orbit, curving above the eyeball, and inserts into the tarsus and skin of the superior eyelid. It is innervated by the superior branch of the oculomotor nerve (III).
Smooth muscles of the orbit
The superior and inferior tarsal muscles insert into the tarsal plate of their respective eyelid. The superior tarsal muscle arises from the levator palpebrae superiorus, but passes more inferiorly to insert just into the tarsal plate. The inferior tarsal muscle arises from the inferior rectus. These muscles are innervated by sympathetic nerves.
Pathological changes
3rd Nerve Palsy
The oculomotor nerve provides somatic motor supply to the superior, inferior and medial rectus as well as the inferior oblique and the levator palpebrae superiorus. Palsy of the oculomotor nerve leads to unopposed abduction and depression of the eyeball ('down and out') as well as ptosis of the eyelid. The oculomotor nerve also supplies parasympathetic input to the pupil, and loss of this input leads to dilatation of the pupil.
4th Nerve Palsy
The trochlear nerve has the longest course of any cranial nerve before reaching the dura. Lesion to the nerve leads vertical diplopia, particularly with medial gaze.
6th Nerve Palsy
Lesions of the abducens nerve lead to unopposed action of medial rectus, leading to medially directed gaze.
Disruption of sympathetic chain in the neck.
The sympathetic nerves supply the tarsal muscles of the orbit. If these nerves are interrupted (often due to lung cancer in the apex of the lung - Pancoast Tumour) the loss of muscle action leads to a partial ptosis of the upper eyelid. The loss of sympathetic input to the eyeball leads to constriction of the pupil due to unopposed parasympathetic action.
Fascia of the orbit
The fascial sheath of the eyeball is a thick capsule which contains the eyeball. It is firmly attached at the corneoscleral junction (limbus) and at the entry of the optic nerve, but only connected by loose connective tissue elsewhere. It contains the ciliary vessels and nerves. It is reflected onto the attachments of the extraocular muscles to form the muscular fascia.
The muscular fascia of the lateral and medial recti continues to the orbital walls to create check ligaments. These prevent excessive abduction/adduction of the eyeball. The fascia of the inferior rectus continuous beyond the insertion of the muscle to attach to the tarsal plate of the inferior eyelid, causing retraction of the eyelid when gaze is directed inferiorly. The fascia of the the superior rectus and levator palpebrae superiorus is attached to the superior conjunctival fornix.
Orbital fat
Between the fascial sheaths of the muscles lies orbital fat, very loose tissue that allows free movement of the eyeball and muscles. The vessels and nerves run through this fatty tissue.
