Orbit

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Frontal bone. Inner surface.

The orbit is a space in the cranium that contains the eye. The anterior of the orbit is covered by the eyelids, which cover and protect the eyes. The orbit also contains the lacrimal apparatus, which serves to moisten the eye, as well as help the eyelids clear the debris.

The eye lies anteriorly in the orbit, surrounded by a fascial sheath (fascia bulbi) which attaches in front to the sclera of the eye, and behind merges with the coverings (meninges) of the optic nerve. The fascial sheath is thickened inferiorly to form a suspensory ligament. Medial and lateral thickenings of the ligament (check ligaments) attach it to the medial and lateral walls of the orbit. The suspensory and check ligaments support the eye, lifting it from the floor of the orbit. The fascial sheath also allows the eye to move freely within the orbit, much of which is filled with fat.


Osteology

Note that the anterior (exposed) margins of the orbit are thick but that the deeper (protected) bones, particularly of the medial wall, are thin.

The skull from the front
Frontal bone. Inner surface.

Muscles

Muscles of the right orbit.

The muscles of the orbit are responsible for the extraocular movements of the eye. The extraocular movements of the eye are controled by the rectus muscles (medial, lateral, superior, and inferior), as well as the superior and inferior oblique muscles. The upper eyelids are raised by the levator palpabrae superioris

Nerves

The optic nerve (Cranial nerve II) is a special sensory nerve responsible for vision. Motor nerves include the oculomotor (III), trochlear (IV), and abducens (VI) nerves.

Sensory nerves are branches of the ophthalmic nerve. They supply the skin of the upper eyelid, forehead, scalp and nose, as well as the mucosa of parts of the nasal cavities and paranasal sinuses. They are the lacrimal (V1), frontal (V1), and naso-ciliary (V1) nerves.

The ciliary ganglion is a parasympathetic ganglion of note.


Blood supply

Ophthalmic artery

  • a branch of the internal carotid artery
  • arises in the cranial cavity
  • Enters the orbit through the optic canal, together with the optic nerve
  • Within the orbit, it gives branches which accompany the various motor and sensory nerves, to supply almost all the structures of the orbit
  • One important branch (central artery of the retina) enters the optic nerve and passes into the eye, where its branches supply the retina
  • The branches are end arteries (i.e., they do not usually anastomose)
  • Occlusion of the central artery leads to blindness

Ophthalmic veins

  • Superior and inferior
  • Valveless
  • Their tributaries accompany branches of the ophthalmic artery and drain areas supplied by the arteries
  • The superior vein communicates anteriorly with the facial vein
  • The inferior vein communicates through the inferior orbital fissure with the pterygoid plexus of veins
  • Both superior and inferior ophthalmic veins also communicate with the cavernous sinus through the superior orbital fissure
  • Because of these various venous connections and because the veins are valveless, there is the potential for the spread of infection through them.

Testing

  • It is also important to note that the movements described here are NOT necessarily the same as those a patient is asked to perform when the functions of individual muscles are tested.
  • For example, to test the functioning of the superior oblique muscle the patient is asked to look downwards when the eye is directed medially
  • However, acting alone the superior oblique will make the eye look down and laterally
  • In order to test the functioning of a given muscle, the actions of other muscles that produce similar movements must be eliminated.

Dissection

Resources