Orbit & eye anatomy l General human anatomy revision for dental students

Orbit & eye anatomy l General human anatomy revision for dental students

 

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Orbit & eye anatomy

 

The Orbital Region

• The orbits are a pair of bony cavities that contain the eyeballs; their associated muscles, nerves, vessels, and fat; and most of the lacrimal apparatus. The orbital opening is guarded by two thin, movable folds, the eyelids.

The Orbit

The orbit is a pyramidal cavity with its base anterior and its apex posterior. 

orbital margin :

• is formed above by the frontal bone, the lateral margin is formed by the processes of the frontal and zygomatic bones, the inferior margin is formed by the zygomatic bone and the maxilla, and the medial margin is formed by the processes of the maxilla and the frontal bone.

Roof:

• Formed by the orbital plate of the frontal bone, which separates the orbital cavity from the anterior cranial fossa and the frontal lobe of the cerebral hemisphere

Lateral wall:

• Formed by the zygomatic bone and the greater wing of the sphenoid

Floor:

• Formed by the orbital plate of the maxilla, which separates the orbital cavity from the maxillary sinus

Medial wall:

• Formed from before backward by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid (which separates the orbital cavity from the ethmoid sinuses), and the body of the sphenoid
• Openings into the Orbital Cavity

Orbital opening:

• Lies anteriorly . About one sixth of the eye is exposed; the remainder is protected by the walls of the orbit.

Supraorbital notch (Foramen):

• The supraorbital notch is situated on the superior orbital margin. It transmits the supraorbital nerve and blood vessels.

Infraorbital groove and canal:

• Situated on the floor of the orbit in the orbital plate of the maxilla; they transmit the infraorbital nerve (a continuation of the maxillary nerve) and blood vessels.

Nasolacrimal canal:

• Located anteriorly on the medial wall; it communicates with the inferior meatus of the nose. It transmits the nasolacrimal duct.

Inferior orbital fissure:

• Located posteriorly between the maxilla and the greater wing of the sphenoid; it communicates with the pterygopalatine fossa. It transmits the maxillary nerve and its zygomatic branch, the inferior ophthalmic vein, and sympathetic nerves.

Superior orbital fissure:

• Located posteriorly between the greater and lesser wings of the sphenoid; it communicates with the middle cranial fossa. It transmits the lacrimal nerve, the frontal nerve, the trochlear nerve, the oculomotor nerve (upper and lower divisions), the abducent nerve, the nasociliary nerve, and the superior ophthalmic vein.

Optic canal:

• Located posteriorly in the lesser wing of the sphenoid ; it communicates with the middle cranial fossa. It transmits the optic nerve and the ophthalmic artery.

Orbital Fascia

• The orbital fascia is the periosteum of the bones that form the walls of the orbit. It is loosely attached to the bones and is continuous through the foramina and fissures with the periosteum covering the outer surfaces of the bones.

• The muscle of Müller, or orbitalis muscle, is a thin layer of smooth muscle that bridges the inferior orbital fissure. It is supplied by sympathetic nerves, and its function is unknown.

Nerves of the Orbit

Optic Nerve

• The optic nerve enters the orbit from the middle cranial fossa by passing through the optic canal. It is accompanied by the ophthalmic artery, which lies on its lower lateral side. The nerve is surrounded by sheaths of pia mater, arachnoid mater, and dura mater. It runs forward and laterally within the cone of the recti muscles and pierces the sclera at a point medial to the posterior pole of the eyeball.

Lacrimal Nerve

• The lacrimal nerve arises from the ophthalmic division of the trigeminal nerve. It enters the orbit through the upper part of the superior orbital fissure and passes forward along the upper border of the lateral rectus muscle. The lacrimal nerve ends by supplying the skin of the lateral part of the upper lid.

Frontal Nerve

• The frontal nerve arises from the ophthalmic division of the trigeminal nerve. It enters the orbit through the upper part of the superior orbital fissure and passes forward on the upper surface of the levator palpebrae superioris beneath the roof of the orbit. It divides into the supratrochlear and supraorbital nerves to supply the skin of the forehead; the supraorbital nerve also supplies the mucous membrane of the frontal air sinus.

Trochlear Nerve

• The trochlear nerve enters the orbit through the upper part of the superior orbital fissure . It runs forward and supplies the superior oblique muscle.

Oculomotor Nerve

• The superior ramus of the oculomotor nerve enters the orbit through the lower part of the superior orbital fissure. It supplies the superior rectus muscle, then pierces it, and supplies the levator palpebrae superioris muscle.

• The inferior ramus of the oculomotor nerve enters the orbit in a similar manner and supplies the inferior rectus, the medial rectus, and the inferior oblique muscles. The nerve to the inferior oblique gives off a branch that passes to the ciliary ganglion and carries parasympathetic fibers to the sphincter pupillae and the ciliary muscle.

 

Nasociliary Nerve

• The nasociliary nerve arises from the ophthalmic division of the trigeminal nerve. It enters the orbit through the lower part of the superior orbital fissure, and ends by dividing into the anterior ethmoidal and infratrochlear nerves.

Branches of the Nasociliary Nerve :

1. The communicating branch to the ciliary ganglion is a sensory nerve.
2. The long ciliary nerves, two or three in number, arise from the nasociliary nerve as it crosses the optic nerve. They contain sympathetic fibers for the dilator pupillae muscle.
3. The posterior ethmoidal nerve supplies the ethmoidal and sphenoidal air sinuses.
4. The infratrochlear nerve passes forward below the pulley of the superior oblique muscle and supplies the skin of the medial part of the upper eyelid and the adjacent
5. part of the nose .
6. The anterior ethmoidal nerve passes through the anterior ethmoidal foramen and enters the anterior cranial fossa on the upper surface of the cribriform plate of the ethmoid. It enters the nasal cavity through a slitlike opening alongside the crista galli. After supplying an area of mucous membrane, it appears on the face as the external nasal branch at the lower border of the nasal bone, and supplies the skin of the nose down as far as the tip

Abducent Nerve

• The abducent nerve enters the orbit through the lower part of the superior orbital fissure . It supplies the lateral rectus muscle.

Ciliary Ganglion

• The ciliary ganglion is a parasympathetic ganglion about the size of a pinhead and situated in the posterior part of the orbit. It receives its preganglionic parasympathetic fibers from the oculomotor nerve via the nerve to the inferior oblique. The postganglionic fibers leave the ganglion in the short ciliary nerves, which enter the back of the eyeball and supply the sphincter pupillae and the ciliary muscle.

Blood Vessels and Lymph Vessels of the Orbit

Ophthalmic Artery

• The ophthalmic artery is a branch of the internal carotid artery after that vessel emerges from the cavernous sinus. It enters the orbit through the optic canal with the optic nerve. It runs forward and crosses the optic nerve to reach the medial wall of the orbit. It gives off numerous branches, which accompany the nerves in the orbital cavity.

Branches of the Ophthalmic Artery

1. The central artery of the retina is a small branch that pierces the meningeal sheaths of the optic nerve to gain entrance to the nerve. It runs in the substance of the optic nerve and enters the eyeball at the center of the optic disc. Here, it divides into branches of end arteries.
2. The muscular branches
3. The ciliary arteries
4. The lacrimal artery to the lacrimal gland
5. The supratrochlear and supraorbital arteries are distributed to the skin of the forehead.

Ophthalmic Veins

• The superior ophthalmic vein communicates in front with the facial vein.
• The inferior ophthalmic vein communicates through the inferior orbital fissure with the pterygoid venous plexus. Both veins pass backward through the superior orbital fissure and drain into the cavernous sinus.

Lymph Vessels

• No lymph vessels or nodes are present in the orbital cavity.