| Orbit (anatomy) | |
|---|---|
| The seven bones that articulate to form the orbit. yellow = Frontal bone green = Lacrimal bone brown = Ethmoid bone blue = Zygomatic bone purple = Maxillary bone aqua = Palatine bone red = Sphenoid bone teal = Nasal bone The nasal bone is illustrated but does not form part of the orbit |
|
| Latin | orbitae |
| Gray's | subject #46 188 |
| MeSH | Orbit |
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket,[1] or it can also be used to imply the contents.[2] In the adult human, the volume of the orbit is 30 ml, of which the eye occupies 6.5 ml.[3]
Contents |
The orbits are conical or four-sided pyramidal cavities, which open into the midline of the face and point back into the head. Each consists of a base, an apex and four walls. They protect the eye from mechanical injury.[4]
The base, which opens in the face, has four borders. The following bones take part in their formation:
The apex lies near the medial end of superior orbital fissure and contains the optic canal which communicates with middle cranial fossa.
The roof (superior wall) is formed by the orbital plate frontal bone and the lesser wing of sphenoid. The orbital surface presents medially by trochlear fovea and laterally by lacrimal fossa
The floor (inferior wall) is formed by the orbital surface of maxilla, the orbital surface of zygomatic bone and the orbital process of palatine bone. Medially near the orbital margin is located the groove for nasolacrimal duct. Near the middle of the floor, located infraorbital groove, which leads to the infraorbital foramen. The floor is separated from the lateral wall by inferior orbital fissure, which connects the orbit to pterygopalatine and infratemporal fossa.
The medial wall is formed by the frontal process of maxilla, lacrimal bone, orbital plate of ethmoid and a small part of the body of the sphenoid.
The Lateral wall is formed by the orbital process of zygomatic and the orbital plate of greater wing of sphenoid. The bones meet at the zygomaticosphenoid suture. The lateral wall is the thickest wall of the orbit. The optic foramen, which contains the optic nerve and the large ophthalmic artery, is at the nasal side of the apex, while a larger entry, the superior orbital fissure, through which veins, motor nerves, and non-visual sensory nerves (e.g., those for pain), among other fissures.
In the orbit, fat tissue, which surrounds the eyeball and its muscles, keeps the rotation smooth with respect to the center of the eye. If excess liquid is collected in the fat cushion tissue, the eye may protrude (known also as exophthalmos). Alternately, the eye may make an illusion of protrusion in extreme fear, not from the contraction of smooth muscle of the orbit, but based on the widening of the eyelids and dilation of the pupil (all commanded by the sympathetic nervous system.[5]
Enlargement of the lacrimal gland, located superotemporally within the orbit, produces protrusion of the eye inferiorly and medially (away from the location of the lacrimal gland). Lacrimal gland may be enlarged from inflammation (e.g. sarcoid) or neoplasm (e.g. lymphoma or adenoid cystic carcinoma).[6]
Tumors (e.g. glioma and meningioma of the optic nerve) within the cone formed by the horizontal rectus muscles produce axial protrusion (bulging forward) of the eye.
Graves disease may also cause axial protrusion of the eye, known as Graves' ophthalmopathy, due to buildup of extracellular matrix proteins and fibrosis in the rectus muscles. Development of Graves' ophthalmopathy may be independent of thyroid function.[7]
In humans, seven bones make up the bony orbit: