Orbit (anatomy)

Orbit
Eye orbit anatomy anterior2.jpg
Diagram of eye with surrounding superior, oblique, medial and inferior rectus muscles; supraorbital foramen shown above the eye, and inferior orbital fissure inferolaterally.
Details
Identifiers
LatinOrbita
MeSHD009915
TAA02.1.00.067
FMA53074
Anatomical terminology

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 millilitres (1.06 imp fl oz; 1.01 US fl oz), of which the eye occupies 6.5 ml (0.23 imp fl oz; 0.22 US fl oz).[3] The orbital contents comprise the eye, the orbital and retrobulbar fascia, extraocular muscles, cranial nerves II, III, IV, V, and VI, blood vessels, fat, the lacrimal gland with its sac and nasolacrimal duct, the eyelids, medial and lateral palpebral ligaments, check ligaments, the suspensory ligament, septum, ciliary ganglion and short ciliary nerves.

Structure

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.[4]

Openings

There are two important foramina, or windows, two important fissures, or grooves, and one canal surrounding the globe in the orbit. There is a supraorbital foramen, an infraorbital foramen, a superior orbital fissure, an inferior orbital fissure and the optic canal, each of which contains structures that are crucial to normal eye functioning. The supraorbital foramen contains the supraorbital nerve, the first division of the trigeminal nerve or V1 and lies just lateral to the frontal sinus. The infraorbital foramen contains the second division of the trigeminal nerve, the infraorbital nerve or V2, and sits on the anterior wall of the maxillary sinus. Both foramina are crucial as potential pathways for cancer and infections of the orbit to spread into the brain or other deep facial structures.

The optic canal contains the optic nerve (cranial nerve II) and the ophthalmic artery, and sits at the junction of the sphenoid sinus with the ethmoid air cells, superomedial and posterior to structures at the orbital apex. It provides a pathway between the orbital contents and the middle cranial fossa. The superior orbital fissure lies just lateral and inferior to the optic canal, and is formed at the junction of the lesser and greater wing of the sphenoid bone. It is a major pathway for intracranial communication, containing cranial nerves III, IV, VI which control eye movement via the extraocular muscles, and the ophthalmic branches of cranial nerve V, or V1. The second division of the trigeminal nerve enters the skull base at the foramen rotundum, or V2. The inferior orbital fissure lies inferior and lateral to the ocular globe at the lateral wall of the maxillary sinus. It is not as important in function, though it does contain a few branches of the maxillary nerve and the infraorbital artery and vein.[5] Other minor structures in the orbit include the anterior and posterior ethmoidal foramen and zygomatic orbital foramen.

Bony walls

The seven bones that 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 (illustrated but not part of the orbit)

The bony walls of the orbital canal in humans do not derive from a single bone, but a mosaic of seven embryologically distinct structures: the zygomatic bone laterally, the sphenoid bone, with its lesser wing forming the optic canal and its greater wing forming the lateral posterior portion of the bony orbital process, the maxillary bone inferiorly and medially which, along with the lacrimal and ethmoid bones, forms the medial wall of the orbital canal. The ethmoid air cells are extremely thin, and form a structure known as the lamina papyracea, the most delicate bony structure in the skull, and one of the most commonly fractured bones in orbital trauma. The lacrimal bone also contains the nasolacrimal duct. The superior bony margin of the orbital rim, otherwise known as the orbital process, is formed by the frontal bone.[6]

The roof (superior wall) is formed primarily by the orbital plate frontal bone, and also the lesser wing of sphenoid near the apex of the orbit. The orbital surface presents medially by trochlear fovea and laterally by lacrimal fossa.[7]

The floor (inferior wall) is formed by the orbital surface of maxilla, the orbital surface of zygomatic bone and the minute 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 primarily by the orbital plate of ethmoid, as well as contributions from the frontal process of maxilla, the lacrimal bone, and a small part of the body of the sphenoid. It is the thinnest wall of the orbit, evidenced by pneumatized ethmoidal cells.[7]

The lateral wall is formed by the frontal process of zygomatic and more posteriorly by the orbital plate of the greater wing of sphenoid. The bones meet at the zygomaticosphenoid suture. The lateral wall is the thickest wall of the orbit, important because it is the most exposed surface, highly vulnerable to blunt force trauma.

Borders

The base, which opens in the face, has four borders. The following bones take part in their formation:

  1. Superior margin: frontal bone and sphenoid
  2. Inferior margin: maxillary bone, palatine and zygomatic
  3. Medial margin: ethmoid, lacrimal bone, sphenoid (body of) and maxilla
  4. Lateral margin: zygomatic and sphenoid (greater wing)
Other Languages
العربية: محجر العين
Bân-lâm-gú: Gán-o
български: Очна орбита
Boarisch: Orbita
brezhoneg: Poull-lagad
čeština: Očnice
Deutsch: Orbita
euskara: Betzulo
فارسی: کاسه چشم
한국어: 안와
հայերեն: Ակնակապիճ
hrvatski: Očna šupljina
italiano: Orbita oculare
қазақша: Көз шарасы
lietuvių: Akiduobė
magyar: Orbitale
македонски: Очна празнина
Nederlands: Oogkas
日本語: 眼窩
polski: Oczodół
português: Órbita (anatomia)
русский: Глазница
slovenčina: Očnica
svenska: Ögonhåla
татарча/tatarça: Күз чокыры
українська: Очна ямка