Orbit and contents
The orbits are bilateral bony structures that resemble hollow quadrangular pyramids with their bases pointed anterolaterally, and their apices extended posteromedially. They are present in the upper half of the facial skeleton, beneath the anterior cranial fossa and anterior to the middle cranial fossa.
Each orbit contains the eyeball and its accessory visual structures: the optic nerve, the extraocular muscles, the lacrimal apparatus, adipose tissue, fascia, and the vessels and nerves supplying these structures.
The bony framework of each orbit consists of seven bones. These bones are the maxilla, zygomatic, frontal, ethmoid, lacrimal, sphenoid, and palatine bones. Together, they give the orbit its distinctive shape.
The roof is formed by the orbital portion of the frontal bone, and a small contribution from the sphenoid bone.
The medial wall is formed by four bones: the maxilla, lacrimal, ethmoid, and the sphenoid (with the orbital plate of the ethmoid bone being the largest contributor).
The lateral wall, on the other hand, is formed by contributions from two bones - the zygomatic and greater wing of sphenoid.
The floor of the orbit is primarily formed by the orbital surface of the maxilla with minute contributions from the zygomatic and palatine bones.
The eyeball contains the optical apparatus of the visual system. It is globe-shaped and located in the anterior part of the orbit.
The eyeball is associated with accessory visual structures that are housed and shielded by the orbit as well. The upper and lower eyelids consist of skin, subcutaneous tissue, voluntary muscles, orbital septum, tarsus, and conjunctiva. They protect the anterior surface of the eyeball when closed.
The lacrimal apparatus is a structure that is involved in the production, movement, and drainage of fluid from the eyeball surface. It consists of the lacrimal gland and its associated ducts, the lacrimal canaliculi, the lacrimal sac and the nasolacrimal duct. It’s divided into an orbital and palpebral part.
Muscles within the orbit include both extrinsic and intrinsic muscles. The extraocular muscles are seven in number. They are the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique. The levator palpebrae is responsible for the raising of the upper eyelid, while the rest are involved in the movement of the eyeball itself (elevation, depression, abduction, adduction). The intrinsic muscles that are within the eyeball control the shape of the lens and the size of the pupil.
The vascular supply of the orbit is by the ophthalmic artery, a branch of the internal carotid artery. It gives off many divisions that include the lacrimal, central retinal, long and short posterior ciliary, muscular, supraorbital, anterior and posterior ethmoidal, medial palpebral, dorsal nasal, and supratrochlear arteries. The two venous channels of the orbits are the superior and inferior ophthalmic veins.
Numerous nerves pass into the orbit and innervate its structures. These nerves are the optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), and the abducent nerve (CN VI). Other nerves include the autonomic nerves and the ophthalmic nerve (V1). All these structures mentioned above are embedded in orbital fat. The eyeballs and muscles are surrounded by the orbital fascia.