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Eye anatomy

Recommended video: Nerves of the orbit [18:28]
Nerves found on the region of the orbit.
Orbit (anterior view)

The eyes are essential for our daily experience, since about 70% of information we gather is by seeing. They are placed within the orbits, two cavities in the upper face, in the anterior surface of the head.

Besides the eyes, the orbits host several structures that support the eyeballs, including muscles, vessels, nerves and a gland. The orbits are specifically designed to allow these neurovascular structures to pass through its walls, from the cranium on their way to the face.

This page will discuss the anatomy of the eye and orbit.

Key facts about the orbit and eyes
Orbit definition Bony cavity within the skull that houses the eye and its associated structures (muscles of the eye, eyelid, periorbital fat, lacrimal apparatus)
Bones of the orbit Maxilla, zygomatic bone, frontal bone, ethmoid bone, lacrimal bone, sphenoid bone and palatine bone
Structure of the eye Cornea, anterior chamber, lens, vitreous chamber and retina
Muscles of the eye Extrinsic: Superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, inferior oblique, levator palpebrae superioris
Intrinsic: Sphincter pupillae, dilator pupillae, ciliaris
Innervation of the eye Vision: Optic nerve (CN II)
Muscles: Oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves
Blood supply of the eye Ophthalmic artery, vorticose veins
Contents
  1. Bones of the orbit
  2. Eyelid anatomy
  3. Lacrimal gland
  4. Eye muscles
  5. Eyeball
    1. Outer layer
    2. Middle layer
    3. Inner layer
    4. Blood supply of the eye
    5. Nerves of the eye
  6. Sources
+ Show all

Bones of the orbit

The bony orbit is made out of seven bones, which include the maxilla, zygomatic bone, frontal bone, ethmoid bone, lacrimal bone, sphenoid bone and palatine bone. The orbit appears as a pyramid, with its base opening anteriorly onto the face, while the apex is pointed posteromedially. The orbit hosts the eyeballs, extraocular muscles, optic nerve, lacrimal apparatus, fat tissue, fascia and vessels that supply these structures.

The walls of the orbit contain several fissures, openings and fossae which are important for housing the orbital structures, and for neurovascular communication of the orbital content with the central nervous system. Some of the most important openings are the superior orbital fissure and its downstairs neighbour, the inferior orbital fissure. More about them and all the other landmarks of the orbital bones can be found in the following article and quizzes.

Eyelid anatomy

The eyelids are soft tissue structures that cover and protect the anterior surface of the eyeball. The anatomy of the eyelid may seem complex, but if we dissolve its multi-layered structure it is actually quite simple: 

  • Skin
  • Subcutaneous tissue
  • Muscle - orbital part of the orbicularis oculi muscle
  • Orbital septum - extensions of the periosteum from the orbital margin, that extends through both eyelids and supports them; 
  • Tarsus - plates of the dense connective tissue present in both eyelids. Superior tarsus is associated with muscles that raise the upper eyelid.
  • Conjunctiva - thin membrane that covers the posterior surface of the eyelid and reflects onto the anterior surface of the eyeball. 

Lacrimal gland

The lacrimal gland is a part of the lacrimal apparatus, which besides the gland consists of its numerous ducts, the lacrimal canaliculi, lacrimal sac and nasolacrimal duct. Everyone who has cried at least once, especially because of sleep deprivation that medical college brings, probably thinks that the lacrimal gland is placed in the medial part of the orbit since the tears run from there. But actually, the gland is placed in the lacrimal fossa at the upper part of the superior orbital wall, and its duct opens at the medial part of the orbit causing the tears to run down our nose when we think about how many anatomy pages we have left and it’s already 5 AM.

So, the function of the lacrimal gland is the production of tears, and it is regulated by the lacrimal nerve, a branch of the ophthalmic nerve (CN V1). Everything about the anatomy of the lacrimal gland and lacrimal apparatus can be found in these learning materials. We made them in a fun and approaching way, so we promise you won't cry: 

Eye muscles

There are two groups of eye muscles:

  • Extraocular muscles that move the eyeballs within the orbit
  • Intrinsic ocular muscles which are within the eyeball itself and control how the eyes accommodate

Six extraocular muscles move the eye: superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique muscles; and one other, levator palpebrae superioris, opens the eyelid. 
 

Extraocular Muscles
Superior rectus Origin - superior part of common tendinous ring (anulus of Zinn)
Insertion - anterior half of eyeball superiorly
Innervation - oculomotor nerve (CN III)
Function - elevation, adduction, internal rotation of eyeball
Inferior rectus Origin - inferior part of common tendinous ring (anulus of Zinn)
Insertion - anterior half of eyeball inferiorly
Innervation - oculomotor nerve (CN III)
Function - depression, adduction, external rotation of eyeball
Medial rectus Origin - medial part of common tendinous ring (anulus of Zinn)
Insertion - anterior half of eyeball medially
Innervation - oculomotor nerve (CN III)
Function - adduction of eyeball
Lateral rectus Origin - lateral part of common tendinous ring (anulus of Zinn)
Insertion - anterior half of eyeball laterally
Innervation - abducens nerve (CN VI)
Function - abduction of eyeball
Superior oblique Origin - body of sphenoid bone
Insertion - superolateral aspect of eyeball (deep to rectus superior, via trochlea orbitae)
Innervation - trochlear nerve (CN IV)
Function - depression, abduction, internal rotation of eyeball
Inferior oblique Origin - orbital surface of maxilla
Insertion - inferolateral aspect of eyeball (deep to lateral rectus muscle)
Innervation - oculomotor nerve (CN III)
Function - elevation, abduction, external rotation of eyeball
Levator palpebrae superioris Origin - lesser wing of sphenoid bone
Insertion - anterior surface of tarsus, skin of upper eyelid
Innervation - oculomotor nerve (CN III)
Function - elevation of upper eyelid

Don’t understand how all these muscles work? You can find out everything about them in the following learning materials.

Eyeball

The eyeball is the main structure within the orbit because it enables us to see. Its importance reflects on its size, making it the largest structure in the orbit. It is something like a little camera inside our head, with all the necessary parts for capturing an image. It is round with an anteriorly convex bulge. The most superficial layer of the convexity is the cornea, which refracts and focuses the details of the image. Posterior to the cornea are the anterior chamber, lens, vitreous chamber and the retina.

You have probably noticed that there is something in the eye called ‘the lens’, which is also present inside our digital cameras. It has the same function as a digital lens, but in the eyeballs it helps in focusing the light onto the retina.

The wall of the eyeball has three layers: an outer (fibrous) layer, a middle (vascular) layer and inner layer.

Let us have a closer look at the individual layers of the eyeball and their components.

Outer layer

The outer (a.k.a. fibrous) layer gives the eyeball its shape and provides resistance. The sclera is the tough opaque wall component of the outer layer covering the posterior five sixths of the eyeball. In addition, it acts as an attachment site for extrinsic and intrinsic muscles of the eye. The anterior one sixth of the eyeball is covered by the transparent avascular cornea instead.

Middle layer

The middle (a.k.a. vascular) layer of the eyeball houses the blood vessels of the eye. This layer specifically consists of the choroid, ciliary body and iris. The choroid is a pigmented layer of connective tissue and blood vessels located between the sclera and the retina. Its main role is to provide the other layers of the eyeball with oxygen and nutrients.

The ciliary body is composed of two different parts: the ciliary muscle and the ciliary processes. The lens of the eye is attached to the ciliary body. Contraction and relaxation of the ciliary smooth muscle will modify the lens’s thickness and focus. The other component of the ciliary body, the ciliary processes, contributes to the formation and secretion of aqueous humor.

Lastly, the final part of the middle layer is the iris, a pigmented circular contractile structure with a central aperture. This opening is the pupil, responsible for transmitting light. The diameter of the pupil is controlled by two involuntary smooth muscles innervated by the autonomic nervous system. The inner sphincter pupillae muscle is arranged in a circular manner around the pupil and is responsible for decreasing the pupil diameter (constricting the pupil) via parasympathetic innervation. The outer dilator pupillae muscle is radially arranged and enables the increase of the pupil diameter (dilating the pupil) via sympathetic innervation.

Inner layer

The inner layer of the eye is formed by the retina, the eye’s light detecting component and the intraoptic part of optic nerve (cranial nerve II). The retina is made up of two separate functional units: the optic and nonvisual parts. The optic part of the retina is light-sensitive and consists of two layers: neural and pigmented layers. The neural layer contains photoreceptors responsible for detecting visual light rays. The pigmented layer is composed of a single layer of cells. This layer is attached to the choroid and strengthens the light-absorbing properties of the choroid while minimizing the scattering of surplus light in the eyeball.

You can study the anatomy of the eyeball in detail through this study unit.

Blood supply of the eye

The eye is supplied by branches of the ophthalmic artery: these are the short posterior ciliary, long posterior ciliary, anterior ciliary and the central retinal arteries. Venous blood is conveyed by the four vorticose veins that drain into the ophthalmic vein.

Read this article about the blood vessels and nerves of the eye and immerse yourself with our learning materials to learn this topic in a more fun way.

Nerves of the eye

The main function of the eye is sight, and the nerve that enables sight is the optic nerve (CN II). Nerves that innervate the extraocular muscles are called bulbomotors and they are the oculomotor (CN III), trochlear  (CN IV) and abducens (CN VI) nerves. The oculomotor nerve also innervates the intrinsic ocular muscles and thus regulates accommodation. The lower eyelid is innervated by the infraorbital nerve, a branch of the maxillary nerve (V3), that passes through the infraorbital foramen. The upper eyelid is supplied by the oculomotor nerve.

Take a look at this video tutorial in order to enhance your knowledge about the nerves of the orbit.

Become a master of the eye anatomy with this specially designed quiz which covers bones and muscles of the orbit and eye anatomy (including neurovasculature)!

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