Eye Anatomy

The eye contains three layers. From outer to inner, they are:

  • The fibrous coat (aka corneoscleral coat)
    • Cornea
    • Sclera.
  • Uvea (aka uveal tract)
    • Iris
    • Ciliary body
    • Choroid (the majority of the uvea)
  • Neural layer: The Retina

THE OUTER LAYER: THE CORNEOSCLERAL COAT

  • Anteriorly, lies the CORNEA which has a pronounced curvature and is transparent to allow for the passage of light.
  • Where the cornea ends, the outer layer becomes the SCLERA, which is opaque, so it blocks the transmission of light.
    • The portion of the sclera we can see is the “white of the eye”; conjunctiva covers it.
  • Further posterior, six EXTRAOCULAR MUSCLES insert into the sclera.

THE BICONVEX LENS

  • It’s transparent and serves to focus a target on the retina, specifically on the area of maximal visual acuity: the fovea centralis of the macula.

THE MIDDLE LAYER: THE UVEA

  • In front of the lens, lies the pigmented IRIS, which forms an adjustable diaphragm to funnel light through the pupil.
    • The PUPIL is the open region within the center of the iris.
  • Posterior to the iris, label the CILIARY BODY.
    • The iridocorneal angle is where the corneal meets the iris; this is also the sceralcorneal junction: the site of the canal of Schlemm, which is fundamental to aqueous humor reabsorption.
  • Posterior to it, lies the CHOROID, which is a thin highly vascular layer sandwiched between the sclera and retina.
    • It nourishes the retina and removes heat produced during phototransduction, which is the process wherein the photoreceptors transform light into neural signal.

CILIARY BODY FUNCTIONS

  • Anchors suspensory ligaments, collectively called ZONULE, which stretch the lens and alter its refractive power.
  • Produces AQUEOUS HUMOR, which is a low- protein, aqueous (ie, watery) fluid.

THE VITREOUS CHAMBER

  • Contains vitreous humor (aka vitreous body).
    • Like aqueous humor, vitreous humor is primarily water, but the presence of glycosaminoglycans and collagen within this substance gives it its gel-like composition, which helps maintain the eye’s shape.

THE NEURAL LAYER: THE RETINA

  • Lies internal to the choroid.
  • Transitions into optic nerve when it exits the eye, posteriorly, at the lamina cribrosa: the retinal fibers become myelinated posterior to the lamina cribrosa.
    • They are unmyelinated within the retina to avoid blocking the passage of light through the retinal layers.
  • The central retinal artery and vein pierce the optic nerve and run through its center.

ANATOMICAL FEATURES OF THE RETINA

  • The optic nerve head.
  • The macula, the area of highest visual acuity (in the center of it, lies the fovea centralis).
  • The ora serrata is the anterior limit of the retina.
    • It’s an important anatomical landmark because it delineates the anterior limit of the retina and choroid, and the posterior limit of the ciliary body.

Anatomical details of the Retina:

  • On the nasal side, lies the optic disc (aka the optic nerve head). It comprises:
    • The neuroretinal rim (which is pink).
    • The optic cup, a pale hole through which the central retinal vessels emanate.
  • In the center of the macula lies the fovea centralis.

THE MENINGEAL LAYERS

  • The sclera becomes:
    • Dura mater (aka dural sheath)
    • Arachnoid mater (aka arachnoid sheath)
  • The pia mater is an extension of the optic nerve.
  • The subarachnoid space lies between the arachnoid mater and pia mater.
    • It allows increased intracranial pressure to translate along the optic nerve and impair its axoplasmic transport, which results in optic disc swelling: called disc edema or, rather, papilledema when it occurs in the setting of increased intracranial pressure.

SUPERFICIAL STRUCTURES OF THE EYE

  • The palpebra is the eyelid.
  • The palpebral fissure is the distance between upper and lower eyelids.
  • The corneal limbus separates the cornea from the sclera.
  • The sclera forms the “white of the eye”.
  • The iris is pigmented.
    • In its center is the pupil.
  • At the lateral extreme, lies the lateral canthus (aka lateral commissure).
  • At the medial extreme, lies the medial canthus (aka medial commissure).
  • The lacrimal caruncle lies at the medial corner of the eye; it produces whitish, oily fluid – “sleep in the eye”.

THE IRIS MUSCLES

  • Iris sphincter muscles are circumferentially-arranged.
    • They are parasympathetically-innervated muscles, which constrict pupil size in bright light.
  • Iris dilator muscles are radially-arranged.
    • They are sympathetically-innervated muscles, which widen pupil size in low light.

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