Eye - Anatomy
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.