Intraocular structures

Tunica Fibrosa

Sclera and episclera

The sclera is a dense, fibrous tissue that forms the outermost layer of the eye.  Its primary functions are for protection and insertion of the extraocular muscles.

Externally, the sclera is covered by episclera, containing vessels and the anterior and posterior plexus.

Cornea

Clear, transparent layer covering the eye.  It is the eye’s main refracting surface- so is avascular.  The outer layer is stratified squamous nonkeratinised epithelium, while the innermost layer is simple cuboidal (substantia propria, bowman membrane and Descemet membrane between).

Uveal Tract

Choroid

Spongy, brown membrane with extensive venous plexuses which contains melanocytes and supplies nutrition to the retina.  It also prevents reflection within the eye (dark, pigment to absorb excess light).

Ciliary body

This encircles the eye anterior to the ora serrata (area where the retina ends).  Made up of smooth muscle for controlling the size of the lens.

Iris

Most anterior section of uvea.  It has a central apeture (pupil) and is peripherally attached to the ciliary body and anteriorly to the lens, separating the anterior chamber from the posterior chamber.  It has both sphincter (smooth muscle- parasympathetic of CN III) and dilator pupillae muscles (sympathetic fibres).

Lens

Crystalline, biconvex structure.  Attached to it are zonular fibres that pass to the ciliary body as the suspensory ligament (zonule).  It is avascular- derives nutrients from the aqueous humor.

Chambers of the eye

Theanterior chamberis bound by the cornea anteriorly, by the lens, iris and ciliary body posteriorly and by the trabecular meshwork laterally, through which aqueous humor drains into the scleral venous sinus (canal of Schlemm).

The posterior chamber is bounded anteriorly by the iris, posteriorly by the lens and zonule fibers and peripheraly by the ciliary processes.

Aqueous humor

Watery fluid that fills the anterior and posterior chambers.  It is secreted partially by the ciliary epithelium and partially by diffusion from ciliary capillaries.  Contains diffusable materials and low protein content.

Scleral venous sinus (Canal of Schlemm)

An annular vessel that encircles the eye, lined by endothelium.  It collects aqueous humor.

Trabecular meshwork

Sponge-like tissue that lies between the anterior chamber and the canal of Schlemm.

Vitreous body

Clear, transparent gel that fills the space between the retina and the lens and adheres to the retina.  Its function is to maintain the eye’s shape and to allow light to pass onto the retina.

Retina

The innermost layer, composed of photoreceptive cells.  In the posterior pole, a shallow depression is termed the fovea centralis- the point of greatest visual acuity, containing only cone cells.  Around the fovea is the macula.  It has several layers:

  • Pigment epithelium (closest to choroid)- functionally absorbs light and prevents reflection through melanin pigment.  It is also important as it releases vitamin A- a rhodopsin (light-sensing chemical) precursor.
  • Layer of rods/cones: Rods are slender, cylindrical cells; where as cones are flask shaped and have larger nuclei.
  • External and internal limiting membrane- the external supports the photoreceptor cells; the internal is the basal lamina of the Muller cells, separating the retina from vitreous body.
  • Outer and inner nucear layers: Outer is composed of nucleated portion of the rod/cone cells.  Inner contains the cell bodies and nuclei of the bipolar neurons and Muller cells.
  • Outer and inner plexiform layers: Outer contains synapses between rod/cones and bipolar cells.  Inner contains synapses between bipolar and ganglion cells.
  • Ganglion cell layer: cell bodies and nuclei of ganglion cells.  Neuroglia also present.
  • Optic nerve fibres: axons of ganglion cells that pass radially to form the optic nerve.
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