Eyeball

• The globe-shaped eyeball occupies the anterior part of the orbit. Its rounded shape is disrupted anteriorly, where it bulges outward.

  • This outward projection represents about one-sixth of the total area of the eyeball and is the transparent cornea
• Posterior to the cornea and in order from front to back are:

  • anterior chamber
  • iris and pupil
  • posterior chamber
  • lens
  • postremal (vitreous) chamber
  • retina.
• The anterior chamber is the area directly posterior to the cornea and anterior to the colored part of the eye (iris).

  The central opening in the iris is the pupil.
• Posterior to the iris and anterior to the lens is the smaller posterior chamber.
• The anterior and posterior chambers are continuous with each other through the pupillary opening.
• The posterior and anterior chambers are filled with a fluid (aqueous humor),

  • which is secreted into the posterior chamber
  • flows into the anterior chamber through the pupil
  • and is absorbed into the scleral venous sinus (the canal of Schlemm) - which is a circular venous channel at the junction between the cornea and the iris
• Clinical Consideration

  The aqueous humor supplies nutrients to the avascular cornea and lens and maintains the intra-ocular pressure.

  • If the normal cycle of its production and absorption is disturbed so that the amount of fluid increases, intraocular pressure will increase. This condition (glaucoma) can lead to a variety of visual problems.
• The lens separates the anterior one-fifth of the eyeball from the posterior four-fifths.

  • It is a transparent, biconvex elastic disc attached circumferentially to muscles associated with the outer wall of the eyeball.
  • This lateral attachment provides the lens with the ability to change its refractive ability to maintain visual acuity.

  The clinical term for opacity of the lens is a cataract.
• The posterior four-fifths of the eyeball, from the lens to the retina, is occupied by the postremal (vitreous) chamber.

  • This segment is filled with a transparent, gelatinous substance—the vitreous body (vitreous humor) - This substance, unlike aqueous humor, cannot be replaced.
• Surrounding the internal components of the eyeball are the walls of the eyeball. They consist of three layers:

  • outer fibrous layer
  • middle vascular layer
  • inner retinal layer
• The outer fibrous layer consists of:

  • sclera posteriorly
  • cornea anteriorly.
• The middle vascular layer consists of:

  • choroid posteriorly
  • continuous with the ciliary body
  • iris anteriorly
• The inner layer consists of:

  • optic part of the retina posteriorly
  • nonvisual retina that covers the internal surface of the ciliary body
  • iris anteriorly
• Glaucoma

  Intraocular pressure will rise if the normal cycle of aqueous humor fluid production and absorption is disturbed so that the amount of fluid increases.

  • This condition can lead to a variety of visual problems including blindness, which results from compression of the retina and its blood supply.
• Cataracts

  With increasing age and in certain disease states the lens of the eye becomes opaque.

  • Increasing opacity results in increasing visual impairment.
  • A common operation is excision of the cloudy lens and replacement with a new man-made lens.
• The fibrous layer of the eyeball consists of two components:

  • sclera covers the posterior and lateral parts of the eyeball, about five-sixths of the surface
  • cornea covers the anterior part
• The sclera is an opaque layer of dense connective tissue that can be seen anteriorly through its conjunctival covering as the “white of the eye.”

  • It is pierced by numerous vessels and nerves, including the optic nerve posteriorly
  • provides attachment for the various muscles involved in eyeball movements.
• The fascial sheath of the eyeball:

  • covers the surface of the sclera externally from the entrance of the optic nerve to the corneoscleral junction
  • internally the surface of the sclera is loosely attached to the choroid of the vascular layer
• Continuous with the sclera anteriorly is the transparent cornea.

  • It covers the anterior one-sixth of the surface of the eyeball and
  • being transparent - allows light to enter the eyeball.
• The vascular layer of the eyeball consists of three continuous parts from posterior to anterior

  • choroid
  • ciliary body
  • iris
• The choroid is posterior and represents approximately two-thirds of the vascular layer.

  • It is a thin, highly vascular, pigmented layer consisting of smaller vessels adjacent to the retina and larger vessels more peripherally.

  It is attached

  • firmly to the retina internally
  • loosely to the sclera externally
• Extending from the anterior border of the choroid is the ciliary body.

  • This triangular-shaped structure, between the choroid and the iris, forms a complete ring around the eyeball.
  • Its components include the ciliary muscle and the ciliary processes
• The ciliary muscle consists of smooth muscle fibers arranged:

  • longitudinally
  • circularly
  • radially

  
  

  Controlled by parasympathetics traveling to the orbit in the oculomotor nerve [III]

  • these muscle fibers, on contraction, decrease the size of the ring formed by the ciliary body
• The ciliary processes are longitudinal ridges projecting from the inner surface of the ciliary body.

  • Extending from them are zonular fibers attached to the lens of the eyeball, which suspend the lens in its proper position and collectively form the suspensory ligament of the lens.
• Contraction of the ciliary muscle decreases the size of the ring formed by the ciliary body.

  • This reduces tension on the suspensory ligament of the lens.
  • The lens therefore becomes more rounded (relaxed) resulting in accommodation of the lens for near vision.
• Ciliary processes also contribute to the formation of aqueous humor.
• Completing the vascular layer of the eyeball anteriorly is the iris.

  • This circular structure, projecting outward from the ciliary body, is the colored part of the eye with a central opening (the pupil).

  Controlling the size of the pupil within the iris:

  • smooth muscle fibers (sphincter pupillae)
  • myoepithelial cells (dilator pupillae)
• Fibers arranged in a circular pattern make up the sphincter pupillae muscle

  • innervated by parasympathetic
  • contraction of its fibers decreases or constricts the pupillary opening
• Contractile fibers arranged in a radial pattern make up the dilator pupillae muscle

  • innervated by sympathetics
  • contraction of its fibers increases or dilates the pupillary opening.
• The inner layer of the eyeball is the retina.

  It consists of two parts.

  • Posteriorly and laterally is the optic part of the retina, which is sensitive to light
  • Anteriorly is the nonvisual part, which covers the internal surface of the ciliary body and the iris.

  The junction between the optic and nonvisual parts of the retina is an irregular line (the ora serrata).
• The optic part of the retina consists of two layers, an outer pigmented layer and an inner neural layer:

  • The pigmented layer is firmly attached to the choroid and continues anteriorly over the internal surface of the ciliary body and iris.
  • The neural layer, which can be further subdivided into its various neural components, is only attached to the pigmented layer around the optic nerve and at the ora serrata.
• It is the neural layer that separates in the case of a detached retina.
• Several obvious features are visible on the posterior surface of the optic part of the retina.
• The optic disc is where the optic nerve leaves the retina.

  • It is lighter than the surrounding retina and branches of the central retinal artery spread from this point outward to supply the retina.
  • As there are no light-sensitive receptor cells in the optic disc, it is referred to as a blind spot in the retina.
• Lateral to the optic disc a small area with a hint of yellowish coloration is the macula lutea with its central depression, the fovea centralis.
• fovea centralis is the thinnest area of the retina and visual sensitivity here is higher than elsewhere in the retina because it has:

  • fewer rods - light-sensitive receptor cells that function in dim light and are insensitive to color
  • more cones - light-sensitive receptor cells that respond to bright light and are sensitive to color
• Layers of the cornea (anteroposteriorly)

  • Corneal epithelium
  • Anterior limiting lamina (Bowman's layer)
  • Substantia Propria (Stroma)
  • Posterior limiting lamina (Descemet's layer)
  • Endothelium
• • Accounts for approximately 10% of the corneal thickness (50 μm) - Corneal Epithelium
  • Lies behind the anterior-most layer, and does not have fibroblasts - Bowman's layer
  • Approximately 500 μm thick and forms the bulk of the cornea - Stroma
  • Covers the most posterior-most layer, regarded as the basement membrane - Descemet's layer
  • Covers the posterior surface of the cornea - Endothelium