LITMIR.BIZ

      Fig. 1.1. The eye in cross section.

       The Fibrous Corneoscleral Coat

      It consists of the cornea and sclera.

      •The Cornea

      This is the anterior most transparent window of the eye. The cornea meets the sclera at the limbus, which is also where the conjunctiva ends. The conjunctiva covers the sclera but does not cover the cornea. The cornea is kept transparent by its avascularity and the innermost monolayer of cells (endothelium) which pumps fluid out of the corneal stroma. The cornea presents a tough barrier to trauma and infection and is responsible for about two-thirds of the eyes refractive power (the other one-third coming from the lens).

      •The Sclera

      This is an opaque white fibrous coat that also protects the eye and maintains its shape due to inherent structural integrity.

       The Uvea (or Uveal Tract)

      The middle vascular pigmented layer of the eye and consists of the iris, ciliary body and choroid.

      •The Iris

      This is a thin contractile circular disc, which is analogous to the diaphragm of a camera. The iris separates the anterior and posterior chambers, which are filled with aqueous humour and are in continuity through an opening, the pupil. The iris is attached by its root at the ‘angle’ (iridocorneal) of the anterior chamber where it merges with the ciliary body and trabecular meshwork. Aqueous humour drains mainly through the trabecular meshwork, which is visible using a mirror within a contact lens called a gonioscope.

      •The Ciliary Body

      This is approximately 6 mm in width and is responsible for the production of aqueous humour. It also contains muscles, which are attached to the zonular ligaments of the lens (changing its shape on contraction to focus or accommodate). It has two parts, the pars plicata and the pars plana. The pars plicata is the anterior part. It is 2 mm long (measured from the limbus) and contains about 70 ciliary processes which are the site of attachment for the aforementioned zonular ligaments. The pars plana is a posterior flat area which is 4 mm long. As the sclera and cornea are relatively rigid, excess production/reduced drainage of aqueous humour or injection of substances into the eye leads to raised intra-ocular pressure (normal is up to 21 mmHg). Intraocular pressure is high immediately after intravitreal injections (can be as high as 60 mmHg). Normalisation of the pressure usually occurs over 30 minutes after injection and is dependent on aqueous humour outflow through the trabecular meshwork.

      •The Choroid

      This highly pigmented and vascular posterior portion lies between the sclera and the retina and extends forward to the ciliary body. Its principal function is to nourish the outer layers of the retina and prevent unwanted light from reflecting back through the retina. It is composed of an outer layer of large calibre blood vessels, which divide into smaller diameter vessels and ultimately form the choriocapillaris (a network of capillaries). These drain into the vortex veins, which ultimately drain into the superior and inferior ophthalmic veins.

       The Retina (Neural Layer)

      This is where photoreception occurs and consists of two primary layers, the inner neurosensory retina and an outer layer called the retinal pigment epithelium. Anatomically the following regions are described:

      •The Macula (Latin ‘patch’, same as macula lutea)

      The area within the main vascular arcades and is 5–6 mm in diameter. Cone photoreceptors are mostly concentrated here for fine resolution (maximum density in the fovea).

      •The Fovea (Latin: ‘pit’)

      The central 1.5 mm diameter area of the macula. The foveola is the central 0.35 mm diameter area of the fovea.

      •The Optic Disc

      1.5 mm in diameter, it contains no normal retinal layers or photoreceptors (thus causing the blind spot) and is the area where nerve fibres of retinal ganglion cells pierce the sclera to enter the optic nerve. The central pale thinned area of the

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