The retina:

It is a thin transparent membrane, placed between the vitreous internally  and the choriod externally. It extends from the edge of the optic nerve disc posteriorly to the ora serrata anteriorly then become continuous with the pigment epithelium of the ciliary body and iris.

Retinal thickness differs between regions. Near the optic disc it is about 0.56mm, change to 0.1mm at the ora serrata. It is the thinnest at the fovea centralis, where it measures about 0.09mm but around this area, in the region of the macula (lutea), it is more like 0.35mm.

Function of the retina:

-         detecting light and movements ( rods photoreceptors)

-         color vision (cone photoreceptors)

the retina is formed of 10 layers:

1-     Pigment epithelium:  single layer of pigmented cells with process.

2-     Rod and cons: light sensitive layer (visual receptors).

3-     External limiting membrane: a fenestrated membrane .

4-     Outer nuclear layer: formed of nuclei of rod ands cones.

5-     Outer plexiform (molecular): synapses between nuclei of rods and cones and bipolar cells.

6-     Inner nuclear layer: contain bipolar cells, horizontal cells, amacrine cells, and nuclei of muller’ s fiber (1^st order neurons).

7-     Inner plexiform layer: synapses between bipolar and ganglion cells.

8-     Ganglion cell layer: (second order neuron).

9-     Nerve fiber layer: Axon of ganglion cells layer and contains the retinal vessel.

10-Internal limiting membrane: it is in contact with vitreous.

Important structures of the retina:

1-     the macula and fovea: it is a dark area 3mm from the temporal side of the optic disc. Its depressed center centre ( fovea centralis), where cones only are present; is the most sensitive part of the retina, as it has 1:1 representation in the visual cortex.

2-     The optic nerve: has only one retinal layer which is the nerve fiber layer. So there is normally a blind  spot in the field of vision corresponding to the optic disc.

3-     Lamina cribrosa: is the posterior scleral opening through which the optic nerve fibers pass. It is traversed by the network of connective tissue and elastic fibers.

The normal fundus:

The optic dicsc:

a-     shape: rounded, physiological cup in its center forming 0.3 of the whole disc

b-    color; pale pink due to a rich capillary network.

c-     edge: regular , sharply defined.

d-    Size: 1.5mm in diameter.

The blood vessels:

a-     C.R.A: emerges from the physiological cup giving upper and lower trunks each divides into nasal and temporal branches.

b-    Veins: follow the arteries, the retinal blood vessels are transparent tubes and what we see is the color of blood through them.

The macula:

3 mm .i.e. 2 disc diameter to the temporal side of the optic disc. Appears dark red, avascular zone with a central yellow foveal reflex.

General background of the fundus:

It is pink in color ( color of the choriod blood vessels ).

Physiology of vision:

It is divided into:

1-     peripheral: for light and movement (rod function).

2-     Central: for details and color ( cone function).

Phases of vision:

1-     physio-chemical: light rays are absorbed by pigment epithelium leading to biochemical and electrical changes in rods (visual purple) and cones.

2-     Physiological: the passage of the nerve impulse ( light stimulus) from rods and cones through the visual pathway th the occipital cortex.

3-     Psyhological: the visual impression recognized and compared with previous impressions.