Internal tunic


The internal tunic is generally referred to as the retina. The retina is the inner layer of the eyeball and it covers the choroid, in part. The internal tunic, which is comprised of both an internal nervous layer and an external pigmented layer, creates an almost encompassing effect on the choroid. The pigmented layer is the visible layer. The pigmented layer reaches back to cover a percentage of the ciliary body and the iris. At the base of the ciliary body, the nervous portion of the tunic layer ends in a jagged edged margin, which is identified as the ora serrata.


Three principles layers created from neurons conjoin to create the nervous layer. Impulses begin with the rod and cone cells, which then continue on to the bipolar neurons, and finally into the ganglion neurons. Light is received in the opposite manner than the impulses are conducted. Light enters the ganglion cells first, then onto the bipolar cells until they finally reach the rod and cone cells for actual visual stimulation.

The rod and cone cells are the photoreceptors of the eyes, the rod cells, which number over 100 million per eyeball, are elongated, slimmer, and are designed for black and white vision. They are located along the peripheral part of the retina, aimed exactly for optimal black and white vision. These cells respond to the dim light sources. Form and movement also stimulate their response, however, rod cells are not attuned for visual acuity.


Internal tunic
Image: Internal Tunic

There are about 7 million cone cells per eyeball. These cells are specifically designed for higher visual acuity and are more susceptible to stimulation provided by daylight, color, motion, and form. The photoreceptors are designed to synapse with the bipolar neurons, which then synapse in form with the ganglion neurons, which then take to the optic nerve path as ganglion axons.


The middle of the retina contains a slight depression, known as the fovia centralis, which holds the cone cells. This is the region where the most accurate vision is produced. Cone cells also exist within the macula lutea, which refers to the yellow area which encompasses the fovia centralis.

At the exact point where the optic nerve meets the eyeball, known as the optic disk, there is a void of photoreceptors. This is of course, a blind spot on the eye. This blind spot is not usually discernable to people, as the eyes are quite active and move about constantly. If an object is viewed with both eyes, most often the object falls into the blind spot of one retina, where the other retina can then pick it up and complete the image.
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