The retina lines the back part of the eye ball. It comes forward to the
ora serrata. It consists of ten layers which have been demonstrated
microscopically. One layer of it passes over the ciliary body and back
part of the iris to the pupil. The fibers of the optic nerve spread
out over the retina. The point of entrance of the optic nerve is the
papilla. It is to the inner side of the posterior pole of the eye. The
retinal vessels emanate there. The macula lutea is the yellow sensitive
spot at the posterior pole of the eye. The fovea is the center of
the macula. The rods and cones constitute the external layer of the
retina. This layer is the light perceiving stratum. For vision to be
perfect all the other layers must be perfectly transparent. The visual
purple is a chemical substance in the rods that gives the retina a
purplish-red color. The light shining into the eye forms images which
are converted into nervous stimuli by chemical action of the visual
purple and by physical changes and fibrillations in the rods and cones.