The retina is a complex peripheral nervous mechanism composed of many elements, the special functions of which are not adequately known. It spreads over the fundus of the eye, but where the nerve pierces the coats of the eyeball there is nothing but nerve fibres, and hence no retina, properly so called, exists at the optic disc.

The structure of the retina varies in different parts, but the following layers can be recognized in most regions (Fig. 231). The exceptions will be mentioned afterward.

Lying next to the hyaloid membrane is the layer of nerve fibres which radiate from the optic disc to the ora serrata near the ciliary region. The fibres spread evenly over the fundus except at the central point (fovea centralis), which they avoid by passing above and below it. These fibres form the inner layer of the retina.

Next to the fibres is a layer of nerve cells, which seem to have one pole connected with a fibre from the optic nerve, while from the other side two or three poles send processes into the adjacent layers of the retina. The cells are numerous near the yellow spot.

Outside the foregoing are four less distinctive layers. The first is broad and granular; next, two layers of peculiar nuclear bodies are found, with a thin, dense one of granular material between them.

Outside these, and separated by a fine limiting membrane, is the terminal layer of the retina. It consists of rods and cones which are connected with those parts of the retina already named, and are embedded in the protoplasm of pigmented epithelial cells, which, on their outer face, show a striking hexagonal outline (Fig. 234). The rods and cones are easily torn away in histological sections from the pigmented epithelium, but the epithelium and rods and cones are so intimately connected in their development and function that they ought to be regarded as a single layer.

Diagrammatic section of retina showing the relation of the different layers in the posterior part of the fundus (not the macula lutea).

Fig. 231. Diagrammatic section of retina showing the relation of the different layers in the posterior part of the fundus (not the macula lutea). (Schultze).

A retinal nerve fibril may be said to have the following course: entering the eyeball from the optic nerve at the porus opticus, it reaches the immediate vicinity of the hyaloid membrane, and runs a certain distance in contact with that membrane; it then turns outward toward the choroid and enters a nerve cell. From the nerve cell pass a couple of filaments which traverse the various granular and nuclear layers - where they probably inosculate with the filaments from other cells - and finally terminate in a rod or a cone. The rods and cones are the ultimate terminals of the nerves, and they lie in the active protoplasm of the peculiar, pigmented epithelial cells.

Showing the course of the fibres of the optic nerve, N.

Fig. 232. Showing the course of the fibres of the optic nerve, N, as they pass along the inner surface of retina, R, to meet the ganglion cells, g, whence special communications pass outward to the layer of rods and cones in the pigment layer p, next the choroid r, within the sclerotic s.

This outer layer, consisting of rods and cones lodged in epithelial protoplasm, is the effective part of the retina. Of this we have the following evidence: -

1. The Anatomical Fact

The Anatomical Fact that the rods and cones must be regarded as the nerve terminals of the optic nerve.

2. That The Macula Lutea

That The Macula Lutea, where the retina is chiefly made up of the cone layer, is very much the most sensitive part, and near the ora serrata, where the rods and cones are less developed, sight is least acute.

3. The Blind Spot

From the facts that where the optic nerve enters the eyeball there are no rods and cones, and though the nerve fibres are fully exposed to the light, they cannot appreciate it, this part, the optic disc, is called the "blind spot." This shows that the nerve fibres are quite insensitive to light, and that we must look to the terminals for its appreciation. The existence of the blind spot can be demonstrated as follows: Shut the left eye, and hold the left thumb, at ordinary reading distance, in front of the other eye. While the right eye is fixed on the left thumb, bring the right thumb to within about four inches, and move it slowly an inch or so, from side to side. A little practice soon enables one to find a place where the right thumb nail disappears. It also can be demonstrated by keeping the right eye, the left being closed, fixed on the small letter "a" and moving the page to or from the eye very slowly; a distance (about 10 inches) may thus be reached when the large letter "A" is quite lost. On approaching the page when "A" is invisible, the letter reappears from the inner side and "x" is first seen; on withdrawing the page it comes into view from the outer side and "o" is first seen. By varying the direction and noting the near and far limits of "A's" being invisible, one can mark out the extent of the fundus which is blind. This blind spot is not noticed in ordinary vision, as we have habitually overcome the deficiency by the experience derived from the use of both eyes since infancy. By rapid movements one eye hides the deficiency, as is found when attempting the experiment just described.

3 The Blind Spot 245

4. Purkinje's Figures

The fact that when the eye is illuminated in a peculiar way we can see the shadow of the blood vessels which lie in the inner layers of the retina thrown upon the outer layer of rods and cones, also shows the latter to be the sensitive part. This phenomenon, known as "Purkinje's figures," can be demonstrated as follows: Close the left eye in a dark room, with an evenly dull-colored wall, and while you stare fixedly at the wall with the right eye turned inward, hold a candle to its outer side so that the light can reach the fundus of the eye from the side. With a little practice the least motion of the candle will bring out an arborescent figure on the wall, which exactly corresponds to the retinal vessels. It may also be seen by arranging a microscope so as to show a bright light, on looking into the instrument and either moving it or the head slightly but constantly, the shadows of the retinal vessels will be clearly seen, as though they were under the instrument.