The eyes of many animals, those of cats, for instance, exhibit a peculiar brilliancy, which is particularly remarkable in the dusk. It was formerly thought that the eyes of such animals emitted light independently, as it was also thought that light could be emitted by the human eye, under the influence of passion. This brilliancy, however, in the eyes of these animals is caused by a carpet of glittering fibres, called the tapetum, which lies behind the retina, and is a powerful reflector. In perfect darkness no light is observed in their eyes, a fact which has been established by very careful experiments; but, nevertheless, a very small amount of light is sufficient to produce the luminous appearance in them.

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CHAPTER IV.

Formation of the Retina-The Blind Spot and the Yellow Spot - Position of the Perception of Vision in the Retina.

THE retina is the nervous organ of the eye, a delicate and highly complicated apparatus, whose mechanism has hitherto been but little understood, whose formation has long been the object of laborious investigation, and is so still. Different layers can be distinguished in the retina when a fine section of it is examined with the microscope. The innermost layer, which lies next to the surface of the vitreous humour, consists of nerve-fibres, in which the optic nerve loses itself. It penetrates all the membranes of the eye at a point a little to one side of the centre, where the blood-vessels also enter the eye, which can be seen with the ophthalmoscope. At the point of entrance of the optic nerve, therefore, the retina consists only of the fibres of the optic nerve, which from this point radiate outwards. One spot, however, which lies in the centre of the retina, is free from nerve-fibres, and surrounded by them in a circular form. On account of its colour it is called the yellow spot. This is the part of the retina with which we see most distinctly; we will therefore speak more fully of its structure.

The layers of the retina are represented in fig. 24 as they have been described by Max Schultze. In the

interior there is a fine limitary membrane, I; then tothe exterior follows-the layer of nerve-fibres, 2;

the layer of nerve-cells, 3, which consists of cells similar to the ganglion cells of the brain; then the granular layer, 4, which consists of a grey indistinct mass of fine granules; then the inner granular layer, 5, consisting of little round grains; then the intermediate granular layer, 6, consisting of a fine granular mass with small fibres ; then follows the outer granular layer, 7, which is exactly like the inner one; and ultimately after a second fine membrane, 8, is found a layer of small Rods and Cones, 9, which has a very peculiar and interesting structure. For the most part it consists of small unconnected transparent rods, which are placed close together like palisades, at right angles to the surface of the retina. From time to time a small rod is seen between them, which expands at the end, and is called a cone. These cones lie very close together only in the midst of the yellow spot, where there is a small depression in the retina. In the yellow spot they are very numerous, but decrease in number towards the edge of the retina.

The rod and cone layer, on account of its regular arrangement, gives us a starting point for conjectures upon the action of vision. The light which enters the eye must undoubtedly pass through all the given layers of the retina, and ultimately reach the rod and cone layer which is covered externally by the black pigment of the choroid, 10. Light can penetrate no further, since it is here absorbed by the black pigment.

The rods and cones, judging from their appearance, have all the properties which are characteristic of sensory organs. On the surface of the retina they form a continuous covering, which closely resembles a regular mosaic, and each part, therefore, of the retina seems to be

provided with a special sensitiveness, imparted by a rod

Fig. 25.

or cone, while at the same time all points are united by an intimate, and, as it were, geometrical connection. These properties agree so closely with the mathematical distinctness of our power of vision, that they immediately give rise to the conjecture, that the action of vision must take place in this mosaiclike formation. If we draw a line to the retina, from a point of the field of vision, through the centre of the eye, it falls upon either a rod or a cone, is delineated on a little square of mosaic, and is perceived by its means alone. Thus we can imagine the entire field of vision to be formed as a fine mosaic upon the retina, in the same manner as a picture worked upon a piece of tapestry.

Of the layers of the retina the light-rays strike the fibres of the optic nerve

first, and from this reason we might be led to suppose