Animal Life and Intelligence

by C. Lloyd Morgan

Fig. 32.—Retina of the eye. Enlarged section of minute fragment.

b., back of retina next the outer coat; l.r.c., layer of rods and cones; i.l., intermediate layers; l.g.c., layer of ganglion-cells; l.n.f., layer of nerve-fibres; f., front of retina, the surface turned towards the pupil.

The human eye is a nearly spherical organ, capable of tolerably free movements of rotation in its socket. What we may call the outer case, which is white and opaque elsewhere, is quite transparent in front. Through this transparent window may be seen the coloured iris, in the centre of which is a circular aperture, the pupil. The size of the pupil changes with the amount of light—it dilates or contracts, according as the light is less or more intense. Just behind it, and still in the front part of the eye, is the transparent lens, the convexity of the anterior surface of which can be altered in the accommodation of the organ for near or far vision. The space between the lens and iris and the corneal window of the eye is filled with a watery fluid. Behind the lens there is a transparent, semi-fluid, jelly-like material, filling the rest of the chamber of the eye. At the back of the eye is spread out the sensitive membrane—the retina. The structure of this membrane is very complicated, and cannot be described here. It is, however, indicated in [Fig. 32]. For our present purpose it is sufficient to note that here are the end-organs of the optic nerve; that these consist of a number of delicate rods and cones; and that these rods and cones do not face in the direction from which the light comes, but face towards the back of the eyeball, where a pigmented substance is developed. The rays of light are thus focussed through the retina on to this pigmented substance; the ends of the rods and cones are stimulated; and the stimulation is handed on, augmented in certain intermediate ganglia, to the delicate transparent nerve-fibres in the front of the retina. These collect to a certain spot, where they pass through the retina to form the optic nerve. Where they pass through the retina there can, of course, be no rods and cones. And in this spot there is no power of vision. It is the blind spot. The reality of its existence can easily be proved. Make a dot on a piece of writing-paper, and about three inches to the left of it place a threepenny or sixpenny bit. Close the right eye, and look with the left eye at the dot. The sixpenny bit will also be seen, but not distinctly. Keep the eye fixed on the dot, and move the head slowly away from the paper. At a distance of about ten inches the coin will completely disappear from view. Its image then falls on the blind spot.

The organ of vision, then, in us consists of an essential sensory membrane, the retina, with its delicate rods and cones; and an accessory apparatus for focussing an inverted image on to the sensitive surface of the retina. The surface is not, however, equally sensitive, or, in any case, does not give an equal power of discrimination, throughout its whole extent. This is seen in the experiment above described. When we look at the dot we see the coin, but not distinctly. The area of clear and distinct vision is, in fact, very small, constituting the yellow spot about 1/12 of an inch (2 millimetres) long, and 1/30 of an inch (.8 millimetre) broad. And even within this small area there is a still more restricted area of most acute sensibility only 1/120 of an inch (.2 millimetre) in diameter. Nevertheless, within this minute area there are some two thousand cones, the rods being here absent. In carefully examining an object we allow this area of acute vision to range over it. Hence the extreme value of that delicate mobility which the eye possesses—a mobility that is accompanied by muscular sensations of great nicety.

We saw that the sense of touch in the tongue is sufficiently delicate to enable us to recognize, as two, points of contact separated by 1/25 of an inch (1.1 millimetre). What, in similar terms, is the delicacy of sight? At what distance apart, on the most delicate part of the retina, can two points of stimulation be recognized as distinct from each other? If the points of stimulation be not less than 1/6000 of an inch (.004 millimetre) apart, they can be distinguished as two. Below this they fuse into one. The diameter of the end of a single cone in the yellow spot is also about 1/6000 of an inch (.0045 millimetre).

With regard to the mode in which the stimulation of the retinal elements is effected, we have no complete knowledge. Certain observations of Boll and Kühne, however, show that when an animal is killed in the dark the retina has a peculiar purple colour which is at once destroyed if the retina be exposed to light. If a rabbit be killed at the moment when the image, say, of a window, is formed on the retina, and the membrane at once plunged in a solution of alum, the image may be fixed, and an "optogram" of the window may be seen on the retina. The discharge of the colour of the retinal purple may be regarded as the sign of a chemical change effected by the impact of the light-vibrations. But in the yellow spot there seems to be no visual purple. It is, indeed, developed only in the rods, not in the cones. Here, probably, chemical or metabolic changes occur without the obvious sign of the bleaching of retinal purple. In the dusk-loving owl the retinal purple is well developed, but in the bat it is said to be absent.

We saw that in the case of hearing the auditory organ is fitted to respond to air-borne vibrations varying from about thirty to thirty thousand per second. And though the details of the process are at present not well understood, it is believed that certain parts of the recipient surface are fitted to respond to low tones, other parts to intermediate tones, and yet others to high tones. Thus the reception is serial. If there be two pianos near each other, accurately in tune, any note struck on one will set the corresponding note vibrating in the other.[FC] The auditory organ may be likened to this second piano. Special parts respond to special tones.

Now, in the case of vision, the conditions are different. The reception cannot be serial. As I range my eye over a flower-bed, I bring the area of distinct vision on to a number of different colours, and these are seen to be distinct, though they are received on the same part of the retinal surface. It might, perhaps, be suggested that special cones were set apart for each shade of colour. But there are only some two thousand cones in the central area of most acute vision, and Lyons silk-manufacturers prepare pattern cards containing as many shades of coloured silks. So that there would be only one cone to each colour. And Herschel thought that the workers on the mosaics of the Vatican could distinguish at least thirty thousand different shades of colour! There are also many phenomena of colour-blending which show that colour-reception cannot in any sense be serial.

How, then, are we to account for our wide range of colour-sensation? Just as the blending by the artist on his palette of a limited number of pigments gives him the wide range of colour seen on his canvas, so the blending of a few colour-tones may give us the many shades we are able to distinguish. The smallest number of fundamental colour-tones which will fairly well account for the phenomena of colour-vision, is three. And these three are red, green, and blue or violet. These are the three so-called primary colours. All others are produced from these elements by blending.

To explain our ability to appreciate differences of colour, then, it is supposed, on the hypothesis of Young and Von Helmholtz, that three kinds of nerve-fibres exist in the retina, the stimulation of which gives respectively, red, green, and violet in consciousness. Professor McKendrick, interpreting Von Helmholtz, gives[FD] the following scheme:—