The science of life : fully illustrated in tone and line and including many diagrams

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

of nerve-fibres with the brain, and before we consider the mental processes accompanying vision we may take note of a curious anatomical fact about the way in which these nerve-fibres are arranged.

By analogy with other sense-organs one would expect the

vessels in that particular part of the retina. From it the nervous fibres run together to a special region of the brain, and, small as it is relatively to the rest of the retina, the fibres from its sensory elements make up a third of the whole thick stalk of the optic nerve.

We can see a

striped sensitive Lo thing distinctly

ends of the rods and cones to point towards — the light and the

nerve-fibres to

lead away in the

opposite direc-

tion. But, as a

matter of fact,

the exact op-

posite is

only when it is focused upon this spot, and so it is natural to conclude that S= such animals as the cow, horse, dog, or mouse

do not see things with anything like the

a 7. an

sory ends

of the re-

tinal cells are turned towards the wall of the eyeball and the nerve-fibres form a layer intervening between them and the light to which they are sensitive. One would expect this nervous layer, interposed between the retinal cells and the light, to fog the images that are cast on the cells; and so

ourselves. A short distance from the yellow spot is the point where the optic nerve leaves the eyeball. ‘The fibres of the retina all converge to this point and are there gathered together and depart. This is the blind spot; at this point there are no sense-cells at all, so no appreciation of the image is possible. Under

it does, over the greater normal conditions the part of the retina, Buin =—=—=—====—===——=== blind spot of one eye is the eyes of men, apes, and =—— SS covered by a sensitive par

monkeys this arrangement is modified in one region, lying nearly in the centre of the back wall of the eyeball, and called the yellow spot because it turns yellow after death. This yellow spot is the spot of distinct vision, and it makes the sight of the ani-

of the other, so its existence is not suspected, but by means of the well-

N. known experiment reproduced as Fig. >? ——__ 50 the reader will be able to convince himself that he 4

possesses a blind spot. King Charles II was

Fig. 51. Illustrating some errors of judgment so entertained by in our visual brain-centres.

this experiment that

mals that possess it a clearer and better

In (1) the four horizontal lines are straight and parallel ; in (2), the shaded areas are equal in height ; in (3), the sloping lines are straight, not stepped ; in (4), the two parts

he used to practise taking off the heads of

thing than that of any other mammals. In this place the nervous layer is reduced to about one-sixth of its normal thickness, so that the fogging of the image is imperceptible. The sensitive cells are very closely packed together, so that the image can be more accurately defined, and there are no blood-

of the horizontal line are equal halves.

his courtiers by this harmless method. There is an important contrast between the mental processes accompanying hearing and those accompanying vision. We have seen that the particular part of the cochlea stimulated by a note depends on its pitch. On the other hand the particular part of the 81