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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

Now, suppose that the head is suddenly moved horizontally from right to left—i.e., in the plane of the lateral canal (a sudden backward glance over the shoulder would involve such a movement). ‘The labyrinth, as we have seen, is full of a lymph-like fluid, and when the head is suddenly moved the inertia of this fluid will make it lag a little behind the movements of the labyrinth itself. There will be a slight circulation of fluid in the lateral canal and, therefore, a bending of the delicate hairs on the sense-cells in the ampulla. ‘This bending stimulates the cells and causes them to dispatch nervous impulses, which are interpreted by the brain as a sensation of horizontal movement. Now, since the three canals lie in planes at right angles to each other, corresponding to the three dimensions of space, any movement of the head will bring about a circulation of fluid in one or more of the canals, and the direction of the movement will determine exactly the relative amount of disturbance in each of the three. That is to say, any movement, if it is vigorous enough, will cause a stimulation of the sense-cells in the ampullae, and by comparing the strength of disturbance in each of the three, the brain is able to judge the direction of the movement. Without this information we should be helpless when we tried to walk or run.

Tt will be noticed that the work-

that we stop abruptly. The inertia of the fluid will work in the usual way; it will tend to go on moving, and will circulate for a little time in the canals after we have come to rest. Evidently, it will now bend the sensitive hairs in the opposite direction, and we get the illusion of rotating in the reverse direction to that in which we were spinning. By putting our forehead down on to the top of a walking-stick, and then moving round it as fast as we can, we get giddy in a vertical instead of a horizontal plane.

On the other hand, there are sense-organs in the ear that apparently do not conform to the general law of contrast. On the wall of the vestibule there are two patches of sensitive cells, one patch lying in a vertical plane and the other in a horizontal plane.

ing of the apparatus depends on Fig. 52. The labyrinth of the inner ear, somewhat

the inertia of the fluid in the canals. It therefore detects, not constant uniform motion, but accelerations and retardations. If we are seated in a railway carriage with the blinds pulled down, as long as the train travels at constant speed we cannot feel whether we are facing the engine or not; but when the train slows down or speeds up the change in velocity is clearly felt. The semicircular canals are like most of our sense-organs ; they feel, not motion, but changes in motion. It is possible to demonstrate this fact by a simple experiment, an experiment that was familiar enough in the days when a waltz was a waltz and not a modified foxtrot. Suppose that we spin round for some minutes, swiftly but regularly. At first, as we accelerate, the fluid in our labyrinths lags a little, and we get the sensation of twisting round ; but after a little time, as a result of its friction against the walls, its circulation ceases and it comes to move exactly as the surrounding labyrinths do. Now, suppose

magnified.

The cochlea on the left ; the semicircular canals on the right.

The structure of these cells is essentially like that of the sense-cells in the ampullae, but in among their hair-like processes there are tiny nodules of carbonate of lime, called the otoliths. The otoliths are constantly pressing on the surrounding hairs, and they constantly stimulate the sense-cells with an intensity depending on the direction of their pressure. Since this direction depends on the position of the head—i.e., on its inclination to the vertical—the stimulation of the cells will also depend on the position of the head. In this way, by streams of impulses from the sense-patches in the vestibule, the brain is constantly informed about the inclination of the head.

At first sight the inner ear seems a very heterogeneous structure, since it detects factors so diverse as sound and position. But there is a principle common to all these organs—cochlea, semicircular canals, and

83