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

BOOK 1

retina stimulated by an object depends on its position. The ear is a sense-organ for perceiving the qualities of sounds; our ideas of the direction from which sounds are coming are generally vague, based, for example, on the relative intensities with which they are heard in the two ears, and which ear hears them first. But the eye is an organ for determining the spatial properties of objects—their positions, shapes, and movements. The brain is constantly elaborating the information received from the eye by means of unconscious associations with past experience. In our judgments of solidity and the three-dimensional shape of objects, for example, we rely to a large extent on their shading and on associations of shading with experiences derived from the touch sense. But we cannot go further into this constant unconscious judging and weighing, nor into the many ingenious optical illusions that illustrate some of the errors that both our optical apparatus and our visual brain-centres can make. One or two such illusions are reproduced herewith. (Fig. 51.)

The characteristic thing about the eye is the precision of the spatial information that it can obtain. By means of its nose a mouse can tell that there is a cat about somewhere : by means of its ears, it can hear him moving and locate him roughly ; but by means of its eyes 1t can get precise information of the enemy's whereabouts—and that, after all, is the most important thing to know.

Sensations of Position and Movement. We have already remarked that the muscles, tendons, and joints are provided with minute sense-organs of elementary structure sensitive to such stimuli as the tension in a muscle, or the degree to which the two faces of a joint slide over each other. By means of these organs our impressions of the relative positions of our parts are derived. If, for example, we close our eyes and allow some other person to move one of our arms into a new position, we have no difficulty in feeling what position the arm has been put into. Such sensations are due to these simple organs in the muscles and joints.

The central nervous system is continually receiving impulses from these organs, and such impulses play a very important part in controlling movements. Obviously, if the central government wants to put the right hand into any particular position, the muscles that must be called into action depend on the position that the movement starts from. During complex acts, such as walking, impulses are constantly streaming

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THE SCIENCE OF LIFE

CHAPTER 3

in from the joints and muscles, informing the central government what stage in the walking process has been reached, so that it may know what to do next. This point may not seem very evident to the reader, but it is amply proved by the symptoms of diseases, such as locomotor ataxy, in which the sensory nerves of the legs are put out of action, while the motor nerves are unimpaired. There is no loss of power in the muscles, but there is a surprising loss of control. Even in the early stages the patient has to look at his legs while he is walking to get some idea of what to do next ; usually, he exaggerates his movements, raising the feet too high at each step and putting them to the ground with a stamp. In the later stages, walking becomes quite impossible ; when an attempt is made the legs are thrown about in all sorts of directions without any effective result. Thus, in spite of wellnourished muscles and intact motor nerves, the patient’s legs are paralysed as far as any useful activity is concerned.

In addition to these important senseorgans there is yet another group, a group of very elaborate and specialized sense-organs, situated in the head, from which we get impressions of the position and movements of the body as a whole.

The labyrinth of the inner ear, as we have seen, is a hollow bag of complicated architecture, embedded in bone. Moreover, only a part of it is concerned with the perception of sounds. Fig. 52 shows the appearance of this labyrinth. On the left is the spirally coiled cochlea, where soundvibrations are converted into touch-stimuli F in the middle there is a bag, the vestibule ; and on the right lie the structures with which we must now concern ourselves.

It will be seen that there are three arching tubes, the semicircular canals, each leading out of the vestibule, sweeping round in a regular half-circle, and opening again into the vestibule. It is important to observe that these three canals lie in three planes at right angles to each other. The posterior canal lies in the plane of the page, while the other two stand out perpendicularly

- from it ; at the same time the planes of the

superior and lateral canals are perpendicular to each other.

Each ,of the three canals has at one end a slight swelling, the ampulla. It is here that the actual sense-cells are located. In each ampulla there is a cluster of cells with long hair-like processes projecting into the cavity of the canal, and continuous below with nerve-fibres.