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

BOOK 1

patches in the vestibule—and one that distinguishes them from all our other senseorgans. We saw that the sense-cells in the cochlea are not sensitive to sounds as such, but that they are touch-cells. Similarly, the sense-cells in the semicircular canals and vestibule are touch-cells, and the parts are so constructed that they are stimulated whenever we move, or whenever our position is abnormal. The inner ear, then, is a complex of several ingenious anatomical devices for stimulating touch-cells whenever certain conditions arise—conditions which themselves do not excite living protoplasm directly. The basal sense in all these cases is touch. Touch and chemical reaction, warmth, cold, and pain are the primary sensations, and all our perceptions of the outer. world, vision, hearing and all, are based on these by processes of elaboration. The sensory process of a man who listens to a sonata of Beethoven differs only in elaboration from the sensory process of stroking a cat, as does that of a man who admires a painting by Michaelangelo from the tasting of a crumb of sugar by a mouse.

§ 6 The Nervous Mechanism and the Brain

Now we are in a position to state certain general ideas about the working of the behaviour-system in the animal bodies we have made our opening subjects of study. Later on we shall take up these general ideas again and scrutinize them much more closely. We have considered the motor mechanism and the organs of sensation. Their action is correlated. How closely and exactly it is correlated, we shall defer until that later discussion. We feel something and we act in response; let that suffice for the present. We may hardly observe that we feel something, and yet we may act in response.

It is customary to distinguish and contrast two aspects of behaviour in reply to a sensory impression. As an example of the first kind we may consider what happens when the hand touches a hot object—the red end of a cigarette, for example. We withdraw the hand, and do so very quickly indeed, for the movement begins before our conscious minds are even aware of the pain. Such an action is called a reflex action, or more compactly a reflex. It is entirely automatic and independent of the mind ; we withdraw our hand, not because we think we are going to be burnt, but simply because we are made

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THE SCIENCE OF LIFE

CHAPTER 3

that way. It is, so to speak, part of our anatomy.

There is another type of response which does not appear to be so automatic as a reflex. It is more complicated. A certain element of memory and consciousness is involved. The reader hears a gong, for example, and makes appropriate preparations for lunch, or he hears the National Anthem and stiffens up as he has been trained to do. The behaviour of the higher animals is twofold in character. Its acts may be of a simple and primary form, part of an inherited and automatic system of reflexes as much a part of one’s constitution as the disposition of one’s arteries or bowels, or they may involve a larger or smaller element of acquired response. There is the simple reflex and there is the reflex complicated by something else less obviously mechanical.

This duality is reflected in our anatomy, for these two systems of response have different seats. In the accompanying figure it can be seen that the greater part of the cranium is filled with an oval, wrinkled mass, the cerebrum. A vertical cleft divides this organ into two halves, the cerebral hemispheres, but the cleft lies in the median plane and is not seen in the figure. The hemispheres are the seat of the mind ; it is here that those processes of memory and association take place which underlie responses of the second kind. The rest of the central nervous system, the spinal cord and certain lower parts of the brain that are overlapped and hidden by the cerebral hemispheres, are concerned with automatic actions only. Only the activities that come up to the cerebrum, and not all of these, are conscious.

By removing the cerebral hemispheres it is possible to get a mindless mammal, and to see just how far one’s many actions are due to the intervention of the higher centres. Such animals have the spinal cord and the higher reflex centres intact, but they lack any power of learning, understanding and memory, and presumably they lack any sort of continuing consciousness. A “decerebrate” frog, for example, can breathe and swallow ; it will sit in the usual position, and when pinched or otherwise stimulated it will jump away, guiding itself by sight so that it avoids obstacles; it turns over at once if it is put on its back. If it is put into water it swims about until it finds an island of some sort, such as a floating piece of wood, when it climbs up and then sits perfectly still. A decerebrate dog can do much the same ; it can breathe