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

BOOK 1

projects into the outer world. The tube of the follicle does not fit tight, and near where it opens to the exterior there is a little belt of branching nerve-fibres which end in swollen blobs. All this is shown in Fig. 40. Now since the hair is loose in its tube, the slightest touch or push upon it tickles these swollen blobs and at once an impulse goes off towards the nervous centres.

This impulse says nothing explicit. It is, so to speak, a rap—nothing more. Any two sensory nerve-fibres are as like as two telegraph wires, and the impulses they transmit are, as far as we can see, identical. The central government has to interpret these messages, and to discriminate between the various kinds of feeling. How can you do so if the messages are all alike? Ifwe could examine all the sensory nerve-fibres in the body we should find that they vary in one respect only—they have different endings. It is in this difference that the answer to our question lies, for an impulse is interpreted by the central nervous system according to the particular nerve-fibres along which it arrives, and the centre or group of cells that receive the impulse. When they get inside the central nervous system different fibres run to different destinations. If, for example, an impulse comes along a fibre that supplies a hair-follicle, it is led to a centre which interprets it as a sensation of touch, and if it comes along a fibre from the sensitive layer of the eye, it is led to another centre which interprets it as a sensation of light. Because of these facts it is quite easy to deceive the central nervous system, and to send it lying messages. For example, we can press on the eyeball and so stimulate the sensory cells there. The reader can do it forthwith. The brain sees spots, clouds, or flashes, of light according to the sharpness of the impact or pressure. Similarly, disease of the nerve of hearing produces sound-sensations, often interpreted as “* Voices,’? and stimulation of the sense-organs of the tongue, by a weak electric current, for example, gives us a taste in the mouth.

A point of general interest is that if the pressure upon that very simple sense-organ the hair is sustained, the stream of messages soon dies away in the nerve. Nervous impulses generally result from a change of conditions ; permanent conditions do not go on producing them. The desirability of that is evident. Our mouse when it creeps out has to be on the alert for approaching enemies. His eyes are sentinels, on the look-out for danger, and evidently they must not be officious with their information ; they

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THE SCIENCE OF LIFE

CHAPTER 3

must not shout continually to the brain, ~ The bed is dim and grey and Mr. Everyman is motionless; Mr. Everyman is motionless ; Mr. Everyman is motionless.” They must be discreet and report only essentials, and so long as Mr. Everyman is motionless he does not matter a rap. The impulses travelling along the optic nerve are more like this, a dying report : “ everything quiet—quiet—quiet—auiet,” lifting suddenly to an arresting announcement“hallo! something moving there on the right!” A little movement in the room has more interest for the eye of a mouse than a whole roomful of still furniture. Its eyes become intent. The same is true of its ears or its nose ; there is little interest in all the customary flavours of Mr. Everyman and his belongings or in any of his familiar breathing exercises, but a sudden ejaculation or a sudden whiff of cat may be of vital importance. It is the changes that matter.

With all the sense-organs that produce precise sensations, a constant unvarying stimulus has little effect, but a change sets the impulses storming. Suppose, for example, that on a cold day we come into a warm room. ‘The sudden rise in temperature affects a set of organs in our skin that are sensitive to warmth ; there is a sudden discharge of impulses, and for a few minutes we feel warm all over. But soon the effect wears off. The temperature stays constant and the warmth-organs calm down; the room seems to get cooler. On stepping out into the cold again there is a similar cycle of events; the sudden change of temperature excites another set of organs, sensitive to cold, but they soon calm down at the constant low temperature and we have the illusion of “warming up.” On. this principle there rests a well-known and very instructive physiological paradox. Take three bowls of water, one hot, one.cold, and one lukewarm. Immerse the right hand in the hot water and the left hand in the cold and wait fora minute orso. There will bea cycle of sensation like that just described ; at first the right hand will feel hot and the left will feel cold, but the sense-organs will soon calm down. Now quickly immerse both hands in the lukewarm bowl. The right hand is suddenly cooled, so its cold-sensitive organs are stimulated ; the left is suddenly warmed, so its warm-sensitive organs are stimulated. Therefore one and the same bowl of water feels hot to the left and cold to the right. Our skins are meant to feel not temperature but changes in temperature.

True, our feelings of warmth and cold