The science of life : fully illustrated in tone and line and including many diagrams, стр. 802

BOOK 8

Gobbo was tormented when at one elbow the devil pressed him to take to his heels and run away from his master, while at the other his conscience urged restraint. In the same way as the heart is controlled, the churning movements of the stomach and bowels can be excited or inhibited by appropriate nerves, and the muscle in the walls of the arteries can be tightened or slackened. To the importance of these and many other similar adjustments (and to the participation therein of internal secretions) we gave our attention in Book 1. In moments of violent exertion the heart and arteries are excited, so that the circulation speeds up and delivers blood more rapidly and at higher pressure, while the digestive organs are inhibited, so that the resources of the body can be concentrated on the voluntary muscles and nerves. In moments of relaxation, on the other hand, the circulation is calmed down, while the bowels churn happily away. Here we remind ourselves that our activities are reflected in this delicate interplay of excitation and inhibition in our chests and bellies because, as we shall shortly see, there is a similar interplay in our brains.

In the very flesh that clothes our limbs and bodies the two processes are continually active. Our voluntary muscles are always braced up and partly contracted in order to support the body and maintain its posture (except of course when we are limply asleep), and whenever a muscle tightens to produce a special movement the opposing muscles must slacken correspondingly. Both these changes are brought about by messages from the central nervous system. When we bend the knee, the bending muscles are excited and those which straighten out the knee-joint are inhibited. Strychnine upsets the nervous system in such a way that these compensating inhibitions are turned into excitation ; in strychnine poisoning, whenever a muscle contracts its opponent contracts too, with the most agonizing convulsions as the result.

In the central nervous system, inhibition can also be traced. The reflex centres can be excited or put temporarily out of action. As with our muscles this is a vitally important fact, for there is always a powerful undercurrent of reflex activity flowing through the nervous system, and if it were not for mutual inhibition the spinal cord would be a hopeless chain of jostling, conflicting reflexes. It is because one reflex can inhibit and temporarily suspend another that our organized automatic activity is possible. ‘To take a simple example, when

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THE SCIENCE OF LIFE

CHAPTER 6

a dog feels an itchy stimulus on one side of its trunk it lifts the nearest hind-leg and scratches the stimulated spot. Also, if one of the feet is hurt (for instance, by a cut, or the sting of an insect) it is reflexly raised, Just as we would automatically withdraw our fingers under the same circumstances. Now suppose that while one hind-foot is busy scratching, the other is suddenly hurtif both reflexes operated at once the animal would evidently fall down. But this never happens. Actually the “scratch reflex ” is inhibited by the ‘‘ flexion reflex’ of the hurt foot, so that the scratching leg is put to the ground to support the animal as the other is raised, even though the stimulating itch continues. The more urgent of the two reflexes has taken precedence over the other. All this is inborn and automatic ; it does not involve the intervention of the brain.

Imagine what the streets of a great city would be like if there were no control over the movements of motor vehicles. It is because of the policemen at the more congested corners, now moving the traffic along and now holding it up, now exciting and now inhibiting the flow, that road-transport can take place at all. So with the nervous flow and circulation of our body-city. Governing and integrating all the more mechanical of our activities we have these two tendencies—excitation and inhibition. Pavlov, already familiarized by his earlier work on the control of blood-pressure and the digestive organs with this balance of excitation and inhibition, extended the idea to the higher centres of the cerebral cortex. The conception of a balance in nervous impulses is by no means new; it is suggested already in such common expressions as “concentration” and “self control.” But it went no further than a half-metaphorical statement before the work of Pavlov. He applied it definitely and precisely, as the statement of a physiological fact. He brought it to the test of measurement in his experimental animals. And following it up, his work developed in the most surprising and illuminating way. As an instance of the part played by inhibition in the cerebral cortex, let us consider what he termed the “ extinction ” of a conditioned reflex.

Suppose that a conditioned reflex has been established in the brain of a dog, so that whenever it hears the sound of an electric buzzer it shows the usual signs of expectation of food. This is done, as we have already seen, by regularly sounding the buzzer and