The science of life : fully illustrated in tone and line and including many diagrams, стр. 801

THE GCORTEX AT WORK

think, the same sort of thing happens on a higher level ; instead of running along the inborn nerve-paths in the spinal cord and brain-stem, impulses are circulating through the labyrinth of tracks that education has laid down in the cerebral hemispheres. In this sense our thinking is a symphony of elaborately associated conditioned reflexes.

But how is the actual thinking done? When we have equipped ourselves with these conditioned reflexes, how do we use them? To this problem we must now turn, with our humbler cousin the dog as a simplified diagram of ourselves.

But first a word about the experimental methods.

As a result of the assiduity with which the case against vivisection has been put before the public, many people have come to think that any experiment on a dog is necessarily a horrible, agonizing performance. We can reassure the reader at once that this at any rate does not apply to the experiments that Wwe are now considering. Let us note as exactly as possible how the animals were treated in the ordinary routine experiments, so that the humane reader can proceed with our brief summary of a brilliant piece of pioneer work without any qualms on the score of cruelty.

First, then, if the laws controlling conditioned reflexes are to be worked out properly, it is necessary to study some response that can be measured with a reasonable degree of accuracy. Then any fluctuations in its intensity can be followed. This requirement is fulfilled by that simple response, mouth-watering. All the animals used in the work had undergone a minor operation, conducted under full anzsthesia and with proper surgical precautions for the welfare of the patient, in such a way that one of the salivary glands discharged on the outside of the cheek instead of into the mouth. Asa dog has several salivary glands, this fraction of the secretion could well be spared. Recovery was rapid, for a dog is more elastic than a man is in this respect, and thereafter the animals showed no sign of discomfort at their treatment.

During the experiment, the dogs had to be held still. This was done by soft loops passing under the arm-pit and groin, in such a way that they could stand comfortably and shift their position, but could not move away. Moreover a little hollow capsule was put on the cheek, which communicated through a long thin rubber tube with a gauge outside the experimental room. The saliva from the operated gland was shed

into the capsule, and its volume could be accurately read off on the gauge. ‘This apparatus involved an amount of discomfort about equal to that produced by a pair of spectacles or a well-fitting denture. That the experience was not a terrifying one is amply evidenced by the fact that several of the animals used to run into the rooms of their own accord, and jump up on to the table where they stood during the experiment. When off duty, they were well cared for.

The rooms in which the experiments were conducted (except in the earliest stages of the investigation) were in a specially designed building, as sound-proof and smell-proof as possible, so that all distracting stimuli were avoided. These details may seem tedious, but it is because of the rigorous experimental methods that the results are so illuminating. Obviously if one is studying the responses of the animal to its environment, the first thing to do is to get an environment that one can regulate with precision. The experimenter himself was outside the room, controlling the experiment by means of electrical and pneumatic controls. By means of a peep-hole he could see how the animals were behaving, and by means of his gauge he could measure their salivary secretion with considerable delicacy and almost watch the thought-processes going on in their brains.

And now for some of the more important results that have emerged, and are still

emerging, from this almost meticulous analysis. § 4 Inhibition and Control The essential idea running through

Pavlov’s work is that the operations of the cerebral hemispheres are controlled by the interplay of two antagonistic processes, excitation and inhibition. This is not a peculiarity of the higher nervous centres. All through the nervous system a similar antagonism can be traced.

We noted in Book 1 that the heart is an automatic pump. It will continue to beat after its removal from the body if it is properly cared for. But in the body its activity is controlled. It quickens or slows and thus regulates its output to suit the needs of the organism. Running down to the heart are two pairs of nerves ; one pair is the bearer of messages which hurry it up, and the other of messages which curb its activity and slow it down—the first excite the heart while the second inhibit it. One is reminded of the dilemma with which poor Launcelot

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