The science of life : fully illustrated in tone and line and including many diagrams, стр. 799

THE CORTEX

watery saliva. At the same time some of them will be conducted along other appropriately arranged fibres to the muscles which enable us to spit things out. All this is a natural and necessary consequence of the way in which the nerve-fibres are arranged in the brain. But the conditioned reflex involves a more tortuous journey for the nervous impulse. Let us consider an example. Suppose that a conditioned reflex is built up in the following way—a whistle is blown and a nasty substance is popped into the mouth, and the procedure is repeated until the sound of the whistle is alone sufficient to call forth watery saliva and expulsive movements of the tongue and _ throat. How has this come about? We know that when the whistle blows impulses stream from the ear to a special part of the brain, and we know when the expulsive response appears that impulses are streaming out from other brain-centres to the appropriate glands and muscles. Manifestly the formation of the conditioned reflex must consist in establishing a connection of some kind between the two centres. A track has been opened between these two centres. The simultaneous excitation of these centres has brought about their mutual connection. As a matter of fact the cerebral hemispheres have played the part of a switchboard, and the two centres have been put into more or less permanent connection with each other. It is this intervention of the cerebrum which is the essential distinction of the conditioned from the unconditioned reflex, and it was the realization of this fact which opened to Pavlov the new vast region of research into cerebral function and mental action he has so ably exploited in the past quartercentury. $3

The Dog as a Simpler Man

Let us try to realize the implications of this conception of the cerebrum as a switchboard for the conditioning of reflexes. If Mr. Everyman runs through his daily routine in his mind and tries to calculate how much of it is determined by his own personal circumstances and experiences, and how much of it is inborn, how much is learnt and how much instinctive, he will get an idea of the importance of conditioned reflexes in his own mental life. But he will probably underestimate that importance. He will

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AT WORK

be disposed to count many things as unconditioned which are in fact conditioned. For example, if little puppies are taken away from their mothers at birth and fed on milk, they do not recognize bread or meat as food the first time they see it. They sniff up to it curiously and taste it experimentally ; behaviour that contrasts very strikingly with the mouth-watering and liplicking of a more experienced dog who sees food. Only when they have eaten a substance once or twice do they give foodreactions at the sight of it. So that the very recognition of food depends on conditioned reflexes. Modern psychology is revealing more and more how responses which seem to us to be instinctive and automatic depend on events of early childhood, events that the passage of years has blotted altogether from our memories.

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HEARING

Fig. 324. The brain shown in Fig. 323, with some of the more important regions of the cerebral cortex indicated.

Compare Figs. 320 and 321,

Because of its very importance and multiplicity of detail the thinking machinery of the human brain is difficult to analyse. Here our remote but sympathetic cousin, the dog, is of great help to us. The mind of a dog is built on the same plan as ours, about as much as its body is built on the same plan as ours. But the mind of the dog is very much less intricate in detail, less fine in its working than the man’s mind. In a dog’s mind we can see many processes at work that are practically identical with those we observe in ourselves, but we see them in a simpler, more readily apprehended form. There is less danger of confusing incidentals with essentials in this simpler mind. ‘This is why these experiments on dogs give such valuable indications of the nature of our own mental machinery.

In the matter of speech, for example, we

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