The science of life : fully illustrated in tone and line and including many diagrams, стр. 798

BOOK 8

to the dog, but after a few repetitions it calls forth signs of expectation—excited movements, licking of the lips, watering of the mouth and a similar, but invisible, watering of the inside of the stomach. In a word, the clicking is now recognized as a signal of impending food.

One might say that the dog has “learnt ” that the metronome means food, or that

CEREBRAL HEMISPHERES

CEREBELLUM

SPINAL CORD

CEREBRAL HEMISPHERE

PITUITARY GLAND

Fig. 323. the left side.

The cerebral hemispheres are better developed than those of a rabbit (Fig. 312), but not so well as those of a chimpanzee or a man.

it has associated the two together. But Pavlov was distrustful of such phrases. He wanted to make a new analysis, using terms of his own invention and definition, and to bring the phenomena as far as possible into line with simpler physiological facts already known with precision. First, then, he remarked that when the new habit was established it became indistinguishable

772

THE SCIENCE OF LIFE

CEREBELLUM

SPINAL CORD

The brain of a dog, seen from above and from

CHAPTER 6

in its action from an ordinary reflex as far as an outside observer could see. The clicking now evoked signs of appetite, precisely as a sudden movement near one of the eyes evokes an automatic blink. ‘The difference was simply that while the latter reflex is inborn in the organism, the former had been acquired and depended upon the peculiar conditions to which the dog had been exposed.

This likeness and this difference are both expressed in Pavlov’s terminology. The “learnt” response he called a conditioned reflex, since it was implanted by external conditions ; the other he called an unconditioned reflex, since it was part of the dog’s natural outfit.

The reader may object to this parallelism on the ground that the reaction to the metronome is much less infallible than the blink-reflex. An animal will only register appetite if the idea of food attracts it, and will ignore the clicking if it is already well filled. But even ordinary reflexes depend on the physiological state of the organism. The sexual reflexes are a good example, appearing in most animals at special seasons only, at the bidding of their internal secretions. On the other hand, a learnt reflex based on physical punishment instead of feeding may become quite as powerful and regular as an inborn reflex. The distinction is not a sound one. These are differences not of kind but of degree. A conditioned reflex is usually less powerful than an unconditioned one. In general a conditioned reflex is slower than an unconditioned one, because it involves a more complicated mechanism in the brain.

It will make this idea clearer if we put it in terms of brain structure. An ordinary reflex depends, as we have learnt, on the way the nervefibres are arranged, just as the course of the electricity in a circuit depends on the way the connections are arranged. Take the reflex of producing a watery saliva whenever an unpleasant substance is put into the mouth. Impulses flow up special nerve-fibres from the senseorgans in the mouth to a centre in the brain. Thence they run away along other fibres to another centre, and thence out to those parts of the salivary glands which make