CHAPTER III
THE BRAIN
I
An animal deprived of the brain is a machine which requires external stimuli in order to move. An uninjured animal is also a machine, but it differs from the other by that power in itself which renders it capable of moving and acting.
When an animal with its brain removed is touched on any point of its body quite lightly, it does not respond at once to this external stimulus, and only when these light touches are often repeated is a reactionary movement excited. There are some very wonderful experiments which made a great impression on me, when I first saw them performed by my friends Kronecker and Stirling in the laboratory in Leipzig. They took a decapitated frog, and fastened between the toes of one of the hind-legs a pen, which made marks on the paper of a rotating cylinder whenever the frog moved. Between the toes of the other leg they fastened the wires of an electric current; a pendulum alternately opening and closing the current in such a manner that an interrupted stimulus was obtained. It was strange to see how the headless frog responded regularly for hours. When stimulated by a weak current (so weak that it could not be felt on the tongue) more numerous repetitions, perhaps thirty, were necessary before the frog responded by a spasmodic movement. If the stimulus were stronger a much smaller number was sufficient to cause reaction, and this continued until life was extinct.
Stimuli accumulate in the spinal cord. We all know it from experience; when we have something in the throat which tickles us, the slight, and at first scarcely perceptible, irritation becomes almost unbearable if it continues, compelling us to cough in order to remove it. As the Italian proverb says, one cannot disguise a cough. Even a slight tickling of the skin has the same effect, and in the functions of reproduction the repetition of slight stimuli produce greater and more ungovernable reflex movements.
There are, however, impressions which remain longer accumulated in the brain before their energy finds expression in muscular activity. Sometimes a part of the nervous system charges itself slowly, like a Leyden jar under the influence of weak electric sparks, the tension of the nerve-cells remaining, as it were, hidden, until suddenly discharged by a contact or some very slight impression. We are astonished; it seems an accidental explosion to us, an effect out of all proportion to the momentary cause, forgetting that fire glows under ashes, that the force had been slowly accumulating, and so we believe we have accomplished the act by means of the will.
The aptitude of the nerve-cells to accumulate and preserve external impressions is such a leading fact in physiology that I do not know any more important one.