4. _The Nervous System_ is not so well known, so far as its functions are concerned, as the circulatory system or the digestive system. Nevertheless, positive discoveries are extremely numerous: the action of the nerves on the glands and on the muscles; the part played by the different portions of the brain; nervous degenerations; the laws governing reflex actions--all this constitutes a formidable body of well-established facts. I do not pretend that everything is known. Alas! No! There are still innumerable truths to be discovered, and serious errors are doubtless most learnedly taught, with many contradictions, much uncertainty, much confusion--all of which simply proves that physiology is not a science whose last chapter has yet been written, that the last word of this science has not yet been pronounced. Nevertheless, blind indeed would the man be who would venture to conclude that physiology was not a science; or to assert that physiology is a science of little importance; that the rôle of the physiologist, from the point of view of the alleviation of human miseries, is null; and that knowledge of physiological facts is useless. Will it be claimed that the doctor has no need of a knowledge of physiology? I will reply by a comparison I am accustomed to make before my medical students when I wish to make them understand the necessity of a sound physiological education. Let us suppose that a watchmaker claimed to be able to cure disordered watches, but at the same time declared himself unable to tell by what springs and by what mysterious mechanism a healthy watch should mark the hour; that watchmaker would inspire me with a very small amount of confidence, and I would not go to him; for, until the contrary is proved to me, I believe that an indispensable condition for repairing a watch when out of order is to know how a watch should work when in good repair. Physiology exists only because there have been physiologists. By that I do not mean to say that all the truths of physiology are due exclusively to vivisection. I only claim that physiology without vivisection would be strangely clumsy, limited to a few empirical facts, and that, if vivisection be proscribed, we must resolutely give up classing physiology among the sciences. We may study the stars and the earth, electricity and heat, geography and history, and are we to be forbidden to study the functions of living matter? Such a proposal is obviously absurd, for of all the sciences accessible to man, physiology is that which is nearest to him.