The second of the facts upon which our knowledge of the relationship between mental life and nervous function is based, consists in the parallel effects of disturbances of their normal condition. Diseases or injuries of the brain are, as a rule, accompanied by disturbances of the mental life. On the other hand, mental disturbances can often be traced to lesions or structural modifications in the brain. This cannot be done in every case; but the actual connection is none the less certain. It is often very difficult to decide whether or not any mental abnormality exists. Expert psychiatrists have for weeks at a time observed men suspected of mental disease without being able to pronounce judgment. Equally difficult is the discovery of material changes in the brain and its elements. Much progress has been made in recent times in this respect; but it is still far from easy to recognize the more delicate changes in nervous structure resulting from disease. Certain abnormalities may never become directly visible although they involve disturbances of function, for instance, abnormalities in the nutrition of the nervous elements or changes in their normal sensitivity. No wonder, then, that for many mental diseases, as hysteria, corresponding material lesions are not yet known. But the correctness of our thesis is so strongly secured by the enormous number of cases in which it has been demonstrated, that no one doubts that it applies also to those cases in which, often for good reasons, its demonstration has thus far been impossible. Of much importance is the particular form of this relationship between brain function and mental life. Popular thought attributes the chief classes of total mental activity to special parts of the brain. Judgment is thought to have its seat behind the thinker’s high forehead. The occipital part of the brain is, according to the medieval philosophers, the organ of memory. And so Gall’s phrenology met with ready acceptance from the public at large, which was delighted to learn that musical ability, mathematical talent, religious sentiment, egotism and altruism, and many other character traits had their special organs in the brain. But anatomists and physiologists have not been able to admit the plausibility of this doctrine. Yet popular thought has, on the other hand, always emphasized the unity of mind. Those who regard its unity as the chief characteristic of mind have for centuries sought for the single point in the brain where the mind can be said to have its seat. If it were distributed all through the brain, would it not be possible to cut the mind into pieces by simply cutting the brain? That both these views of the relation between brain and mind are inadmissible has become certain. Since about forty years ago the truth in this matter has been known. But to understand it clearly it is necessary first to familiarize ourselves with the construction of the nervous system. QUESTIONS