and body are connected wherever an action is performed. I have the will to grasp for the book before me, and obediently my arm performs the movement; the muscles contract themselves, the whole physical apparatus comes into motion through the preceding mental fact. The same holds true where no special will act arouses the muscles. If a thought is in my mind and it discharges itself in appropriate words, those words are after all as physical facts the movements of lips and tongue and vocal cords and chest; in short, a whole system of physical responses has set in through a mental experience. But the same thought may be the starting-point for many other bodily changes; it may make me blush, and that means that large groups of blood-vessels become dilated; or I may get pale, the blood-vessels are contracted. Or I may cry, the lachrymal gland is working; or it may spoil my appetite, the membranes of my stomach cease to produce; or my muscles may tremble, or my skin may perspire; in short, my whole organism may resound with mental excitement which some words may set up.
But it is not only the impression of outer stimuli and the expression of inner thoughts in which mind and body come together. Daily life teaches us, for instance, how our mental states are dependent upon most various bodily influences. If the temperature of the blood is raised in fever, the mental processes may go over into far-reaching confusion; if hashish is smoked, the mind wanders to paradise, and a few glasses of wine may give a new mental optimism and exuberance; a cup of tea may make us sociable, a dose of bromide may annihilate the irritation of our mind, and when we inhale ether, the whole content of consciousness fades away. In every one of these cases, the body received the chemical substance, the blood absorbed and carried it to the brain, and the change in the brain was accompanied by a change in the mental behavior. Even ordinary sleep at night presents itself surely as a bodily state—the fatigued brain cells demand their rest, and yet at the same time the whole mental life becomes entirely changed. It is not difficult to carry over such observations of daily life to the more exact studies of the psychological laboratory and to examine with the subtle means of the psychological experiment the mental variations which occur with changes of physical conditions. We might feel, without instruments, that our ideas pass on more easily after a few cups of strong coffee, but the laboratory may measure that with its exact methods and study in thousandth parts of a second, the quickening or retarding in the flow of ideas. Every subjective illusion is then excluded, our electrical clocks, which measure the rapidity of mental action and of thought association, will show then beyond doubt how every change in the organism influences the processes of the mind. Bodily fatigue and indigestion, physical health and blood circulation, everything, influence our mental make-up. In the same way it is the laboratory experiment which shows by the subtlest means that every mental state produces bodily effects where we ordinarily ignore them. As soon as we apply the equipment of the psychological workshop, it is easy to show that even the slightest feeling may have its influence on the pulse and the respiration, on the blood circulation and on the glands; or, that our thoughts give impulse to our muscles and move our organs when we ourselves are entirely unaware of it.
Again we may turn in another direction. Pathology shows us how every physical disablement of the brain is accompanied by mental processes. If the blood supply to the brain is cut off, we faint; a blow on the head may wipe out the memory of the preceding hours, and a hemorrhage in the brain, the bursting of a blood vessel which destroys groups of brain cells, produces serious defects in the mental content. A tumor in the brain may completely change the personality; the bodily disease of certain convolutions in the brain brings with it the loss of the power of speech; paralysis of the brain dissolves the whole mental personality. Physical inhibition in the growth of the brain involves, on the mental side, feeble-mindedness and idiocy. Of course, all this is not sufficient to bring out a definite parallelism between special mental functions and special physical processes, as the phenomena are extremely complex. If a patient who has suffered from a mental disturbance dies, and his brain is examined, there is no simple correlation before us. It may be difficult to diagnose exactly the mental symptoms. If we have heard that the man was unable to read, we do not know from that what really happened in his brain. He may not have read because he did not see the words, or because the letters were confusing, or because he had lost memory for the meaning, or because he had lost the impulse to speak the words, or because he felt unable to turn his attention, or because the impulse to read aloud was not carried out by his organism, or because an inner voice told him that it is a sin to read, or for many similar reasons; and yet each one represents psychologically an entirely different situation. On the other hand, on the physical side, the destruction is probably not confined to one particular spot. Complications have crept over to other places or the disturbance in one part works as inhibitory influence on other brain parts, or a tumor may press on a far-removed part, or the disturbance may be one which cannot be examined with our present microscopic means. In short, we have always a complex mental situation and a complex physical one, and to find definite correlations may be possible only by the comparison of very many cases.
Other methods, however, may supplement the pathological one. The comparative anatomist shows us that the development of the central nervous system in the kingdom of animals goes parallel to the development of the mental functions, and that it is not only a question of progress along all lines. Any special function of the mind may have in certain animal groups an especially high development, and we see certain parts correspondingly developed. The dog has certainly a keener sense of smell than the man—the part of the brain which is in direct connection with the olfactory nerve is correspondingly much bulkier in the dog's brain than in the human organism. Here too, of course, research may be carried to the subtlest details and the microscope has to tell the full story. Not the differences in the big structure, but the microscopical differences in the brain cells of special parts are to be held responsible. But comparison may not be confined to the various species of animals; it may refer not less to the various stages of man. The genetic psychologist knows how the child's mind develops in a regular rhythm, one mental function after another, how the first days and first weeks and first months in the infant's life have their characteristic mental possibilities, and no mental function can be anticipated there. The new-born child can taste milk, but cannot hear music. The anatomist shows us that correspondingly only certain nervous tracts have the anatomical equipment by which they become ready for functioning. Most of the tracts at first lack the so-called medullar sheath, and from month to month new paths are provided with this physical equipment.
Finally we have the experiment of the physiologist. His vivisectional experiments, for instance, demonstrate that the electrical stimulation of a definite spot on the surface of a dog's brain produces movements which we should ordinarily take as expressions of mental states, movements of the front legs or of the tail, movements of barking or whining. On the other hand, the dog becomes unable to fulfill the mental impulses if certain definite parts