Not further than thirty or forty years ago it was very generally maintained and taught that the psychical activity of mind on the one side, and the chemical or physical changes which take place in the brain and the nerves on the other side, belong to two quite distinct domains, separated by a wide gap which can by no means be bridged over. Our sensations, our emotions, our thoughts, it was said, and the material changes which may go on in the nervous system, are not only two distinct sets of facts-they are two quite separate worlds, "separate in existence." Consequently, if physiologists should ever succeed in tracing each electrical current and each chemical change produced in the brain and the nerves whilst a sensation is awakened and thoughts besiege our mind, they nevertheless would add absolutely nothing to our knowledge of sensations and thoughts; still less to their interpretation. Facts of psychology can not be explained by facts of physics or chemistry. Current ideas, however, are rapidly changing upon this point. It lies beyond contest that from a mass of psycho-physiological investigations which have been made within the last thirty years, something new has continually been learned about man's psychical life-something that could not be learned from mere psychological self-observation. And gradually, even the strictest psychologists have grown accustomed to the idea that in the researches of physiologists they will find, to say the least, a most precious aid for their own investigations. One group of such researches-into the gradual evolution of senses in the animal worldwas analyzed last year in this Review; and the new light that was thrown by these researches upon the complicated nature of our own sensations, as well as upon the evolution of what we de scribe as the conscious state of mind, was indicated. Now we have to analyze another group of epoch-making discoveries relative to the finer structure of the nerve-system, and to see what may be learned from them about a still higher sphere of mental activity, namely, the associations of ideas and mechanism of thought. a The ambition of modern physiologists will be best understood from some such illustration as he following. Suppose a flash of lightning strikes our eyes, and we see a thunderbolt striking a tree in our neighborhood. Immediately, and quite unconsciously, we may stop in our walk, turn pale, or lift our hand as if to protect our eyes. Next we may make movement-run, let us say, towards the some quite conscious tree to ascertain whether a child which we saw a moment before in that direction has not been struck by the thunderbolt. Or the reminiscence of friend who has had a narrow escape in a similar circumstance may be awakened all of a sudden. Or we may set thinking about the rain which is coming, and is much wanted for the crops, or about electricity and the lightning, or about the beauty of the cause of suddenly illuminated landscape, and so on. Now, our sensations in this case, and our subsequent emotions, conscious actions, and thoughts may, of course, be described and studied by the psychologist; in fact nearly all the domain of psychology can be strolled this simple case. But then the physiologist steps in. He wants to know, in his turn, what changes, chemical physical, took place in the retina of our eye as it was struck by light; what nerves were irritated next, and to what parts of the brain and the spinal cord the nerve-current was transmitted; in which way these or those muscles of the arm, or such blood-vessels of the face, were contracted; what took place in the cells of the brain, and in which way the conscious run towards the tree was originated; by what mechanism the old, dormant reminiscences of a friend, or the familiar associations of lightning with rain, with electricity, or over in or with the beauty of a landscape, were awakened; in which spots of the brain were these associations stored, and how was it that once more they came to consciousness? an The problem is immense, and is imbued with the deepest interest. It matters little what are one's particular views upon "matter" and "mind." Once it is admitted that for each sensation, emotion, or thought there is equivalent process which goes on in the brain and the nerves-and that much is now admitted on all sides both processes must be known in full. They may be described as simply "parallel," but "separate in existence," and not in the least independent-that would be the dualist's view; or they may be considered, by the monist, as the two aspects, inner and outer, of the very same process; or the psychical process may be considered as a result of what took place in the brain and the nerves -such would be the materialist's view; but all three-the dualist, the monist, and the materialist-are equally interested in knowing both processes in all their details. This is, in fact, what science aims at at the present time. The task is, however, beset with almost incredible difficulties, and one of the chief among them was for a very long time the impossibility of making out the finer structure of nerve-tissues. In all sciences dealing with life it has been lately found out that a grosso modo study of the organs is utterly insufficient; that in order to understand nutrition and growth, reproduction and heredity-life in a word-attention must be turned to the wonderful phenomena which go on in the tiny microscopical cells. The same became necessary in psycho-physiology: the tiny nerve-cells, each of which leads its own life, while all are thoroughly connected together, had to be studied. Not further back than ten years ago that study met with almost insuperable obstacles. The nerve-cells were found to be surrounded with such an inextricable tissue of finest nerve-fibres that it seemed almost hopeless to disentangle the tissue. Imagine a thick felt, which is composed not of thick hair, but of millions of finest microscopical fibres ramifying in all directions, and try to follow in it each separate fibre! Various roundabout methods were tried, and the most astounding was that certain anatomists (especially His) succeeded to some extent in disentangling that network, at least for the white bundle of nerves. But the grey substance of the brain and the spinal cord defied all their efforts. Then came, in 1885, the welcome news that the Italian professor, Golgi, had discovered a new method of staining microscopical preparations, which enabled him to trace separate nervefibres in the grey tissue as well. The method soon was tested, slightly im proved upon, and in the hands of such anatomists as His, Lenhossék. van Gehuchten, Retzius, Sala, and especially the Spanish anatomist, S. Ramón y Cajal, and the veteran histologist Kölliker, it soon yielded quite unexpected results. In less than ten years the felt was disentangled; and the intimate structure of the brain and the spinal cord-their grey and white substances alike—the nerve-ganglia, and the nerve-system altogether, appeared under a quite new aspect. It is firmly established now that the different parts of the nerve-system consist of millions of microscopical nerveunits, which are all built upon the same fundamental plan. The name of neurons has been proposed for these units, and is now pretty generally accepted. Like all other cells, the nervecells consist of protoplasm, and have a nucleus and a still smaller nucleolus; while they vary, of course, very much in size, and their shape may be round or stellar, or roughly triangular. A typical nerve-cell differs, however, from all other cells in that it has, as a rule, two sorts of outgrowths. On the one side it gives origin to short ramifications of naked grey protoplasm, which may be covered with protoplasm granules or send out short side branches, so that they resemble a microscopic moss; these ramifications have received the name of dendrons. From the other side of the cell issues a nerve-fibre-one or their protoplasmic more. As a rule the fibre is longer than the dendrons: it may attain any length, from a mere fraction of onetenth of an inch to two or three feet. Its ramifications are also much longer, and it essentially differs from the den drons in being smooth and consisting, as a rule, of a thin thread of grey nerve-matter enclosed in a thin sheath of yellow, greasy, protective matter. The fibres end in a great number of thin tree-like, unsheathed ramifications. It has been remarked, moreover, that wherever the functions of the nervecells are well determined, the naked moss-like dendrons convey outside irritations towards the cell; while sheathed nerve-fibres convey the nervecurrent from the cell-to the muscles, to the tissues, or to other nerve-cells. the A nerve-cell, with moss-like ramifications of naked protoplasm (dendrons) on the one side, and with one or more sheathed nerve-fibres issuing from it on the other side-such is, then, the typical neuron. This is the fundamental unit out of which the nerve-system is built up. In a frog it will have but few and short ramifications on both sides; but in a lizard, in a rat, and the more so in man, the number of ramifications will be very great. Again, in the embryo of a mammalian the embryonal nerve-cell (neuroblast) will be simply an oval sac provided with but one thread-like appendage; but as the animal grows, two sorts of appendages appear, and they ramify more and more, in proportion as full mental development is attained. And, what is still more striking, whether we take those big cells in the grey cortex of the brain which are the organs of the highest psychical faculties (the so-called "psychical cells"), or any other nerve-cells, they maintain the same essential features. The differences are in the number of their ramifications, and in their connections with other neurons. The infinite variety of man's psychical life, his sensations, his emotions, his conscious and unconscious movements, and his thoughts have thus no other arena for their development but these nerve-cells, with outgrowths, and their nerve-fibres. An inexhaustible variety of psychical acts is achieved, not through a corresponding variety of elements, but through the countless multitude of connections which can be established between these millions of cells. When the flash of lightning makes us move our hand or run, laugh, or cry, and think of this or that subject, these manifold results are obtained in accordance with the various connections that are established between the ramifications of thousands of cells. Neurons of the brain and the spinal cord transmit nerve-impulses to each other, and nerve-currents are sent through other neurons to the arms and legs, to different muscles of the face, to the heart, to the blood-vessels, and to different other cells of the brain itself. And out of this multitude of connections a very small part only will reach our consciousness-what we call our "I"-while the immense number will be simply automatic and lie beyond any control. The linkage of the neurons deserves special attention. In the simplest case a neuron may receive irritations through its dendrons, and transmit them through its nerve-fibre to a muscle, which consequently contracts. More often, however, the ramified ends of the nerve-fibre do not yet reach a muscle; they spread amidst and around the dendrons of another neuron, and the nerve-current has to pass through a second neuron before it reaches the muscle. Again, the nerve-fibre of the second neuron often divides into two branches which run in opposite directions: one of them goes, for instance, up the spinal cord, reaches the brain, and there envelops with its twigs the dendrons of a psychical cell; while the other branch goes down the spinal cord and reaches another neuron, out of which a fibre runs towards a muscle or to some other neuron connected with some other part of the body. The countless combinations which may arise in this way and the complexity of results can easily be imagined. The three chief parts of the central nerve-system of man-the brain, the spinal cord, and the nerve-gangliaare thus closely connected together. Nay, within the brain itself countless fibres connect its different parts and regions; while thousands of nerves connect the organs of senses, as well as all muscles and all the inner organs, and even each hair of the skin, with some part of the central system. Surely it is by no means an easy task to find out the paths of the nerve-currents within that amazingly complicated network. But they have been followed, and are now known to a great extent. An illustration will perhaps better represent the complexity of these connections, and show their characters. Suppose the skin of the right hand is irritated by, let us say, a burn. The end-ramifications of some nerve-fibre, which exist in every portion of the skin, at once transmit the irritation in wards, to a ganglion cell, located near the spinal cord. From it a nerve-impulse is sent along another nerve-fibre, which enters, let us say, the spinal cord, and there envelops with its endbranches the dendrons of some neuron. The central nerve-system has thus been rendered aware of the irritation of the skin, and in some way or another it will respond to it. The nerve-current, after having reached the cell of that spinal cord neuron, immediately issues from it along a nerve-fibre; and if that fibre runs towards a striated muscle of, let us say, the other hand, our left hand may touch or scratch the burned spot without our "I" being aware of that action; it is a simple reflex action. But the nerve-fibre of that same cell may divide into two main branches, and while one of them runs to the muscle of the left hand the other branch runs up the spinal cord and reaches (either directly or through an intermediate neuron) one of the big pyramidal cells of the grey cortex of the brain. The ramifications of this branch envelop the dendrons of the brain cell and transmit the impulse to it. Then our "I" becomes conscious of the sensation in the right hand, and we may-quite consciously this time-ex amine the burn. However, the pyra midal cell in the grey cortex is connected, through its dendrons and fibres, with many other cells of the brain, and all these cells are also started into activity. But the big pyramidal cells, in some way unknown, are the recipients and keepers of formerly received im pressions; and as they are stimulated, associations of previously impressed images-that is, thoughts-are generated. A familiar association between a burn and oil may thus be awakened. and we put some oil on the burn. At the same time the nerve-impulse was also transmitted to that row of ganglia (the so-called vaso-motor system) which is connected with the heart, the intestines, and all other inner organs, as also with the blood-vessels, the glands, and the roots of the hair. And if the burn was severe, and very painful, the activity of the heart may resent it, as also the blood-vessels: we may turn pale, shed tears, and so on. Now, if we analyze this illustration (which only represents one out of scores of possible results of an outward irritation), we see that various hypotheses may be made for explaining how an undoubtedly physical nerve-impulse (the burn) could be transformed into complicated psychical processes, each of which also ended in physical facts (contraction of muscles, tears, and so on). The dualist, the monist, and the materialist will each defend his own hypothesis. But the paths which the nerve-impulse follows, and the activity which it starts in a number of cells of the brain, the spinal cord and the ganglia are hypotheses no more. They are facts which have been ascertained by scores of direct observations. The electrical effects of the nerve-impulse, as it is transmitted along such and such nerves, have been measured; its transmission from this or that cell of the cord to these or those cells of the brain has been tracked step by step: nay, as will presently be mentioned, the activity of the stimulated nerve-cells in the brain and elsewhere has actually been observed, and the effects of fatigue in nerve-cells have been studied of the skin will be transmitted in this way, and not in another, to such cells in such cases, and start into activity such cells of the brain, this is speculation no longer. It is a fact of natural science, firmly established, and verified in different ways by a mass of mutually controlled observations. II. in detail. And when the anatomist At the same time, in the front segment maintains that an irritation of the animal-its head-the vision and the smell organs are specified more or less; nerve-matter grows round the inner ends of the nerves of these sense organs, and a sort of brain is developed; while in the high articulates (bees, wasps, ants) there is already a real brain, connected by nerve-fibres with all other parts of the body. And yet each of the segments of the body being possessed of its own ganglia, maintains its own consciousness. The body of a beheaded grasshopper continues for a short time to lay eggs, and the head of an ant, separated from the trunk during one of their battles, remains, as the great explorer of ants, Professor Forel, remarks, for a few seconds "capable of distinguishing between an inhabitant of its own nest and a stranger, and behaves accordingly"—that is, tries to fight the latter. The consequences of these remarkable researches and discoveries are evidently numerous and important. Each neuron is, then, a separate unit, and can be compared to an amoeba-like organism, possessed of its own life, its own irritability, and its own capacity to receive irritations from without and to answer them. And, starting with loose aggregations of nerve-units of the simplest typesuch as are found in both the lowest animals and the embryos of the highest ones-it became possible to trace step by step the evolution of the nerve-system in the animal series, and the parallel evolution of psychical faculties-not yet in all details, but in all its essential features. con At a very low grade, in very low animals, the neurons are already found grouped into nerve-ganglia, and there they already become connected together and associated. Consequently, a very vague sort of sciousness, derived from irritations transmitted to a group of associated nerve-cells, already makes its appearance. Then, going a step further, to the lower articulate animals, such as worms, which consist of separate segments, a succession of nerve-ganglia is found-one pair in each segment; and as that series of articulates is ascended higher and higher, nerve-fibres are discovered, which issue from separate ganglia, and run lengthways, connecting them together. Accord ingly, a sort of common consciousness of the whole being, of the whole chain of segments, is evolved, and every one can easily ascertain it himself by observing the behavior of these animals. It is only in vertebrates that the brain gradually attains a prevailing influence over the entire nerve-system, and that such anatomical features are discovered in it as correspond to a higher psychical activity. Only in birds and mammals, but not yet in fishes and reptiles, the inner ends of the nerves of vision become connected with the grey cortex of the brain by a great number of nerve-fibres; and consequently the bird, and still more the mammal, not only sees, but understands what it sees, and interprets it in connection with previously accumulated experience. Compare in fact the sparrow, which at once notices that the small scraps of paper which you throw him, instead of grain, are no food, with the hungry snake, which stops in the midst of its pursuit of a mouse or a frog, and even glides over the latter, simply because the one and the other have stopped running or jumping. The bird knows what it sees, and associates it with what it has seen before, while the snake is nearly devoid of that power. It is known, moreover, that in the human infant the anatomical connection between the nerves of vision and the grey cortex is established dur |