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.

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. Immeḍiately, 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 some quite conscious movement-run, let us say, towards the 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 a 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 cause of lightning, or about the beauty of the 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 over in this simple case. But then the physiologist steps in. He wants to know, in his turn, what changes, chemical or 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

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?

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 an 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 improved 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