been proposed for these units, and is now pretty generally accepted. Like all other cells, the nerve-cells 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 more. As a rule the fibre is longer than the dendrons: it may attain any length, from a mere fraction of one-tenth of an inch to two or three feet. Its ramifications are also much longer, and it essentially differs from the dendrons 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 nerve-cells are well determined, the naked moss-like dendrons convey outside irritations towards the cell; while the sheathed nerve-fibres convey the nerve-current from the cell-to the muscles, to the tissues, or to other nerve-cells. 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.5 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 nervecells, they maintain the same essential features. The differences are in the number of their ramifications, and in their connections with * If the comparison were not so rough, a neuron could almost be compared (to give an idea of its shape without a drawing) to a microscopical radish, of which the body would correspond to the cell, the leaves to the dendrons, and the root and rootlets to the nerve-fibres. As has been remarked by Professor Schäfer (Nature, 13th of August, 1896), the name of neuron is not etymologically correct, and ought to be applied to the nerve-fibre alone. In this case one would say that the nerve-cell, or the nerve-unit, consists of a cell-body (we call it nerve-cell), with dendrons on the one side and a neuron (we say a nerve-fibre) issuing on the other side. This nomenclature has been followed with full success by Mr. Donaldson (The Growth of the Brain). 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 their protoplasmic outgrowths, and their nervefibres. 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 nerveimpulses 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-ganglia are thus closely connected together. Nay, within the brain itself countless fibres connect its different parts and regions; • It may as well be remembered here that our central nerve-system consists of three chief parts: the brain (or rather the encephalon), the spinal cord, and the so-called vaso-motor system (rows of nerve-ganglia, which run parallel to the spinal cord). Bundles of nerves issue from each of these three main parts towards all organs of senses and all parts of the body. The encephalon, or brain, consists in its turn of two hemispheres which make the brain proper (cerebrum), the small or hind-brain (cerebellum), the mid-brain, the bridge and the bulb (or after-brain), which connects the encephalon with the spinal cord. Each hemisphere consists, again, of white matter (chiefly nerve-fibres), completely covered by a cortex of grey matter (chiefly cells). Specks of grey matter exist also in other parts of the brain, and the spinal cord consists of two columns of grey matter, surrounded by white matter. 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 inwards, to a ganglion cell, located near the spinal cord. From it a nerve-impulse is sent along another nervefibre, which enters, let us say, the spinal cord, and there envelops with its end-branches 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 nervecurrent, 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 -examine the burn. However, the pyramidal 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 impressions; 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 bloodvessels, 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.3 case. There exists still some confusion about these cells; but I take the simplest For this part of the nerve-system see Kölliker's admirable address, delivered before the sixty-sixth Congress of German Naturalists and Physicians, held in Vienna in September 1894. Now, if we analyse 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 in detail. And when the anatomist maintains that an irritation 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.9 II 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 amœba-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 type 9 S. Ramón y Cajal, Les nouvelles idées sur la structure du système nerveux, translated with additions from the Spanish (Paris 1895), where a full bibliography, containing the works of Golgi, His, Lenhossék, van Gehuchten, &c., is given, and his Croonian lecture before the Royal Society (Transactions and Nature, 15th of March, 1894, vol. lix., p. 465); Kölliker's Handbuch der Gewebelehre des Menschen, 6th edition, 1893-96; and Michael Foster's Textbook of Physiology, vols. iii. and iv., may be taken as the safest guides. Also the two excellent American reviews, Journal of Psychology and Psychological Review, and the Journal of Anatomy and Physiology, Mind and Brain, published in this country. Déjérine's big work, Le Cerveau (Paris, 1895), is a treasury of splendid drawings from the microscopic preparations. Of a mass of works, Lenhossék's Der feinere Bau des Nervensystems, Wien, 1895, and S. Exner's Entwurf einer physiologischen Erklärung der psychischen Erscheinungen, Wien, 1895, deserve a special mention. S. Ramón y Cajal's new work, Beiträge zum Studium der Medulla oblongata &c., is only known to the present writer from abstracts. For the general reader, the two volumes, 1895 and 1896, of L'Année pyschologique, edited by Binet and Beaunis (general reviews by Forel and Azoulay), can especially be recommended, as also H. H. Donaldson's The Growth of the Brain, London, 1895, and Dr. Waller's article in Science Progress, May 1895. -such 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. 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 consciousness, derived from irritations transmitted to a group of associated nerve-cells, already makes it 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. Accordingly, a sort of common consciousness of the whole being, of the whole chain of segments, is evolved, and everyone can easily ascertain it himself by observing the behaviour of these animals. At the same time, in the front segment 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. 10 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 10 Forel, in a general review of the subject in Binet and Beaunis, Année psychologique, ii. p. 25. |