RECENT observations regarding the connections and relations of nerve cells and nerve fibres, with which the names of Ehrlich, Golgi, Ramon y Cajal, and Retzius are chiefly connected, are interesting and important. Nerve cells possess two kinds of processes, the protoplasmic processes or dendrites, and the axis cylinders or axons (neuraxons). Each process ends by arborescent branches. When nerve cells communicate with nerve cells it is not by direct communication of their processes, but the arborescent endings articulate with other arborescent endings, or else form a reticulum around the cells themselves and so influence them. This mode of connection is called by Foster a synapsis ( and = I clasp). Each cell with its processes down to their final arborescences is independent and constitutes a unit of nerve tissue (the neurone of Waldeyer). Nerve cells may, in this way, by means of their processes articulating with the processes of other cells, be brought into relation with many other cells. The arborescent expansions are thus the recipient parts of the nervous system. The grey matter of this system contains the cell processes with their ramifications and expansions, and the white matter is the prolongation of axis cylinders, which again are processes of the cells and therefore parts of the cells.

The nutrition of nerve cells and their expansions is, like that of other cells, dominated by the nucleus, and when a process is cut off from the nucleus it suffers in its nutrition.

The simplest idea of a nervous system is that of a central ganglion with afferent or centripetal fibres and efferent or centrifugal ones. An approach to this simplest form of nervous system is afforded us in the case of the heart. We have here in the substance of the organ certain ganglia, which possess on the one hand centripetal fibres coming chiefly from the endocardium, and on the other hand centrifugal fibres passing to the muscular fibres of the heart. It is to be presumed that impressions conveyed from the endocardium induce the development of impulses which are conveyed by the centrifugal fibres to the muscle and bring about its contraction.

But these intrinsic ganglia of the heart, although forming with their connections a complete nervous system, are not entirely isolated and independent. They are under the command of higher centres which control their action and through them affect the contractions of the heart. From these higher centres fibres reach the heart by two paths, by the vagus and by the sympathetic, and by means of these fibres the action of the intrinsic ganglia is restrained or stimulated.

Taking a general survey of the nervous system, we find that, among the innumerable centres, there are grades or orders to be recognized, the lower or simpler being under the control of the higher and more complex. Leaving aside the peripheral centres and the sympathetic system, we may fitly illustrate this in the case of the cerebro-spinal axis.

In the Spinal cord there are, chiefly in the anterior cornua, groups of ganglion cells which form distinct individual centres. Many of these appear to be of the simplest kind, representing, as it were, single muscles or limited groups of muscles. The stimulation of such simple centres would produce no properly co-ordinated movements, but simply the contraction of a muscle or muscles. But in the cord itself there are centres of a higher order than this, representing, not single muscles or very limited groups, but more considerable groups of associated muscles, so that movements of some complexity are brought about by their stimulation. The centres of lower order are under the control of the higher, and it is to be presumed that the higher, in bringing about movements, do not act directly on the muscles, but stimulate in the first instance the lower centres, which then act directly on the muscles. Even the higher centres in the cord are, as compared with those in the brain, of a very low order, and are only capable of effecting such simple actions as the extension of the toes, the drawing up of the leg, etc.

The Medulla oblongata may be regarded as simply an extension upwards of the spinal cord. Its centres are scarcely of a higher order than those of the cord, and the movements which may be effected by it alone are of the simplest character. In it are massed the great centres which have the control of the respiratory movements, and the contraction and dilatation of the blood-vessels. The muscles of the tongue, mouth, pharynx, etc., are represented here, as are those of the arms, legs, and trunk in the spinal cord.

Passing to the centres next in order above the cord and medulla oblongata we reach the so-called Middle brain, including the centres in the pons varolii, the corpora quadrigemina, and, as perhaps of a still higher order, the cerebellum. Many animals can go through very elaborate movements when deprived of all parts above this middle brain. A pigeon can fly, a frog can leap, and a rabbit can run. There is, however, a want of spontaneity in the movements, which present many of the characters of complex reflex or automatic actions. A rabbit will remain quiet till its foot is pinched, and will then set about running. The movements effected by means of the middle brain require the action of the same muscles as those in which the spinal cord alone is concerned, but the combinations are more complex and the grouping of the muscles more intricate. In effecting these more complex movements the higher centres act in the first place on the lower, and, through them, on the muscles, the lower centres in the cord being thus a necessary link in the chain.

In man the middle brain appears to be much less independent than in the lower animals. In many animals, as we have seen, a stimulus coming from the periphery may induce such complex acts as flying, leaping, running, but it is not so in man. If the centres for such acts are situated in the middle brain in man, they are so dependent on the higher centres that when their connection with these is severed they are only able to act very imperfectly. A certain degree of independence is shown in man by the fact that when a person, completely paralyzed on one side by the connection being divided between the middle and upper brain, yawns, the paralyzed arm will often move in an exaggerated fashion entirely independently of the will. Yawning is an exaggerated inspiration, and in order to elevate the chest the arm is stretched upwards and backwards so as to bring the pectoral muscle into action on the chest wall. When we have command of ourselves we can control these movements, but when the middle brain is disconnected the paralyzed arm may act in an exaggerated fashion.

The Basal ganglia of the cerebrum form a series of centres of a very high order. When such animals as the dog and cat are deprived of all centres higher than the corpus striatum they are capable of running about, these movements being, of course, automatic. But in man, and also in monkeys, although the general movements of the body may be regarded as gathered together in these ganglia, they are not sufficient for the more complex acts of locomotion, etc. The movements of the body, although represented in a complex form in these ganglia, are represented higher up in a still more complex form, and at the same time the lower centres are less independent of these higher ones.

In the Convolutions of the cerebral hemispheres we have the highest order of centres, and in man the Motor area may be taken to form the seat of all the centres which are concerned with the more complex voluntary acts. In the motor convolutions we have the movements of the body as it were written larger, occupying much more space than in the corpus striatum, and more individualized.

In regard to Sensation, we are not to look for a succession of centres such as we have in the case of motion. There are peripheral organs of a highly specialized character, which are engaged in the transmission of the various special kinds of sensation. Between these and the highest centres there are virtually no others interposed, the intervening structures being only concerned in conduction, perhaps with arrangements for fortifying the impressions as they are conducted through greatly elongated paths. Besides the apparatus engaged in sensation, there are afferent fibres which are related to reflex actions, and probably the same fibres to some extent subserve both functions.

In studying the various diseases of the nervous system it will be necessary to carry these physiological considerations along with us, and in the case of each disease it will be needful to take into account the effect which it will have on the physiological action.

Lesions occurring in nervous structures produce various effects. They may irritate the centres either directly or by means of their communicating fibres. If a Motor centre be irritated there will be muscular movements, spasm, convulsion. If a Sensory centre be irritated there will be subjective sensations as of sight, smell, touch, hearing, taste. If a Mental centre be irritated there will be subjective mental phenomena, that is, mental phenomena which are beyond the control of the individual, peculiar thoughts, illusions, etc. On the other hand, lesions may destroy centres, in which case we shall have paralysis of motion (akinesia), or loss of sensation (anaesthesia), or mental degeneracy.

Lesions which are large and palpable are often called Coarse lesions, as where a tumour or a clot destroys or irritates, or does both. Coarse lesions are thus distinguished from those finer changes which are matter for microscopic observation. In some cases, indeed, the existence of actual physical changes is matter of inference, the anatomical demonstration of them being not yet furnished.

Literature

An excellent exposition of the construction of the nervous system is given in Herbert Spencer's Principles of Psychology, vol. i.; also in many papers by Hughlings Jackson, whose influence in advancing the pathology of the nervous system has been very great. A systematic study of the physiology of the nervous system in Ferrier's excellent work, The Functions of the Brain, 2nd ed., 1886. See also, for diseases of the nervous system, Boss, A Treatise on Diseases of Nervous System, 2nd ed., 1883 (which contains frequent references); and Gowers, A Manual of Diseases of the Nervous System, 1893 and 1899.