Having thus acquired some knowledge of the anatomical and microscopical characters of the nervous system, we may proceed to consider the purposes it fulfils in the body. Of the two constituents of the nervous system, cells and fibres, the cells are regarded as the organs by means of which impressions are perceived and registered, and impulses to motion or secretion generated, whilst the fibres are mere conductors extending between the cells and the particular tissues to which the nerve-fibres issuing from the cells are distributed. Those nerves which conduct impressions from one or other of the organs of sense - eye, ear, mouth, nose, skin, etc. - to the spinal cord or brain are called sensory or afferent fibres. Those which conduct impulses from the cord or brain to the muscles or glands are named "efferent" or "motor" nerves. The rapidity with which the conduction of impressions or impulses is effected is very considerable. It is obvious that if the knowledge of the proximity of food or of danger, communicated by the senses, which are the outposts of the nervous system, is to prove of service, it is necessary that the information should be both accurate and prompt. The accuracy is provided for by the special attributes of the several senses. By daylight, the eye affords most of the information required, though in the majority of animals the ears are constantly on the alert against the approach of an invidious foe. At night the faculties of hearing and smell are those which are specially exercised, and in many predatory animals their acuteness rises to a height of which we can form but a faint idea.
By whichever of the senses the impressions are conveyed to the cells of the central nervous system, it is important that the muscular responses should be effected with promptitude.
At first sight it might appear impossible to acquire any definite knowledge of the swiftness with which sensory impressions of objects affecting the animal, and the motor impulses by which it responds to them, are propagated. The speed of thought is proverbial, yet by the application of electrical currents, the rapidity of which may be regarded as covering short distances and intervals with no appreciable loss of time, conclusions have been arrived at showing that nervous changes, currents or waves, travel at a much slower speed than the electric current.
It has been ascertained that nervous impulses, whether in an afferent or in an efferent nerve, that is, whether sensory or motor, in animals as different as a frog and a horse or man, travel at the rate of about a hundred feet per second, or nearly at the rate of twenty-two miles per hour.
The most careful researches into the elements which enter into the chemical composition of nerve tissue afford no insight into the extraordinary properties it possesses. In the living state it may be regarded as a kind or form of protoplasm, but when dead and submitted to analysis it only presents those elements with which we are familiar in the proteids - carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, with salts of calcium, sodium, potassium, and magnesium. Complex substances known as protagon and neurokeratin may be obtained by analysis, but these are but the caput mortuum of the active, living, sentient material, which receives impressions, retains and reproduces them, and can liberate impulses. We are yet far from being able to predict function from chemical composition or molecular arrangement. It is interesting to notice that the gray substance of the cord and brain, which by common consent is acknowledged to be the active part, contains no less than from eighty-five to ninety per cent of water, whilst the white part, chiefly composed of fibres, contains only about seventy per cent. The reaction of nervous tissue is alkaline to test-paper.
Speaking generally, three parts are recognizable in every nerve-fibre: the origin, which is usually from a cell in one of the nerve centres; the course, which is longer or shorter in correspondence with the part of the body supplied; and the termination, which presents special modifications, in accordance with the special organ of sense to which the nerve is distributed, if it be a sensory nerve, or the muscle or gland, if it be a motor nerve.
The agent exciting- a nerve to action is named a stimulus. Some stimuli, as electricity and mechanical irritation, seem to be able to excite all nerves to action, but, as a rule, each nerve responds, or responds best, to its own proper stimulus. Thus the undulations of light excite specifically the nerve terminations in the retina of the eye, the vibrations of sound those in the ear. The change, or information of the change, exciting the nerve endings, say, from blue to red in the case of light, is propagated along the nerve-fibre till it reaches a special sensory cell in the nerve centre. The cell, if well - nourished and not already exhausted, is excited, and a wave of force is liberated from it which may be propagated to a neighbouring cell, and may expand itself through it in producing ideas, or movement, or secretion. The stimuli with which we are most familiar, besides those of electricity, light, and sound already mentioned, are those of contact or of a mechanical nature, those of a chemical nature, as odours and tastes, those of temperature, and those proceeding from the exercise of the will. The last always originate within the nerve centre, and the impulses they awaken travel from that centre and expend themselves on muscles or glands. It is possible that some animals may have nerves capable of responding to stimuli of which we have no more conception than a man born blind has of light, as they certainly have nerves which recognize variations in the intensity of ordinary stimuli that are imperceptible to us. Thus the presence of water, or the proximity of one of their own species, is recognized by many animals when quite imperceptible to man. A stimulus may be so feeble that it fails to be propagated to the central nerve-cells, and is then said to be insufhcient or inoperative; but though our application of the stimulus may not be thus propagated with such intensity as to excite the cell to discharge itself, it may awaken its activity if frequently repeated, and cause it to respond, just as a touch which fails to awaken a sleeper, will do so if repeated. This constitutes the summation of a stimulus. The chemical stimuli are represented by the various mineral and vegetable acids, by alkalis, by ethereal and alcoholic liquids. As a rule, any chemical substance that produces sensation when applied to a sensory nerve, will cause contraction of muscle when applied to a motor nerve, but experiment has shown that the motor nerves are more strongly affected than the sensory by alkaline solutions.