Innervation

“ While all tissues are irritable, and react on being stimulated, each tissue has its special mode of reaction. The secreting-cell reacts differently from the muscle-cell. The reaction of197 the nerve is the innervation of a centre or a muscle; the reaction of an innervated centre is sensation; of a muscle, contraction. There are three aspects of neural reaction: excitation, propagation of the disturbance, and innervation. The first is expressed by irritability, the second by conductibility, the third by sensibility ”

“ He dwells upon the remarkable power exerted by the mind "upon any organ or tissue to which the attention is directed, to the exclusion of other ideas, the mind gradually passing into a state in which, at the desire of the operator, portions of the nervous system can be exalted in a remarkable degree, and others proportionately depressed; and thus the vascularity, innervation and function of an organ or tissue can be regulated and modified according to the locality and nature of the disorder. ”

“ By the separation of a main branch of one of the nerves of sensation, although all the operations of life will still proceed, the organ to which that nerve goes is dead to its particular sense. In like manner, if one of the nerves of volition is divided, the member will not obey the inclination of the brain. It is evident, therefore, although many of the great phenomena of vital force are dependent on the nervous system, and the paralysis of a member ensues upon the separation or the disease of a nerve, that the nerves are but the channels through which certain influences are carried. ”

“ The transfer of this condition of the nerve-fiber to the central parts of the nervous system, especially to the brain. These are the mechanical antecedents for sensation. They are susceptible of physical treatment. They may, and often do, operate without any sensation arising; more than this, they may operate so as to produce a reflex activity, causing violent motions, still without the faintest appearance of sensation. It is plain, then, that to know anything about sensation we must pass from physiology to personal experience. ”

“ The Brain, or Sensorium.—The brain or other nerve-centre sits in darkness, surrounded by a bony protecting box. To this main nerve-centre, or sensorium, come the nerves from all parts of the body that have sensation, the external skin as well as the special organs of sight, hearing, taste, and smell. With these come nerves bearing sensations of pain, temperature, muscular effort—all kinds of sensation which the brain can receive. These nerves are the sole sources of knowledge to any animal organism. Whatever idea its brain may contain must be built up through these nerve-impressions. ”

“ We are thus kept endlessly informed as to the existence of parts, because the impressions which reach the brain are, by a law of our being, referred by us to the part from which they came. Now, when the part is cut off, the nerve-trunks which led to it and from it, remaining capable of being impressed by irritations, are made to convey to the brain from the stump impressions which are as usual referred by the brain to the lost parts, to which these nerve-threads belonged. In other words, the nerve is like a bell-wire. ”

“ How surprising, then, it is to have revealed to us only cells and fibers! The brain is the least solid and most unsubstantial looking organ in the body. Eighty per cent of water, seven of albumen, some fat, and a few minor substances constitute the instrument which rules the world. Strangest of all, the brain, which is the seat of sensation, is itself without sensation. Every nerve, every part of the spinal cord, is keenly alive to the slightest touch, yet "the brain may be cut, burned, or electrified without producing pain." ”

“ Dependence of Mind on Nerves and Brain. We have seen how in a general way the mind is dependent on the body. We have seen how in a more intimate way it is dependent on the special sense organs. But the part of the body to which the mind is most directly and intimately related is the nervous system. The sense organs themselves are merely modifications of the nerve ends together with certain mechanisms for enabling stimuli to act on the nerve ends. The eye is merely the optic nerve spread out to form the retina and modified in certain ways to make it sensitive to ether vibrations. ”

“ The substance of nervous organization is alike in all animals. In some it acts in a limited manner, and without volitional control; in others, it acts in more varied modes, and it manifests some power of volitional control and volitional rest, as well as of involuntary movement. But in all animals the substance of which nervous organization is composed—the substance which acts in producing movement, whether voluntary or involuntary—is the same kind of physical structure. In the higher animals, the great nerve-center is the organ called the brain. ”

“ From this, we advance to the nerve centre,—the brain,—to which the nerves of sensibility run up, and from which the nerves of motion come forth. Here also there is identity in the nature of the organ, while there is variety in its size, with more or less complicated plans of arrangement, according to the extent of the nerve system of which it is the central organ. Still keeping to the human body for illustration, we may find in the most complex organism known to us illustration of what holds good in the main so far as essential structure is concerned. ”

“ The several works which these nerves do are called the senses. By means of these sensory nerves we receive impressions from the external world, sensations of heat, cold, roughness, good and bad odors, taste, sound, and the color and form of things. Thus impressions of the external world are made upon the brain, and it is these impressions that stimulate the brain to action. The brain worked on by them, through ways that we know not of, governs the muscles, sends commands by the motor nerves, and rules the body as a conscious, intelligent will. ”

“ First, let us recall something of what we know of the anatomy and localization of function in the brain, and of the nature of ordinary sleep. The brain, as you know, is a complicated organ, made up internally of nerve masses, or ganglia, of which the central and underlying masses are connected with the automatic functions and involuntary actions of the body (such as the action of the heart, lungs, stomach, bowels, etc.) , while the investing surface shows a system of complicated convolutions rich in gray matter, thickly sown with microscopic cells, in which the nerve ends terminate. ”

“ A nerve starts from every particle of his body, so to speak, which transmits the different impressions it receives to the brain. If terrestrial man were endowed with such a nervous system, looking into his organism through the intermediary of the nerves, he would see how food transforms itself into chyle, the latter into blood, blood into flesh, muscular, nervous substance, etc.: he would see himself! But we are very far from that, the centre of our perceptions being obstructed by nerves, thickened by cerebral lobes and optic thalami. ”

“ There is no reason, in short a priori, for supposing that a state of the sensorial particles of the nerves cannot be the cause of sensation, and that the sensation must be the effect of a state equally unknown, of apparently similar particles, in that other part of the general sensorial organ, which we have denominated the brain. Sensation, indeed, is prevented by decay, or general disease of the brain, or by separation of the nerve, or pressure on it, in any part of its course. ”

“ To admit that all nerves have a common property, and that their functional relations depend on the organs which they innervate, demands small relinquishment of cherished opinions. But to admit that all nerve-centres have a common property, and that their functional relations depend on their anatomical connections, is to sweep away at once a mass of theoretic interpretations which from long familiarity have acquired an almost axiomatic force. That the brain, and the brain only, is the source and seat of Sensibility is the postulate of modern Physiology. ”

“ I developed the psycho-physiological theory of the subconscious, traced the causation and nature of subconscious activities, and worked out the laws of normal and abnormal suggestibility. The following pertains to our present subject:
The nervous centers of man's nervous system, if classified as to function, may be divided into inferior and superior. The inferior centers are characterized by reflex and automatic activities. A stimulus excites the peripheral nerve-endings of some sense-organ. At once a nerve-current is set up in the afferent nerves. ”

“ Suppose a child of intelligent parents were ushered into the world without a nerve leading from his otherwise perfect brain to any portion of his body, with no optic nerve to transmit the glorious sensations from the eye, no auditory nerve to conduct the vibrations of the mother’s voice, no tactile nerves to convey the touch of a hand, no olfactory nerve to [Pg 207] rouse the brain with the delicate aroma from the orchards and the wild flowers in spring, no gustatory, thermal or muscular nerves. ”

“ As the whole nervous system of our body has numerous communications between all its parts, it perceives in its totality something of all the stimulations of its parts, and the consciousness which accompanies them. In this manner there arises in the centre where all the nerve ducts of the whole body meet, i.e., in the brain, a total consciousness composed of innumerable partial consciousnesses, having evidently for its object only the processes of its own organism. ”

“ We must suppose, in short, that physical processes constitute a locked system, every subsequent state of which is fully accounted for by the immediately preceding state; that the nervous system is a highly complex mechanism for adapting the movements of an animal to environing conditions in a manner useful to the animal; and that consciousness is a parallel manifestation, concomitant to certain processes in the highest nerve-centres. ”

“ Traversing the body from its extreme tissues to the gray vesicular substance composing the spinal cord and covering the surface and convolutions of the brain, are two sets of white, fibrous nerves. One set, the afferents, bring in sensation, all kinds of tidings, from the out world of matter. The other set, the efferents, carry out volition, all kinds of decrees, from the in world of mind. Without an afferent nerve no influence of the world can reach the mind; and without an efferent nerve no conclusion of the mind can reach the world. ”

“ This they do by supplying special structures through which the stimuli can act to the best advantage upon the nerve endings. Moreover, there are different kinds of sense organs, and these cause the neurons to be sensitive to different kinds of stimuli. Acting through the sense organs adapted for receiving them, light, sound, heat, cold, and odors all act as stimuli for starting impulses. Indeed, the arrangement is so complete that the nervous system is subjected to the action of external stimuli in some form practically all the time. ”

“ In order to render these opinions understood by those who have never read any of the ancient writers of medicine, from a mistaken idea of their not being worth the reading, I must premise, that they considered the nerves to be the conductors of a certain subtile fluid, by means of which the muscles are made to contract, and all the animal functions are performed; this they denominated the animal spirits, and were of opinion that it was secreted from the blood by the brain. ”

“ Every bodily movement or function that you can start or stop at will, even to such seemingly unconscious acts as winking, walking, etc., is controlled through the cerebro-spinal system. All other functions of the body, including the great vital processes, such as heart pulsation and digestion, are performed unconsciously, are beyond the direct control of the will, and are governed through the sympathetic nerve system. ”

“ We must picture to ourselves, then, in the animal organism, a multitude of nerve-fibres running inwards from all the end-organs of the special senses, from the muscles, and from the internal organs, and all converging on the central nervous system. And we must picture to ourselves a multitude of nerve-fibres passing outwards from the central system, and diverging to supply the muscles, glands, and other organs which are to respond to the stimulation from without. ”

“ Nerves are conductors for nerve-currents. When a nerve-current is started in a sense organ, it passes on through the path of least resistance.
Now, some nerves are so organized and connected through inheritance as to offer small resistance. This forms a ready-made connection between stimulus and response. Muscular responses that are connected with their stimuli through inherited bonds, by inherited nerve structure, are called instincts. Those that are connected by acquired bonds are called habits. Sucking, crying, laughing, are instinctive acts. Adding, typewriting, piano playing, are habits. ”

“ There is nothing so plentifully, and nothing more curiously interwoven with every other substance, as the nerves; and so amazingly quick is their sensation, that the very instant a single nerve, even the smallest and remotest in the system, is affected, the soul, or the common sensorium, is sensible of the identical part so affected. ”

“ The brain and nervous system are divided into distinct portions, each having different offices to perform, and each acting independently of the others; as, for example, one portion is employed by the mind in thinking, and in feeling pleasurable or painful mental emotions; another in moving the muscles; while the nerves that run to the nose, ears, eyes, tongue, hands, and surface generally, are employed in seeing, hearing, smelling, tasting, and feeling all physical sensations. ”