1911 Encyclopædia Britannica/Muscle and Nerve

by Charles Scott Sherrington

MUSCLE AND NERVE (Physiology).^[1] Among the properties of living material there is one, widely though not universally present in it, which forms the pre-eminent characteristic of muscular cells. This property is the liberation of some of the energy contained in the chemical compounds of the cells in such a way as to give mechanical work. The Muscle. mechanical work is obtained by movement resulting from a change, it is supposed, in the elastic tension of the framework of the living cell. In the fibrils existing in the cell a sudden alteration of elasticity occurs, resulting in an increased tension on the points of attachment of the cell to the neighbouring elements of the tissue in which the cell is placed. These yield under the strain, and the cell shortens between those points of its attachment. This shortening is called contraction. But the volume of the cell is not appreciably altered, despite the change of its shape, Contracti-bility. for its one diameter increases in proportion as its other is diminished. The manifestations of contractility by muscle are various in mode. By tonic contraction is meant a prolonged and equable state of tension which yields under analysis no element of intermittent character. This is manifested by the muscular walls of the hollow viscera and of the heart, where it is the expression of a continuous liberation of energy in process in the muscular tissue, the outcome of the latter’s own intrinsic life, and largely independent of any connexion with the nervous system. The muscular wall of the blood-vessels also exhibits tonic contraction, which, however, seems to be mainly traceable to a continual excitation of the muscle cells by nervous influence conveyed to them along their nerves, and originating in the great vaso motor centre in the bulb. In the ordinary striped muscles of the skeletal musculature, e.g. gastrocnemius, tonic contraction obtains; but this, like the last mentioned, is not autochthonous in the muscles themselves; it is indirect and neural, and appears to be maintained reflexly. The receptive organs of the muscular sense and of the semicircular canals are to be regarded as the sites of origin of this reflex tonus of the skeletal muscles. Striped muscles possessing an autochthonous tonus appear to be the various sphincter muscles.

Another mode of manifestation of contractility by muscles is the rhythmic. A tendency to rhythmic contraction seems discoverable in almost all muscles. In some it is very marked, for example in some viscera, the spleen, the bladder, the ureter, the uterus, the intestine, and especially in the heart. In several of these it appears not unlikely that the recurrent explosive liberations of energy in the muscle tissue are not secondary to recurrent explosions in nerve cells, but are attributable to decompositions arising sua sponte in the chemical substances of the muscle cells themselves in the course of their living. Even small strips of the muscle of the heart, if taken immediately after the death of the animal, continue, when kept moist and Warm and supplied with oxygen, to “beat” rhythmically for hours. Rhythmic contraction is also characteristic of certain groups of skeletal muscles, e.g. the respiratory. In these the rhythmic activity is, however, clearly secondary to rhythmic discharges of the nerve cells constituting the respiratory centre in the bulb. Such discharges descend the nerve fibres of the spinal cord, and through the intermediation of various spinal nerve cells excite the respiratory muscles through their motor nerves. A form of contraction intermediate in character between the tonic and the rhythmic is met in the auricle of the heart of the toad. There slowly successive phases of increased and of diminished tonus regularly alternate, and upon them are superposed the rhythmic “beats” of the pulsating heart.

“The beat,” i.e. the short-lasting explosive contraction of the heart muscle, can be elicited by a single, even momentary, application of a stimulus, e.g. by an induction shock. Similarly, such a single stimulus elicits from a skeletal muscle a single “beat,” or, as it is termed, a “twitch.” In the heart muscle during a brief period after each beat, that is, after each single contraction of the rhythmic series, the muscle becomes inexcitable. It cannot then be excited to contract by any agent, though the inexcitable period is more brief for strong than for weak stimuli. But in the skeletal, voluntary or striped muscles a second stimulus succeeding a previous so quickly as to fall even during the continuance of the contraction excited by a first, elicits a second contraction. This second contraction starts from whatever phase of previous contraction the muscle may have reached at the time. A third stimulus excites a third additional contraction, a fourth a fourth, and so on. The increments of contraction become, however, less and less, until the succeeding stimuli serve merely to maintain, not to augment, the existing degree of contraction. We arrive thus by synthesis at a summation of “beats” or of simple contractions in the compound, or “tetanic,” or summed contraction of the skeletal muscles. The tetanic or summed contractions are more extensive than the simple, both in space and time, and liberate more energy, both as mechanical work and heat. The tension developed by their means in the muscle is many times greater than that developed by a simple twitch.

Muscle cells respond by changes in their activity to changes in their environment, and thus are said to be “excitable.” They are, however, less excitable than are the nerve cells which innervate them. The change which excites them is termed a stimulus. The least stimulus which suffices to excite is known as the stimulus of Excitability. threshold value. In the case of the heart muscle this threshold stimulus evokes a beat as extensive as does the strongest stimulus; that is, the intensity of the stimulus, so long as it is above threshold value, is not a function of the amount of the muscular response. But in the ordinary skeletal muscles the amount of the muscular contraction is for a short range of quantities of stimulus (of above threshold value) proportioned to the intensity of the stimulus and increases with it. A value of stimulus, however, is soon reached which evokes a maximal contraction. Further increase of contraction does not follow further increase of the intensity of the stimulus above that point.

Just as in a nerve fibre, when excited by a localized stimulus, the excited state spreads from the excited point to the adjacent unexcited ones, so in muscle the “contraction,” when excited at a point, spreads to the adjacent uncontracted parts. Both in muscle and in nerve this spread is termed conduction. It is propagated along the muscle fibres of the skeletal muscles at a rate of about 3 metres per second. In the heart muscle it travels much more slowly. The disturbance travels as a wave of contraction, and the whole extent of the wave-like disturbance measures in ordinary muscles much more than the whole length of any single muscle fibre. That the excited state spreads only to previously unexcited portions of the muscle fibre shows that even in the skeletal variety of muscle there exists, though only for a very brief time, a period of inexcitability. The duration of this period is about 1/600 of a second in skeletal muscle.

When muscle that has remained inactive for some time is excited by a series of single and equal stimuli succeeding at intervals too prolonged to cause summation the succeeding contractions exhibit progressive increase up to a certain degree. The tenth contraction usually exhibits the culmination of this so-called “staircase effect.” The explanation may lie in the production of CO₂ in the muscle. That substance, in small doses, favours the contractile power of muscle. The muscle is a machine for utilizing the energy contained in its own chemical compounds. It is not surprising that the chemical substances produced in it by the decomposition of its living material should not be of a nature indifferent for muscular life. We find that if the series of excitations of the muscle be prolonged. beyond the short stage of initial improvement, the contractions, after being Well maintained for a time, later decline in force and speed, and ultimately dwindle even to vanishing point. This decline is said to be due to muscular fatigue. The muscle recovers on being allowed to rest unstimulated for a while, and more quickly on being washed with an innocuous but non-nutritious solution, such as ·6%, NaCl in water. The washing seems to remove excreta of the muscle’s own production, and the period of repose removes them perhaps by diffusion, perhaps by breaking them down into innocuous material. Since the muscle produces lactic acids during activity, it has been suggested that acids are among the “fatigue substances” with which muscle poisons itself when deprived of circulating blood. Muscles when active seem to pour into the circulation substances which, of unknown chemical composition, are physiologically recognizable by their stimulant action on the respiratory nervous centre. The effect of the fatigue substances upon the contraction of the tissue is manifest especially in the relaxation process. The contracted state, instead of rapidly subsiding after discontinuance of the stimulus, slowly and only partially wears off, the muscle remaining in a condition of physiological “contracture.” The alkaloid veratrin has a similar effect upon the contraction of muscle; it enormously delays the return from the contracted state, as also does epinephrine, an alkaloid extracted from the suprarenal gland.

Nervous System.—The work of Camillo Golgi (Pavia, 1885 and onwards) on the minute structure of the nervous system has Neuron led to great alteration of doctrine in neural physiology. It had been held that the branches of the nerve cells, that is to say, the fine nerve fibres—since all nerve fibres are nerve cell branches, and all nerve cell Neuron Theory. branches are nerve fibres—which form a close felt-work in the nervous centres, there combined into a network actually continuous throughout. This continuum was held to render possible conduction in all directions throughout the grey matter of the whole nervous system. The fact that conduction occurred preponderantly in certain directions was explained by appeal to a hypothetical resistance to conduction which, for reasons unascertained, lay less in some directions than in others. The intricate felt-work has by Golgi been ascertained to be a mere interlacement, not an actual anastomosis network; the branches springing from the various cells remain lifelong unattached and unjoined to any other than their own individual cell. Each neuron or nerve cell is a morphologically distinct and discrete unit connected functionally but not structurally with its neighbours, and leading its own life independently of the destiny of its neighbours. Among the properties of the neuron is conductivity in all directions. But when neurons are linked together it is found that nerve impulses will only pass from neuron A to neuron B, and not from neuron B to neuron A; that is, the transmission of the excited state or nervous impulse, although possible in each neuron both up and down its own cell branches, is possible from one nerve cell to another in one direction only. That direction is the direction in which the nerve impulses flow under the conditions of natural life. The synapse, therefore, as the place of meeting of one neuron with the next is called, is said to valve the nerve circuits. This determinate sense of the spread is called the law of forward direction. The synapse appears to be a weak spot in the chain of conduction, or rather to be a place which breaks down with comparative ease under stress, e.g. under effect of poisons. The axons of the motor neurons are, inasmuch as they are nerve fibres in nerve trunks, easily accessible to artificial stimuli. It can be demonstrated that they are practically indefatigable—repeatedly stimulated by electrical currents, even through many hours, they, unlike muscle, continue to respond with unimpaired reaction. Peripheral Fatigue. Yet when the muscular contraction is taken as index of the response of the nerve, it is found that unmistakable signs of fatigue appear even very soon after commencement of the excitation of the nerve, and the muscle ceases to give any contraction in response to stimuli applied indirectly to it through its nerve. But the muscle will, when excited directly, e.g. by direct application of electric currents, contract vigorously after all response on its part to the stimuli (nerve impulses) applied to it indirectly through its nerve has failed. The inference is that the “fatigue substances” generated in the muscle fibres in the course of their prolonged contraction injure and paralyse the motor end plates, which are places of synapsis between nerve cell and muscle cell, even earlier than they harm the contractility of the muscle fibres themselves. The alkaloid curarin causes motor paralysis by attacking in a selective way this junction of motor nerve cell and striped muscular fibre. Non-myelinate nerve fibres are as resistant to fatigue as are the myelinate.

The neuron is described as having a cell body or perikaryon from which the cell branches—dendrites and axon—extend, and it is this perikaryon which, as its name implies, contains the nucleus. It forms the trophic centre of the cell, just as the nucleus-containing part of every cell is the trophic centre of the whole cell. Any part of the cell Solidarity
of Neuron. cut off from the nucleus-containing part dies down: this is as true of nerve cells as of amoeba, and in regard to the neuron it constitutes what is known as the Wallerian degeneration. On the other hand, in some neurons, after severance of the axon from the rest of the cell (spinal motor cell), the whole nerve cell as well as the severed axon degenerates, and may eventually die and be removed. In the severed axon the degeneration is first evident in a breaking down of the naked nerve filaments of the motor end plate. A little later the breaking down of the whole axon, both axis cylinder and myelin sheath alike, seems to occur simultaneously throughout its entire length distal to the place of severance. The complex fat of the myelin becomes altered chemically, while the other components of the sheath break down. This death of the sheath as well as of the axis cylinder shows that it, like the axis cylinder, is a part of the nerve cell itself.

In addition to the trophic influence exerted by each part of the neuron on its other parts, notably by the perikaryon on the cell branches, one neuron also in many instances influences the nutrition of other neurons. When, for instance, the axons of the ganglion cells of the retina are severed by section of the optic nerve, and thus their influence upon the nerve cells of the visual cerebral centres is set aside, the nerve cells of those centres undergo secondary atrophy (Gadden’s atrophy). They dwindle in size; they do not, however, die. Similarly, when the axons of the motor spinal cells are by severance of the nerve trunk of a muscle broken through, the muscle cells undergo “degeneration”—dwindle, become fatty, and alter almost beyond recognition. This trophic influence which one neuron exerts upon others, or upon the cells of an extrinsic tissue, such as muscle, is exerted in that direction which is the one normally taken by the natural nerve impulses. It seems, especially in the case of the nexus between certain neurons, Tonic Activity of Neurons. that the influence, loss of which endangers nutrition, is associated with the occurrence of something more than merely the nervous impulses awakened from time to time in the leading nerve cell. The wave of change (nervous impulse) induced in a neuron by advent of a stimulus is after all only a sudden augmentation of an activity continuous within the neuron—a transient accentuation of one (the disintegrative) phase of the metabolism inherent in and inseparable from its life. The nervous impulse is, so to say, the sudden evanescent glow of an ember continuously black-hot. A continuous lesser “change” or stream of changes sets through the neuron, and is distributed by it to other neurons in the same direction and by the same synapses as are its nerve impulses. This gentle continuous activity of the neuron is called its tonus. In tracing the tonus of neurons to a source, one is always led link by link against the current of nerve force—so to say, “up stream”—to the first beginnings of the chain of neurons in the sensifacient surfaces of the body. From these, as in the eye, ear, and other sense organs, tonus is constantly initiated. Hence, when cut off from these sources, the nutrition of the neurons of various central mechanisms suffers. Thus the tonus of the motor neurons of the spinal cord is much lessened by rupture of the great afferent root cells which normally play upon them. A prominent and practically important illustration of neural tonus is given by the skeletal muscles. These muscles exhibit a certain constant condition of slight contraction, which disappears on severance of the nerve that innervates the muscle. It is a muscular tonus of central source consequent on the continual glow of excitement in the spinal motor neuron, whose outgoing end plays upon the muscle cells, whose ingoing end is played upon by other neurons—spinal, cerebral and cerebellar.

It is with the neural element of muscle tonus that tendon phenomena are intimately associated. The earliest-studied of these, the “knee-jerk,” may serve as example of the class. It is a brief extension of the limb at the knee-joint, due to a simple contraction of the extensor muscle, elicited by a tap or other short mechanical stimulus applied to the muscle fibres through the tendon of the muscle. The jerk is obtainable only from muscle fibres possessed of neural tonus. If the sensory nerves of the extensor muscle be severed, the “jerk” is lost. The brevity of the interval between the tap on the knee and the beginning of the resultant contraction of the muscle seems such as to exclude the possibility of reflex development. A little experience in observations on the knee-jerk imparts a notion of the average strength of the “jerk.” Wide departures from the normal standard are met with and are symptomatic of certain nervous conditions. Stretching of the muscles antagonistic to the extensors—namely, of the flexor muscles—reduces the jerk by inhibiting the extensor spinal nerve cells through the nervous impulses generated by the tense flexor muscles. Hence a favourable posture of the limb for eliciting the jerk is one ensuring relaxation of the hamstring muscles, as when the leg has been crossed upon the other. In sleep the jerk is diminished, in deep sleep quite abolished. Extreme bodily fatigue diminishes it. Conversely, a cold bath increases it. The turning of attention towards the knee interferes with the jerk; hence the device of directing the person to perform vigorously some movement, which does not involve the muscles of the lower limb, at the moment when the light blow is dealt upon the tendon. A slight degree of contraction of muscle seems the substratum of all attention. The direction of attention to the performance of some movement by the arm ensures that looseness and freedom from tension in the thigh muscles which is essential for the provocation of the jerk. The motor cells of the extensor muscles, when preoccupied by cerebral influence, appear refractory. T. Ziehen has noted exaltation of the jerk to follow extirpation of a cortical centre.

Although the cell body or perikaryon of the neuron, with its contained nucleus, is essential for the maintenance of the life of the cell branches, it has become recognized that the actual process and function of “conduction” in many neurons can, and does, go on without the cell body being directly concerned in the conduction. Conduction in Neurons. S. Exner first showed, many years ago, that the nerve impulse travels through the spinal ganglion at the same speed as along the other parts of the nerve trunk—that is, that it suffers no delay in transit through the perikarya of the afferent root-neurons. Bethe has succeeded in isolating their perikarya from certain of the afferent neurons of the antennule of Carcinus. The conduction through the amputated cell branches continues unimpaired for many hours. This indicates that the conjunction between the conducting substance of the dendrons and that of the axon can be effected without the intermediation of the cell body. But the proper nutrition of the conducting substance is indissolubly dependent on the cell branches being in continuity with the cell body and nucleus it contains. Evidence illustrating this nexus is found in the visible changes produced in the perikaryon by prolonged activity induced and maintained in the conducting branches of the cell. As a result the fatigued cells appear shrunken, and their reaction to staining reagents alters, thus showing chemical alteration. Most marked is the decrease in the volume of the nucleus, amounting even to 44% of the initial volume. In the myelinated cell branches of the neuron, that is, in the ordinary nerve fibres, no visible change has ever been demonstrated as the result of any normal activity, however great—a striking contrast to the observations obtained on the perikarya. The chemical changes that accompany activity in the nerve fibre must be very small, for the production of CO₂ is barely measurable, and no production of heat is observable as the result of the most forced tetanic activity.

The nerve cells of the higher vertebrata, unlike their blood cells, their connective tissue cells, and even their muscle cells, early, and indeed in embryonic life, lose power of multiplication. The number of them formed is definitely closed at an early period of the individual life. Although, unlike so many other cells, thus early sterile for Growth in Nervous System. reproduction of their kind, they retain for longer than most cells a high power of individual growth. They continue to grow, and to thrust out new branches and to lengthen existing branches, for many years far into adult life. They similarly possess power to repair and to regenerate their cell branches where these are injured or destroyed by trauma or disease. This is the, explanation of the repair of nerve trunks that have been severed, with consequent degeneration of the peripheral nerve fibres. As a rule, a longer time is required to restore the motor than the sensory functions of a nerve trunk.

Whether examined by functional or by structural features, the conducting paths of the nervous system, traced from beginning to end, never terminate in the centres of that system, but pass through them. All ultimately emerge as efferent channels. Every efferent channel, after entrance in the central nervous system, subdivides; Cerebral Cortex. of its subdivisions some pass to efferent channels soon, others pass further and further within the cord and brain before they finally 'reach channels of outlet. All the longest routes thus formed traverse late in their course the cortex of the cerebral hemisphere. It is this relatively huge development of cortex cerebri which is the pre-eminent structural character of man. This means that the number of “longest routes” in man is, as compared with lower animals, disproportionately great. In the lower animal forms there is no such nervous structure at all as the cortex cerebri. In the frog, lizard, and even bird, it is thin and poorly developed. In the marsupials it is more evident, and its excitation by electric currents evokes movements in the musculature of the crossed side of the body. Larger and thicker in the rabbit, when excited it gives rise in that animal to movements of the eyes and of the fore-limbs and neck; but it is only in much higher types, such as the dog, that the cortex yields, under experimental excitation, definitely localized foci, whence can be evoked movements of the fore-limb, hind-limb, neck, eyes, ears and face. In the monkey the proportions it assumes are still greater, and the number of foci, for distinct movements of this and that member, indeed for the individual joints of each limb, are much more numerous, and together occupy a more extensive surface, though relatively to the total surface of the brain a smaller one.

Experiment shows that in the manlike (anthropoid) apes the differentiation of the foci or “centres” of movement in the motor field of the cortex is even more minute. In them areas are found whence stimuli excite movements of this or that finger alone, of the upper lip without the lower, of the tip only of the tongue, or of one upper eyelid by itself. The movement evoked from a point of cortex is not always the same; its character is determined by movements evoked from neighbouring points of cortex immediately antecedently. Thus a point A will, when excited soon subsequent to point B, which latter yields protrusion of lips, itself yield lip-protrusion, whereas if excited after C, which yields lip-retraction, it will itself yield lip-retraction. The movements obtained by point-to-point excitation of the cortex are often evidently imperfect as compared with natural movements—that is, are only portions of complete normal movements. Thus among the tongue movements evoked by stigmatic stimulation of the cortex undeviated protrusion or retraction of the organ is not found. Again, from different points of the cortex the assumption of the requisite positions of the tongue, lips, cheeks, palate and epiglottis, as components in the act of sucking, can be provoked singly. Rarely can the whole action be provoked, and then only gradually, by prolonged and strong excitation of one of the requisite points, e.g. that for the tongue, with which the other points are functionally connected. Again, no single point in the cortex evokes the act of ocular convergence and fixation. All this means that the execution of natural movements employs simultaneous co-operative activity of a number of points in the motor fields on both sides of the brain together.

The accompanying simple figure indicates better than any verbal description the topography of the main groups of foci in the motor field of a manlike ape (chimpanzee). It will be noted from it that there is no direct relation between the extent of a cortical area and the mass of muscles which it controls. The mass of muscles in the trunk is greater than in the leg, and in the leg is greater than in the arm, and in the arm is many times greater than in the face and head; yet for the last the cortical area is the most extensive of all, and for the first-named is the least extensive of all.

Diagram of the Topography of the Main Groups of Foci in the Motor Field
of Chimpanzee.

The motor field of the cortex is, taken altogether, relatively to the size of the lower parts of the brain, larger in the anthropoid than in the inferior monkey brains. But in the anthropoid brain still more increased even than the motor field are the great regions of the cortex outside that field, which yield no definite movements under electric excitation, and are for that reason known as “silent.” The motor field, therefore, though absolutely larger, forms a smaller fraction of the whole cortex of the brain than in the lower forms. The statement that in the anthropoid (orang-outan) brain the groups of foci in the motor fields of the cortex are themselves separated one from another by surrounding inexcitable cortex, has been made and was one of great interest, but has not been confirmed by subsequent observation. That in man the excitable foci of the motor field are islanded inexcitable surface similarly and even more extensively, was a natural inference, but it had its chief basis in the observations on the orang, now known to be erroneous.

In the diagram there is indicated the situation of the cortical centres for movement of the vocal cords. Their situation is at the lower end of the motor field. That they should lie there is interesting, because that place is close to one known in man to be associated with management of the movements concerned in speech. When that area in man is injured, the ability to utter words is impaired. Not that there is paralysis of the muscles of speech, since these muscles can be used perfectly for all acts other than speech. The area in man is known as the motor centre for speech; in most persons it exists only in the left half of the brain and not in the right. In a similar way damage of a certain small portion of the temporal lobe of the brain produces loss of intelligent apprehension of words spoken, although there is no deafness and although words seen are perfectly apprehended. Another region, “the angular region” is similarly related to intelligent apprehension of words seen, though not of words heard.

When this differentiation of cortex, with its highest expression in man, is collated with the development of the cortex as studied in the successive phases' of its growth and ripening in the human infant, a suggestive analogy is obvious. The nervous paths in the brain and cord, as they attain completion, come to be furnished more and more with fibres that are fully myelinate. At the beginning of its history each is unprovided with myelinate nerve fibres. The excitable foci of the cerebral cortex are well myelinated long before the unexcitable are so. The regions of the cortex, whose conduction paths are early completed, may be arranged in groups by their connexions with sense-organs: eye-region, ear-region, skin and somaesthetic region, olfactory and taste region. The areas of intervening cortex, arriving at structural completion later than the above sense-spheres, are called by some association-spheres, to indicate the view that they contain the neural mechanisms of reactions (some have said “ideas”) associated with the sense perceptions elaborated in the several sense-spheres.

The name “motor area” is given to that region of cortex whence, as D. Ferrier’s investigations showed, motor reactions of the facial and limb muscles are regularly and easily evoked. This region is often called the sensori-motor cortex, and the term somaesthetic has Sensori-motor Centres. also been used and seems appropriate. It has been found that disturbance of sensation, as well as disturbance of movement, is often incurred by its injury. Patients in whom, for purposes of diagnosis, it has been electrically excited, describe, as the initial effect of the stimulation, tingling and obscure but locally-limited sensations, referred to the part whose muscles a moment later are thrown into co-ordinate activity. The distinction, therefore, between the movement of the eyeballs, elicited from the occipital (visual) cortex, and that of the hand, elicited from the cortex in the region of the central sulcus (somaesthetic), is not a difference between motor and sensory, for both are sensori-motor in the nature of their reactions; the difference is only a difference between the kind of sense and sense-organ in the two cases, the muscular apparatus in each case being an appanage of the sensual.

That the lower types of vertebrate, such as fish, e.g. carp, possess practically no cortex cerebri, and nevertheless execute “volitional” acts involving high co-ordination and suggesting the possession by them of associative memory, shows that for the existence of these phenomena the cortex cerebri is in them not essential. In the dog it has been proved that after removal from the animal of every vestige of its cortex cerebri, it still executes habitual acts of great motor complexity requiring extraordinarily delicate adjustment of muscular contraction. It can walk, run and feed; such an animal, on wounding its foot, will run on three legs, as will a normal dog under similar mischance. But signs of associative memory are almost, if not entirely, wanting. Throughout three years such a dog failed to learn that the attendant’s lifting it from the cage at a certain hour was the preliminary circumstance of the feeding-hour; yet it did exhibit hunger, and would refuse further food when a sufficiency had been taken. In man, actually gross sensory defects follow even limited lesions of the cortex. Thus the rabbit and the dog are not absolutely blinded by removal of the entire cortex, but in man destruction of the occipital cortex produces total blindness, even to the extent that the pupil of the eye does not respond when light is flashed into the eye.

Examination of the cerebellum by the method of Wallerian degeneration has shown that a large number of spinal and bulbar nerve cells send branches up into it. These seem to end, for the most part, in the grey cortex of the median lobe, some, though not the majority, of them decussating across the median line. The organ seems Cerebellum. also to receive many fibres from the parietal region of the cerebral hemisphere. From the organ there emerge fibres which cross to the opposite red nucleus, and directly or indirectly reach the thalamic region of the crossed hemisphere. The pons or middle, peduncle, which was regarded, on the uncertain ground of naked-eye dissection of human anatomy, as commissural between the two lateral lobes of the cerebellum, is now known to constitute chiefly a cerebro-cerebellar decussating path. Certain cerebellar cells send processes down to the cell-group in the bulb known as the nucleus of Deiters, which latter projects fibres down the spinal cord. Whether there is any other or direct emergent path from the cerebellum into the spinal cord is a matter on which opinion is divided.

Injuries of the cerebellum, if large, derange the power of executing movements, without producing any detectable derangement of sensation. The derangement gradually disappears, unless the damage to the organ be very wide. A reeling gait, oscillations of the body which impart a zigzag direction to the walk, difficulty in standing, owing to unsteadiness of limb, are common in cerebellar disease. On the other hand, congenital defect amounting to absence of one cerebellar hemisphere has been found to occasion practically no symptoms whatsoever. Not a hundredth part of the cerebellum has remained, and yet there has existed ability to stand, to walk, to handle and lift objects in a fairly normal way, without any trace of impairment of cutaneous or muscular sensitivity. The damage to the cerebellum must, it would seem, occur abruptly or quickly in order to occasion marked derangement of function, and then the derangement falls on the execution of movements. One aspect of this derangement, named by Luciani astasia, is a tremor heightened by or only appearing when the muscles enter upon action—“intention tremor.” Vertigo is a frequent result of cerebellar injury: animals indicate it by their actions; patients describe it. To interpret this vertigo, appeal must be made to disturbances, other than cerebellar, which likewise occasion vertigo. These include, besides ocular squint, many spatial positions and movements unwonted to the body: the looking from a height, the gliding over ice, sea-travel, to some persons even travelling by train, or the covering of one eye. Common to all these conditions is the synchronous rise of perceptions of spatial relations between the self and the environment which have not, or have rarely, before arisen in synchronous combination. The tactual organs of the soles, and the muscular sense organs of limbs and trunk, are originating perceptions that indicate that the self is standing on the solid earth, yet the eyes are at the same time originating perceptions that indicate that the solid earth is far away below the standing self. The combination is hard to harmonize at first; it is at least not given as innately harmonized. Perceptions regarding the “me” are notoriously highly charged with “feeling,” and the conflict occasions the feeling insufficiently described as “giddiness.” The cerebellum receives paths from most, if not from all, of the afferent roots. With certain of these it stands associated most closely, namely, with the vestibular, representing the sense organs which furnish data for appreciation of positions and movements of the head, and with the channels, conveying centripetal impressions from the apparatus of skeletal movement. Disorder of the cerebellum sets at variance, brings discord into, the space-perceptions contributory to the movement. The body’s movement becomes thus imperfectly adjusted to the spatial requirements of the act it would perform.

In the physiological basis of sense exist many impressions which, apart from and devoid of psychical accompaniment, reflexly influence motor (muscular) innervation. It is with this sort of habitually apsychical reaction that the cerebellum is, it would seem, employed. That it is apparently devoid of psychical concomitant need not imply that the impressions concerned in it are crude and inelaborate. The seeming want of reaction of so much of the cerebellar structure under artificial stimulation, and the complex relay system revealed in the histology of the cerebellum, suggest that the impressions are elaborate. Its reaction preponderantly helps to secure co-ordinate innervation of the skeletal musculature, both for maintenance of attitude and for execution of movements.

Sleep.—The more obvious of the characters of sleep (q.v.) are essentially nervous. In deep sleep the threshold-value of the stimuli for the various senses is very greatly raised, rising rapidly during the first hour and a half of sleep, and then declining with gradually decreasing decrements. The muscles become less tense than in their waking state: their tonus is diminished, the upper eyelid falls, and the knee-jerk is in abeyance. The respiratory rhythm is less frequent and the breathing less deep; the heart-beat is less frequent; the secretions are less copious; the pupil is narrow; in the brain there exists arterial anaemia with venous congestion, so that the blood-flow there is less than in the waking state.

It has been suggested that the gradual cumulative result of the activity of the nerve cells during the waking day is to load the brain tissue with “fatigue-substances” which clog the action of the cells, and thus periodically produce that loss of consciousness, &c., which is sleep. Such a drugging of tissue by its own excreta is known Theories of Sleep. in muscular fatigue, but the fact that the depth of sleep progressively increases for an hour and more after its onset prevents complete explanation of sleep on similar lines. It has been urged that the neurons retract during sleep, and that thus at the synapses the gap between nerve cell and nerve cell becomes wider, or that the supporting cells expand between the nerve cells and tend to isolate the latter one from the other. Certain it is that in the course of the waking day a great number of stimuli play on the sense organs, and through these produce disintegration of the living molecules of the central nervous system. Hence during the day the assimilatory processes of these cells are overbalanced by their wear and tear, and the end-result is that the cell attains an atomic condition less favourable to further disintegration than to reintegration. That phase of cell life which we are accustomed to call “active” is accompanied always by disintegration. When in the cell the assimilative processes exceed dissimilative, the external manifestations of energy are liable to cease or diminish. Sleep is not exhaustion of the neuron in the sense that prolonged activity has reduced its excitability to zero. The nerve cell just prior to sleep is still well capable of response to stimuli, although perhaps the threshold-value of the stimulus has become rather high, whereas after entrance upon sleep and continuance of sleep for several hours, and more, when all spur to the dissimilation process has been long withheld, the threshold-value of the sensory stimulus becomes enormously higher than before. The exciting cause of sleep is therefore no complete exhaustion of the available material of the cells, nor is it entirely any paralysing of them by their excreta. It is more probably abeyance of external function during a periodic internal assimilatory phase.

Two processes conjoin to initiate the assimilatory phase. There is close interconnexion between the two aspects of the double activity that in physiological theory constitute the chemical life of protoplasm, between dissimilation and assimilation. Hering has long insisted on a self-regulative adjustment of the cell metabolism, so that action involves reaction, increased catabolism necessitates after-increase of anabolism. The long-continued incitement to catabolism of the waking day thus of itself predisposes the nerve cells towards rebound into the opposite phase; the increased catabolism due to the day’s stimuli induces increase of anabolism, and though recuperation goes on to a large extent during the day itself, the recuperative process is slower than, and lags behind, the disintegrative. Hence there occurs a cumulative effect, progressively increasing from the opening till the closing hours. The second factor inducing the assimilative change is the withdrawal of the nervous system from sensual stimulation. The eyes are closed, the maintenance of posture by active contraction is replaced by the recumbent pose which can be maintained by static action and the mere mechanical consistence of the body, the ears are screened from noise in the quiet chamber, the skin from localized pressure by a soft, yielding couch. The effect of thus reducing the excitant action of the environment is to give consciousness over more to mere revivals by memory, and gradually consciousness lapses. A remarkable case is well authenticated, where, owing to disease, a young man had lost the use of all the senses save of one eye and of one ear. If these last channels were sealed, in two or three minutes time he invariably fell asleep.

If natural sleep is the expression of a phase of decreased excitability due to the setting in of a tide of anabolism in the cells of the nervous system, what is the action of narcotics? They lower the external activities of the cells, but do they not at the same time lower the internal, reparative, assimilative activity of the cell that in natural sleep goes vigorously forward preparing the system for the next day’s drain on energy? In most cases they, seem to Narcotics. lower both the internal and the external activity of the nerve cells, to lessen the cell’s entire metabolism, to reduce the speed of its whole chemical movement and life. Hence it is not surprising that often the refreshment, the recuperation, obtained from and felt after sleep induced by a drug amounts to nothing, or to worse than nothing. But very often refreshment is undoubtedly obtained from such narcotic sleep. It may be supposed that in the latter case the effect of the drug has been to ensure occurrence of that second predisposing factor mentioned above, of that withdrawal of sense impulses from the nerve centres that serves to usher in the state of sleep. In certain conditions it may be well worth while by means of narcotic drugs to close the portals of the Senses for the sake of thus obtaining stillness in the chambers of the mind; their enforced quietude may induce a period in which natural rest and repair continue long after the initial unnatural arrest of vitality due to the drug itself has passed away.

Hypnotism.—The physiology of this group of “states” is, as regards the real understanding of their production, eminently vague (see also Hypnotism). The conditions which tend to induce them contain generally, as one element, constrained visual attention prolonged beyond ordinary duration. Symptoms attendant on the hypnotic state are closure of the eyelids by the hypnotizer without subsequent attempt to open them by the hypnotized subject; the pupils, instead of being constricted, as for near vision, dilate, and there sets in a condition superficially resembling sleep. But in natural sleep the action of all parts of the nervous system is subdued, whereas in the hypnotic the reactions of the lower, and some even of the higher, parts are exalted. Moreover, the reactions seem to follow the sense impressions with such fatality, that, as an inference, absence of will-power to control them or suppress them is suggested. This reflex activity with “paralysis of will” is characteristic of the somnambulistic state. The threshold-value of the stimuli adequate for the various senses may be extraordinarily lowered. Print of microscopic size may be read; a watch ticking in another room can be heard. judgment of weight and texture of surface is exalted; thus a card can in a dark room be felt and then re-selected from the re-shuffled pack. Akin to this condition is that in which the power of maintaining muscular effort is increased; the individual may lie stiff with merely head and feet supported on two chairs; the limbs can be held outstretched for hours at a time. This is the cataleptic state, the phase of hypnotism which the phenomena of so-called “animal hypnotism” resemble most. A frog or fowl or guinea-pig held in some unnatural pose, and retained so forcibly for a time, becomes “set” in that pose, or rather in a posture of partial recovery of the normal posture. In this state it remains motionless for various periods. This condition is more than usually readily induced when the cerebral hemispheres have been removed. The decerebrate monkey exhibits “cataleptoid” reflexes. Father A. Kircher’s experimentum mirabile with the fowl and the chalk line succeeds best with the decerebrate hen. The attitude may be described as due to prolonged, not very intense, discharge from reflex centres that regulate posture and are probably intimately connected with the cerebellum. A sudden intense sense stimulus usually suffices to end this tonic discharge. It completes the movement that has already set in but had been checked, as it were, half-way, though tonically maintained. Coincidently with the persistence of the tonic contraction, the higher and volitional centres seem to lie under a spell of inhibition; their action, which would complete or cut short the posture-spasm, rests in abeyance. Suspension of cerebral influence exists even more markedly, of course, when the cerebral hemispheres have been ablated.

But a potent—according to some, the most potent—factor in hypnotism, namely, suggestion, is unrepresented in the production of so-called animal hypnotism. We know that one idea suggests another, and that volitional movements are the outcome of ideation. If we assume that there is a material process at the basis of ideation, we may take the analogy of the concomitance between a spinal reflex movement and a skin sensation. The physical “touch” that initiates the psychical “touch” initiates, through the very same nerve channels, a reflex movement responsive to the physical “touch,” just as the psychical “touch” may be considered also a response to the same physical event. But in the decapitated animal we have good arguments for belief that we get the reflex movement alone as response; the psychical touch drops out. Could we assume that there is in the adult man reflex machinery which is of higher order than the merely spinal, which employs much more complex motor mechanisms than they, and is connected with a much wider range of sense organs; and could we assume that this reflex machinery, although usually associated in its action with memorial and volitional processes, may in certain circumstances be sundered from these latter and unattendant on them—may in fact continue in work when the higher processes are at a standstill—then we might imagine a condition resembling that of the somnambulistic and cataleptic states of hypnotism.

Such assumptions are not wholly unjustified. Actions of great complexity and delicacy of adjustment are daily executed by each of us without what is ordinarily understood as volition, and without more than a mere shred of memory attached thereto. To take one’s watch from the pocket and look at it when from a familiar clock-tower a familiar bell strikes a familiar hour, is an instance of a habitual action initiated by a sense perception outside attentive consciousness. We may suddenly remember dimly afterwards that we have done so, and we quite fail to recall the difference between the watch time and the clock time. In many instances hypnotism seems to establish quickly reactions similar to such as usually result only from long and closely attentive practice. The sleeping mother rests undisturbed by the various noises of the house and street, but wakes at a slight murmur from her child. The ship’s engineer, engaged in conversation with some visitor to the engine-room, talks apparently undisturbed by all the multifold noise and rattle of the machinery, but let the noise alter in some item which, though unnoticeable to the visitor, betokens importance to the trained ear, and his passive attention is in a moment caught. The warders at an asylum have been hypnotized to sleep by the bedside of dangerous patients, and "suggested” to awake the instant the patients attempt to get out of bed, sounds which had no import for them being inhibited by suggestion. Warders in this way worked all day and performed night duty also for months without showing fatigue. This is akin to the “repetition” which, read by the schoolboy last thing overnight, is on waking “known by heart.” Most of us can wake somewhere about a desired although unusually early hour, if overnight we desire much to do so.

Two theories of a physiological nature have been proposed to account for the separation of the complex reactions of these conditions of hypnotism from volition and from memory. R. P. H. Heidenhain’s view is that the cortical centres of the hemisphere are inhibited by peculiar conditions attaching to the initiatory sense stimuli. W. T. Preyer’s view is that the essential condition for initiation is fatigue of the will-power under a prolonged effort of undivided attention.

Hypnotic somnambulism and hypnotic catalepsy are not the only or the most profound changes of nervous condition that hypnosis can induce. The physiological derangement which is the basis of the abeyance of volition may, if hypnotism be profound, pass into more widespread derangement, exhibiting itself as the hypnotic lethargy. This is associated not only with paralysis of will but with profound anaesthesia. Proposals have been made to employ hypnotism as a method of producing anaesthesia for surgical purposes, but there are two grave objections to such employment. In order to produce a sufficient degree of hypnotic lethargy the subject must be made extremely susceptible, and this can only be done by repeated hypnotization. It is necessary to hypnotize patients every day for several weeks before they can be got into a degree of stupor sufficient to allow of the safe execution of a surgical operation. But the state itself, when reached, is at least as dangerous to life as is that produced by inhalation of ether, and it is more difficult to recover from. Moreover, by the processes the subject has gone through he has had those physiological activities upon which his volitional power depends excessively deranged, and not improbably permanently enfeebled.  (C. S. S.) 

1. The anatomy of the muscles is dealt with under Muscular System, and of the nerves under Nerve and Nervous System.