The Nervous System In Health And Disease
( Originally Published 1908 )
THE cellular theory has been one of the most stimulating movements in modern science. This theory considers all living organisms, whether animals or plants, as composed of an immense number of independent, yet interacting, units, called cells. A cell may be defined as a microscopic, anatomical unit, limited by a membrane or wall and containing a semi-liquid substance called protoplasm. Within this protoplasm lies the most vital portion of the cell, from the standpoint of heredity and energy, known as the nucleus. This protoplasm has been defined by Huxley as "the physical basis of life." Cells can be seen and studied by means of the higher powers of the microscope. Simple as they may seem, yet their complexity is marvelous, and the rôle played by them in development, inheritance, disease, and physiological function is of the highest importance. In the middle of the nineteenth century, Virchow, the great German pathologist, applied the cell theory to the disease processes of living beings, and so fruitful has this concept been, that medicine has uninterruptedly advanced along these lines. The various bacteria are really very minute cells. Plants and trees are composed of millions of identical cells, but it is in living animals that we find the greatest diversity of cell-forms, and of special functions of individual groups of cells. Each organ has its special cells; the liver cells secrete bile; the cells of the stomach secrete gastric juice, etc. The list might be indefinitely extended. But it is in the central nervous system that the cell has reached its highest development in form and function. A great variety of cells can be found in the various portions of the brain and spinal cord, and these, with their interlacing network of fibers, each an offshoot from an individual cell, form a complexity of nerve tracts that is positively bewildering even to the trained neurologist. During recent years the anatomy and physiology of the central nervous system has attracted an immense amount of attention. To take an entire library on nerve anatomy, physiology, and pathology, with all the technicalities, and to condense this materia' within the limits of a single chapter, is a formidable task. We must therefore ask the indulgence of our readers for all sins of omission that this chapter may contain. Everything in the living animal organism, muscular movement, secretion, excretion, breathing, the heart-beat, even consciousness itself, is dependent on the central nervous system. Thus the nervous system may be called the master tissue. To the ultimate units of this master tissue, the nerve cell itself, we will first direct our attention.
A. The Normal Nerve Cell. In Figs. 1 and 2 are represented normal nerve cells from the brain and spinal cord. One of these (FIG. I) is known as a pyramidal cell from its shape; the other (Fig. 2) is an anterior horn cell from the spinal cord. In Fig. 3 is another pyramidal nerve cell, but a different variety of stain has been used. Nerve cells vary in shape and size, from 1/250 to 1/3000 of an inch in diameter. Their number is enormous, the figures amounting to hundreds of millions. When stained by one of the aniline dyes (methylene blue), a normal nerve cell shows the following characteristics. In the center of the cell there is an oval, light area, within which lies a round, dark body. This light area is known as the nucleus; the dark body within is called the nucleolus. Outside the nucleus and within the cell body are a number of irregularly shaped granules, arranged rather concentrically and called the Nissl bodies, after a distinguished German neurologist. Running through each nerve cell and passing into the various processes of the cell are a large number of fine thread-like structures, called neuro-fibrils (not shown in the illustration). These neuro-fibrils are perhaps the conducting substance of the nervous system.
Two varieties of processes come from the cell body. One of these, called the dendrites, divides and sub-divides in an antler-like fashion, and thus spreads over a considerable territory. The other, called the axis-cylinder, is smooth, but instead of being grouped around the cell like the dendrite, it runs out a considerable distance, varying from the smallest fraction of an inch to the length of the spinal cord. A nerve cell can have many dendrites, but it can have only one axis-cylinder. Upon the dendrites may be seen minute buds or swellings called gemmules (see Fig. 3). The entire cell, with its axis-cylinder, dendrites, gemmules, etc., is known as a neurone. Each neurone is an independent anatomical and physiological unit, but they all inter-lace and connect with one another in a most complex and bewildering manner (see Fig. 4). Every neurone, in spite of its independence, acts in conjunction with several other neurones, and wide communications are possible by means of the dendrites and axis-cylinders. The supporting tissue of the neurones is known as the neuroglia. The axis-cylinder is composed of three layers; an inner layer .of fine fibers, a middle protecting layer arranged in segments and known as the medullary sheath, and an outer thin membrane called the neurilemma. The nerve cells make up the gray matter of the brain and spinal cord; the white matter is composed of the axis-cylinders. Nerve impulses are carried to the cell by the dendrites and away from the cell by the axis-cylinder.
The grouping of the axis-cylinders makes up the central nervous pathways and the nerves themselves, which either communicate with central cells in the brain or cord (point of origin), or with the skin or various organs of the body (point of distribution). Like all cells of the body, the nutrition of the nerve cell is dependent on its blood supply.
B. The Diseased Nerve Cell. In organic diseases of the nervous system marked changes are found, not only in the nerve cell, but also in the nerve fibers. Figures 5 and 6 show very common appearances of diseased nerve cells in the brain and spinal cord, such as is pre-eminently found in chronic alcoholism and alcoholic neuritis. Observe that the nucleus is no longer central, but is pushed to one side, while the Nissl bodies have almost completely disappeared and are replaced by a fine dust-like substance. Fig. 7 A shows the appearance of a nerve cell when its blood supply is cut off, the dendrites and gemmules having in great part disappeared. In Fig. 7B is shown the destructive effect of ricin poisoning. Note the spindle-shaped swellings on the dendrites and the complete disappearance of the gemmules. The above are only a few of the changes that can take place in various diseases. In senile dementia, general paralysis, locomotor ataxia, hemorrhage or softening of the brain or spinal cord, not only are the cells diseased, but the nerve fibers likewise suffer destruction. Fever of any kind has an especially deleterious effect upon the nerve cell, and this accounts for the delirium of fever. Alcohol, morphine or cocaine, the bites of certain ven omous snakes or spiders, in tetanus or lockjaw, and in certain forms of melancholia, marked changes are found in the nerve cell. Chronic alcoholism, leading to alcoholic insanity or dementia or to alcoholic neuritis, is particularly prone to injure the nerve cell. One man's drink flies to his head, another to his heels, but both the incoherent speech and the staggering gait of the intoxicated individual are due to the influence of alcohol on the central nervous system. In certain severe states of exhaustion and fatigue, the tired nerve cell becomes shrunken and diminished in size. When a nerve fiber is cut or diseased, its connected cell shows a characteristic change (Figs. 5 and 6). Now in all functional nervous diseases, such as the various forms of neurasthenia, hysteria, and psychasthenia, the nerve cells appear absolutely normal. In these diseases we are dealing, not with any anatomical changes in the cell, but with a disordered function of the cell, a change in its physiology.
C. Nerve Physiology. We have briefly reviewed the anatomy of the nerve cell and its fibers in health and disease. Before taking up the general features of consciousness and cerebral localization, it will be necessary to consider some essential facts of nerve physiology. Nerve tissue possesses two fundamental characteristics. One of these is nerve conduction, whereby motion, sensation, and reflex action become possible; the other chief function of nerve tissue is the storing up of impressions and reproducing them in the same order. This latter forms what is known as associative memory. The brain is the organ of mind or of consciousness, while the spinal cord may be called the organ of reflex action. Of course this is true only within certain limits, as the various functions are not isolated phenomena, but there exists a considerable overlapping. It is generally believed, however, that the spinal cord possesses no consciousness.
The reproduction of impressions is due to the peculiar characteristics of the protoplasm of the nerve cell, which in many ways acts like the cylinder of a phonograph. Nerve conduction takes place in the axis-cylinder and this axis-cylinder resembles a copper wire through which a current of electricity is transmitted. The medullary sheath surrounding the axis-cylinder acts like an insulating substance, in the same way as an electric wire is insulated with gutta percha. If the protoplasm of the nerve cell is diseased, it cannot store up impressions and therefore cannot reproduce them. Under these conditions, either a loss of memory or amnesia results, or there is a state of mental enfeeblement or dementia. When the axis-cylinder is cut or diseased, conduction becomes impossible, and there results loss of motion (paralysis), or loss of sensation (anaesthesia), or loss of reflex action.
Certain nerves and their endings in the internal organs have an isolated function of their own. The olfactory nerve, for instance, can only react to odors; the auditory nerve to air vibrations which are appreciated as sound in the central nervous system; the optic nerve and the retina can only react to the ether waves which become sensations of light. The optic nerve cannot react to sound vibrations nor the auditory nerve to ether waves. In other words, we cannot see sound, nor hear light. Simple as this may seem, yet this absolute reaction of a nerve to a fixed stimulus forms one of the most fundamental facts in modern neurology, what is known as the doctrine of the specific energies of nerves.
If the finger touches a sharp point or a hot object, it is drawn quickly away. There is a sensation of burning or of pain. Both sensation and motion have taken place. But if the nerves in the arm had been previously cut, thus paralyzing the arm and abolishing sensation, no pain or burning would have been felt, neither would the strongest effort of the will have sufficed to have drawn the arm away. What does this mean? It means that the feeling of pain does not reside in the fine nerve filaments of the finger or in the large nerve trunks of the arm, but in the brain itself. The nerves merely conduct the physical stimulus of a sharp point or a hot object to the brain and there it is felt as pain. It is the brain, on feeling the painful sensation, that wills that the arm be drawn away. This impulse is conducted from the brain to the spinal cord and down the nerves of the arm. A short but appreciable and easily measurable length of time is needed for this act, before the pain is felt in the central nervous system, appreciated in consciousness, transformed into the idea that the arm must be pulled away to prevent further injury, the reaction chosen and the arm quickly moved. The act appears simple, but in reality it is quite complex. It takes time. It is really a movement of protection, a reflex action. This choice of reaction in the brain is known as the will.
Nerve conduction can be demonstrated in another way. If one of the large leg muscles of a frog with its attached nerve be excised and an electric current applied to the nerve, the muscle will contract. This can be repeated until the muscle becomes fatigued. In frequently repeated stimulation, the muscular contractions will follow one another very rapidly and a condition called tetanus will result. But if the nerve be cut or the place where the nerve is attached to the muscle (the so-called motor end plate) be poisoned with curare (Indian arrow poison), conduction is interfered with and there-fore no muscular contraction can take place.
Reflex actions are very important in studying and diagnosing the organic diseases of the nervous system. What is known as the knee jerk is perhaps the most important reflex from the standpoint of diagnosis. If the patellar tendon just below the knee cap be struck a quick and moderately sharp blow with a rubber hammer, or the finger-tips, there results a contraction of the thigh muscles and the leg is thrown forward. This is the knee jerk. It has nothing to do with consciousness or sensation as when a finger is burnt, but is purely a reflex action. The stimulus from the blow is carried along the nerve trunks of the leg to the spinal cord, there it goes over to the nerve fibers that connect with the muscles, is transformed to a motor impulse which causes the muscles to contract and the kick results. This connection of nerve with spinal cord, and of this latter with the muscles again, forms what is known as the reflex arc. If this arc is interrupted along any portion of its path, in the nerves or in the spinal cord, the knee jerk remains absent. This especially occurs in locomotor ataxia, a disease of the spinal cord.
Other nerves, such as those which control respiration and the heart-beat, seem to act automatically from the brain centers themselves. The nerves which carry sensation are called sensory nerves; those which preside over muscular movements are called motor nerves. Other nerves control the secretion of various juices of the body, such as the saliva or gastric juice. These are called secretory nerves. The so-called trophic nerves govern the nutrition of the tissues.
An example of mixed nerve conduction may be cited. When light falls on the retina, not only is the sensation of light itself produced, but the pupil of the eye narrows in an automatic manner, without the control of the will. Here we seem to be dealing, not only with a sensory reaction (sensation of light), but one which is reflex as well (the contraction of the pupil). The knee reflex can be controlled; the pupillary reflex cannot be controlled.
It has been shown that a nerve transmits impulses by means of its axis-cylinder much as a copper wire transmits electricity, and also that a certain period of time is required for the passage of the nerve impulse. The popular phrase "quick as thought" has therefore no foundation in fact. Thought is slow when compared to the rate of movement of light (186,000 miles per second) or even sound (1Q91 feet per second). In man the rate of transmission along nerve fibers is about thirty-three meters (r o8 feet) per second; in the frog it is about twenty-eight meters (92 feet) per second. By this we see that the reactions to pain or other sensations are not instantaneous as is popularly supposed, but that an appreciable length of time is necessary for stimuli to be appreciated in consciousness as such, and for sensory impulses to be trans-formed into motor reactions. When something is "willed" this time interval becomes lengthened. The reaction time is greatly increased in old age, where not only the rate of nerve transmission, but the mental processes, become slower.
The storing up of impressions in the central nervous system is based upon the principle that certain molecular changes are produced, which continue after the original stimulus has been removed. Of the simpler examples, the best are those of after images on the retina, or the "feeling" of a day's skating remaining in the limbs long after the exercise has ceased. The highest example of the storing up of external impressions is found in the phenomena of memory.
D. The Anatomy and Functions of the Nervous System. The nervous system is composed of the brain, spinal cord, the peripheral and cranial nerves, and the sympathetic nerves. The course of the central pathways in the brain and spinal cord and the ramifications of the peripheral nerves are very complex and bewildering. These central pathways, however, have been followed and mapped out in diseases of the central nervous system, in its embryological development and also in experiments on animals, until now the majority of these are fairly well recognized. The pathways themselves are made up of the axis-cylinders that come from nerve cells. Although these axis-cylinders seem to run in all directions, yet each central pathway is sharply limited, being derived from cells that occupy fixed areas in the brain. Each central pathway is called a system. Now in diseases of the cells or of the brain tissue that contains the cells, these fiber systems undergo degeneration and by means of proper staining methods can be followed along their course. In embryological development, the fiber systems of the brain do not develop or become medullated at the same period, and here also we have a method of following the course of the brain fibers. This last fact is of great importance in the architecture of the nervous system, as was pointed out by the German neurologist Flechsig.
A nervous system is the property of all vertebrates and can be found in a rudimentary form in invertebrates very low in the animal scale, such as the jelly fish. The nervous system has well been called the master tissue, and to it may be applied the quaint description of the heart by William Harvey, the immortal discoverer of the circulation of the blood. "It is the household divinity which, discharging its functions, nourishes, cherishes, quickens the whole body, and is indeed the foundation of life, the source of all action."
The brain is contained within the skullcap in order that its delicate structure may be well protected from injury. Further protection is afforded by three membranes, with which the brain is entirely surrounded. The most external of these membranes, and lying just beneath the inner table of the skull, is called the dura mater. Beneath this is the arachnoid membrane, which is a very fine structure made up almost entirely of interlacing small blood-vessels, and so-called because it resembles a spider's web. Beneath the arachnoid, and covering the entire surface of the brain, dipping down even into its fissures, is the pia mater. The spinal cord lies within the spinal column, which is made up of the individual vertebral bones superimposed upon one an-other like a pile of checkers. Between each vertebra is a cartilaginous membrane. This not only is protection afforded the spinal cord, but there is also an extreme degree of elasticity, so that the body may be bent in all directions. Like the brain, the spinal cord is surrounded by the same three membranes. The nerve tracts of the spinal cord run lengthwise, and thus connect the brain centers (the cerebrum or hemispheres), and the medulla, with the nerves that go to the muscles.
The brain has several subdivisions. Chief of these are the two hemispheres or the cerebrum, which are connected by a bridge of white substance called the corpus callosum. Within the hemispheres can be found the central motor tracts. The cerebrum itself is divided into lobes, the frontal, parietal, occipital, temporal, and within one of the great fissures of the brain that separates the parietal from the temporal portions lies another lobe called the Island of Reil. This is not visible externally. Below the cerebrum is the pons Varolii, below this the medulla, and just back of the medulla lies the cerebellum. The medulla contains the centers for respiration and the control of the heart-beat, while the cerebellum presides over equilibrium.
The sympathetic nervous system consists of chains of ganglia, or aggregations of nerve cells, lying on each side of the spinal column. At the base of the brain lie the so-called cranial nerves. Some of these are for the special senses, such as the olfactory nerve for smell, the optic nerve for sight, the auditory nerve for hearing; some supply the motions of the eyeball, and the mimic movements of the face; still others control vital functions, such as swallowing and the heart-beat.
The cerebrum is made up of white and gray matter, the former consisting of nerve fibers, the latter of nerve cells. The gray matter is called the cortex and lies in a folded manner on the surface of the brain, thus greatly increasing its area. Within the spinal cord this order is reversed; it is the white matter that is external and the gray matter within. The prominent raised folds of gray matter in the brain are called convolutions; the grooves between the convolutions are called fissures. In the lower animals, even in the dog and the higher apes, and in cases of lack of brain development, such as idiocy, the cortex is comparatively smooth. In dementia the convolutions are small or atrophied. Thus the area of gray matter and the size of the convolutions can be taken as a measure of intelligence. The brain weighs on the average about 1415 grams (50 ounces) in the adult male, and about 136o grams (44.5 ounces) in the adult female. In idiots and imbeciles, in dementia and in cases of low intelligence, the weight is much less, sometimes sinking as low as 900 grams (28.1 ounces). In individuals of great intellectual capacity, such as in Daniel Webster or in the naturalist Cuvier, there was a great increase in weight.
Within the spinal cord there is a minute central canal, which expands into several connecting chambers in the brain, known as ventricles. These ventricles act as kind of a drainage system. The activity of the brain depends upon the blood supply, which is quite complex. The arteries of the brain are very ingeniously arranged in a hexagonal shape at the base of the brain. This hexagon of arteries is known as the circle of Willis. If one artery becomes plugged, the circulation can go on through the other branches.
The nerve tracts of the brain are very complex. Some of these control motion, such as the pyramidal tract, others control sensation, while still others, known as the association fibers, seem to be at the basis of intellect and associative memory. In right-handed individuals the intellectual functions are localized on the left side of the brain. This is due to the fact that most of the fiber tracts cross over to the other side. A right-handed person therefore may be said to be left-brained.
The brain is the organ of consciousness or mind; the spinal cord is merely a conducting mechanism for the control of motion and sensation by the higher brain centers. If a pigeon or frog be deprived of its brain, it will live, but its actions will resemble a machine. In other words it will become a reflex automaton, originating nothing, learning nothing. Not a trace of memory can be found. It will show nothing but motor activity. Food put before it will be unnoticed, but if the food be placed in the mouth, it will be swallowed. In man, when the brain is profoundly diseased, as in dementia or idiocy, there can be found some of the phenomena of the brainless animal. When sleep results in a healthy individual, there is a low degree of consciousness, other-wise dreaming could not take place. If there is a hemorrhage in the brain, or the head be struck a severe blow, complete unconsciousness results. All these facts prove that the mechanism of consciousness depends upon the integrity of the brain tissue.
Cerebral localization, or the mapping out of various functions on the surface of the brain, has made amazing strides in the last quarter century. This has been the result partly of autopsies on pathological brain lesions (hemorrhages, tumors, softenings) in man, and partly of excision and electrical stimulation experiments on the cortex of dogs and monkeys. Figure 8 is a diagrammatic representation of the principal areas on the surface of the brain. The area marked A controls the movements of the various muscles of the body. Here are localized the large motor cells whose axis-cylinders dip down in a fan-like manner into the white matter of the brain, curve together in the portion known as the internal capsule, and finally, after a more or less devious course, they reach the medulla. Here the fibers from the right side of the brain cross over to the left and vice versa, and then pursue their way down the spinal cord. This system of fibers in the brain and cord is known as the pyramidal tract; the place in the medulla where they cross to the opposite side is called the decussation of the pyramids. A hemorrhage in any portion of the pyramidal tract in the brain causes, therefore, a complete or partial paralysis of the opposite portion of the body. This hemorrhage is accompanied by a sudden loss of consciousness and is known as an apoplectic shock.
The portion behind the motor area, marked B, is the cortical center for sensation of all kinds and the recognition of the nature of objects by touch. In the blind deaf-mutes, such as Laura Bridgman or Helen Keller, this center is greatly developed, as their education was entirely along the line of touch sensations. C is known as Broca's convolution. It is the center for motor speech and is localized on the left side of the brain. A lesion here causes what is known as motor aphasia, in which the patient becomes dumb or is able to make only a few meaningless sounds, yet can perfectly comprehend what is said. D is the hearing center. Disease of this center causes complete deafness. E is the center where auditory memories or the memories of the sound of words are stored up. Disease of this portion of the brain causes what is known as sensory aphasia. The patient can talk freely, can hear what is said, but the words are meaningless to him. He hears the words but does not comprehend them; it is as if one spoke in a foreign language of which he was completely ignorant. At F the visual memories are stored up. In a lesion here, objects are seen but not recognized; it causes what is known as mind blindness. G is the visual center, where all the fibers of the optic nerve terminate. Disease here, on account of the peculiar crossing of the optic nerve, causes a blindness of one half of each eye. H is the so-called writing center. In a lesion of this portion of the brain there is inability to write, known as agraphia. I is the cerebellum, the center of equilibrium. Diseases of the cerebellum cause the individual to lose all sense of the co-ordination of muscular movements necessary for the equilibrium of the body and therefore he reels like a drunken man. The higher psychical centers for reason, memory, and association seem to be localized in the frontal lobe of the brain, anterior to the motor area (A) and above the motor speech center (C). In dementia and in idiocy this is the portion that is most profoundly diseased. The mapping-out of these centers in the brain has been of great practical value in the localization of brain tumors and their successful treatment by surgical operation.
As the individual progresses from childhood to adult life, there is an increase in the weight of the brain and in the number of association fibers. In old age the brain atrophies or grows smaller and the nerve cells degenerate. The speech center is highly developed in man, for with-out speech all abstract reasoning and thought are manifestly impossible. All great thinkers and writers seem to have possessed a large vocabulary, while the language of the savage consists of only a few hundred words. The poet has well said,
"He gave man speech and speech created thought, Which is the measure of the universe."
Unless it has been abused to an irreparable point, so that organic changes occur, the nervous system possesses great recuperative powers. Nervous fatigue is much benefited by sleep, while protracted insomnia or a restless, broken night, has a particularly pernicious effect on the general feeling of well-being, not only in the neurasthenic state, but in perfect health as well. Fatigue may take on a distinctly pathological aspect such as restlessness, muscular twitchings, tremors; various fancies may take possession of the mind which may degenerate into pathological obsessions. As the fatigue increases, there develops that peculiar feeling of incapacity and the disinclination for physical and muscular exercise, symptoms which are very characteristic of the neurasthenic states. The restlessness of school children at the end of a morning or afternoon session, as shown in the movement of the lips, eye-brows, forehead, the picking at the fingers, is due solely to fatigue. Thus we see how necessary it is that fatigue be neutralized by periods of relaxation, either physical exercise or sleep.
Many theories have been proposed to explain sleep, but all of these, when critically examined, are found to have their vulnerable points. The most pertinent interpretation of sleep, from the purely biological standpoint, is that of Claparède, who concludes that sleep is a function of defense, a physiological device, its purpose being to protect the organism against fatigue. Hence the need of sleep is frequently felt before fatigue sets in. This is an admirable mechanism, splendidly fitted to neutralize activity with repose, as it has been shown by Hodge that prolonged activity leading to severe fatigue induces distinct organic changes in the nerve cells, and that after a certain period of rest the cells resume their normal appearance.