A Practical Physiology Part 30

The cord also transmits by means of its motor nerves the commands of the brain to the voluntary muscles, and so causes movement. Thus, when the cord is divided at any point, compressed, as by a tumor or broken bone, or disorganized by disease, the result is a complete loss of sensation and voluntary movement below the point of injury. If by accident a man has his spinal cord injured at some point, he finds he has lost all sensation and power of motion below that spot. The impulse to movement started in his brain by the will does not reach the muscles he wishes to move, because traveling _down_ the spinal cord, it cannot pa.s.s the seat of injury.

So the impression produced by p.r.i.c.king the leg with a pin, which, before pain can be felt, must travel up the spinal cord to the brain, cannot reach the brain because the injury obstructs the path. The telegraph wire has been cut, and the current can no longer pa.s.s.

277. The Spinal Cord as a Conductor of Impulses. The ident.i.ty in structure of the spinal nerves, whether motor or sensory, and the vast number of nerves in the cord make it impossible to trace for any distance with the eye, even aided by the microscope and the most skillful dissection, the course of nerve fibers. The paths by which the motor impulses travel down the cord are fairly well known. These impulses originate in the brain, and pa.s.sing down keep to the same side of the cord, and go out by nerves to the same side of the body.

The sensory impulses, however, soon after they enter the cord by the nerve of one side, cross in the cord to the opposite side, up which they travel to the brain. Thus the destruction of one lateral half of the cord causes paralysis of motion on the _same side_ as the injury, but loss of sensation on the _opposite side_, because the posterior portion destroyed consists of fibers which have crossed from the opposite side.

Experiment proves that if both roots of a spinal nerve be cut, all those parts of the body to which they send branches become paralyzed, and have neither sense of pain nor power of voluntary movement. The parts might even be cut or burned without pain. It is precisely like cutting a telegraph wire and stopping the current.

[Ill.u.s.tration: Fig. 119.--The Base of the Brain.

A, anterior lobe of the cerebrum; B, olfactory nerve; C, sphenoid portion of the posterior lobe; D, optic chiasm; E, optic tract; F, abducens; H, M, hemispheres of the cerebellum; K, occipital portion of the occipital lobe; L, fissure separating the hemispheres; N, medulla oblongata; O, olivary body; P, antenor pyramids; R, pons Valoru; S, section of olfactory nerve, with the trunk removed to show sulcus in which it is lodged; T, anterior extremity of median fissure ]

Experiment also proves that if only the posterior root of a spinal nerve be cut, all sensation is lost in the parts to which the nerve pa.s.ses, but the power of moving these parts is retained. But if the anterior root alone be divided, all power of motion in the parts supplied by that nerve is lost, but sensation remains. From these and many other experiments, it is evident that those fibers of a nerve which are derived from the anterior root are motor, and those from the posterior root sensory, fibers. Impulses sent _from_ the brain and spinal cord to muscles will, therefore, pa.s.s along the anterior roots through those fibers of the nerves which are derived from these (motor) roots. On the other hand, impressions or sensations pa.s.sing _to_ the brain will enter the spinal cord and reach the brain through the posterior or sensory roots.

278. The Spinal Cord as a Reflex Center. Besides this function of the spinal cord as a great nerve conductor to carry sensations to the brain, and bring back its orders, it is also an independent center for what is called reflex action. By means of its sensory nerves it receives impressions from certain parts of the body, and on its own authority sends back instructions to the muscles by its motor nerves, without consulting the brain. This const.i.tutes reflex action, so called because the impulse sent to the spinal cord by certain sensory nerves is at once reflected or sent back as a motor impulse to the muscles.

This reflex action is a most important function of the spinal cord. This power is possessed only by the gray matter of the cord, the white substance being simply a conductor.

The cells of gray matter are found all along the cord, but are grouped together in certain parts, notably in the cervical and lumbar regions. The cells of the anterior horns are in relation with the muscles by means of nerve fibers, and are also brought into connection with the skin and other sensory surfaces, by means of nerve fibers running in the posterior part of the cord. Thus there is established in the spinal cord, without reference to the brain at all, a reflex mechanism.

279. Reflex Centers. For the purpose of ill.u.s.tration, we might consider the body as made up of so many segments piled one on another, each segment presided over by a similar segment of spinal cord. Each bodily segment would have sensory and motor nerves corresponding to its connection with the spinal cord. The group of cells in each spinal segment is intimately connected with the cells of the segments above and below.

Thus an impression reaching the cells of one spinal segment might be so strong as to overflow into the cells of other segments, and thus cause other parts of the body to be affected.

Take as an example the case of a child who has eaten improper food, which irritates its bowels. Sensory nerves of the bowels are disturbed, and powerful impressions are carried up to a center in the spinal cord. These impressions may now overflow into other centers, from which spasmodic discharges of nerve energy may be liberated, which pa.s.sing to the muscles, throw them into violent and spasmodic contraction. In other words, the child has a fit, or convulsion. All this disturbance being the result of reflex action (the spasmodic motions being quite involuntary, as the brain takes no part in them), the child meanwhile is, of course, entirely unconscious and, however it may seem to be distressed, really suffers no pain.

Scattered along the entire length of the spinal cord, especially in the upper part, are groups of nerve cells which preside over certain specific functions of animal life; that is, definite collections of cells which control definite functions. Thus there are certain centers for maintaining the action of the heart, and the movements of breathing; and low down in the cord, in the lumbar regions, are centers for the control of the various abdominal organs.

Numerous other reflex centers are described by physiologists, but enough has been said to emphasize the great importance of the spinal cord as an independent nerve center, besides its function as a conductor of nervous impulses to and from the brain.

280. The Brain as a Reflex Center. The brain, as we have just stated, is the seat of consciousness and intelligence. It is also the seat of many reflex, automatic, and coordinating centers. These give rise to certain reflex actions which are as entirely independent of consciousness as are those of the spinal cord. These acts take place independently of the will, and often without the consciousness of the individual. Thus, a sudden flash of light causes the eyes to blink, as the result of reflex action. The optic nerves serve as the sensory, and the facial nerves as the motor, conductors. The sudden start of the whole body at some loud noise, the instinctive dodging a threatened blow, and the springing back from sudden danger, are the results of reflex action. The result ensues in these and in many other instances, without the consciousness of the individual, and indeed beyond his power of control.

281. The Importance of Reflex Action. Reflex action is thus a marvelous provision of nature for our comfort, health, and safety. Its vast influence is not realized, as its numberless acts are so continually going on without our knowledge. In fact, the greater part of nerve power is expended to produce reflex action. The brain is thus relieved of a vast amount of work. It would be impossible for the brain to serve as a "thinking center" to control every act of our daily life. If we had to plan and to will every heart-beat or every respiration, the struggle for life would soon be given up.

The fact that the gray cells of the spinal cord can originate a countless number of reflex and automatic activities is not only of great importance in protecting the body from injury, but increases vastly the range of the activities of our daily life.

Even walking, riding the bicycle, playing on a piano, and numberless other such acts may be reflex movements. To learn how, requires, of course, the action of the brain, but with frequent repet.i.tion the muscles become so accustomed to certain successive movements, that they are continued by the cord without the control of the brain. Thus we may acquire a sort of artificial reflex action, which in time becomes in a way a part of our organization, and is carried on without will power or even consciousness.

So, while the hands are busily doing one thing, the brain can be intently thinking of another. In fact, any attempt to control reflex action is more apt to hinder than to help. In coming rapidly down stairs, the descent will be made with ease and safety if the spinal cord is allowed entire charge of the act, but the chances of stumbling or of tripping are very much increased if each step be taken as the result of the will power. The reflex action of the cord may be diminished, or inhibited as it is called, but this power is limited. Thus, we can by an effort of the will stop breathing for a certain time, but beyond that the reflex mechanism overcomes our will and we could not, if we would, commit suicide by holding our breath. When we are asleep, if the palm of the hand be tickled, it closes; when we are awake we can prevent it.

[Ill.u.s.tration: Fig. 120.--Dr. Waller's Diagrammatic Ill.u.s.tration of the Reflex Process.

From the sentient surface (1) an afferent impulse pa.s.ses along (2) to the posterior root of the spinal cord, the nerve fibers of the posterior root ending in minute filaments among the small cells of this part of the cord (3). In some unknown way this impulse pa.s.ses across the gray part of the cord to the large cells of the anterior root (5), the cells of this part being connected by their axis-cylinder with the efferent fibers (6). These convey the stimulus to the fibers of the muscle (7), which accordingly contract. Where the brain is concerned in the action the circuit is longer through S and M.]

Experiment 131. _To ill.u.s.trate reflex action by what is called knee-jerk._ Sit on a chair, and cross the right leg over the left one.

With the tips of the fingers or the back of a book, strike the right ligamentum patellae. The right leg will be raised and thrown forward with a jerk, owing to the contraction of the quadriceps muscles. An appreciable time elapses between the striking of the tendon and the jerk. The presence or absence of the knee-jerk may be a most significant symptom to the physician.

282. The Sympathetic System. Running along each side of the spine, from the base of the skull to the coccyx, is a chain of nerve knots, or ganglia. These ganglia, twenty-four on each side, and their branches form the sympathetic system, as distinguished from the cerebro-spinal system consisting of the brain and spinal cord and the nerves springing from them. The ganglia of the sympathetic system are connected with each other and with the sensory roots of the spinal nerves by a network of gray nerve fibers.

At the upper end the chain of each side pa.s.ses up into the cranium and is closely connected with the cranial nerves. In the neck, branches pa.s.s to the lungs and the heart. From the ganglia in the chest three nerves form a complicated network of fibers, from which branches pa.s.s to the stomach, the liver, the intestines, the kidneys, and other abdominal organs. A similar network of fibers is situated lower down in the pelvis, from which branches are distributed to the pelvic organs. At the coccyx the two chains unite into a single ganglion.

Thus, in general, the sympathetic system, while intimately connected with the cerebro-spinal, forms a close network of nerves which specially accompany the minute blood-vessels, and are distributed to the muscles of the heart, the lungs, the stomach, the liver, the intestines, and the kidneys--that is, the hollow organs of the body.

283. The Functions of the Sympathetic System. This system exercises a superintending influence over the greater part of the internal organs of the body, controlling to a certain extent the functions of digestion, nutrition, circulation, and respiration. The influence thus especially connected with the processes of organic life is generally different from, or even opposed to, that conveyed to the same organs by fibers running in the spinal or cranial nerves. These impulses are beyond the control of the will.

[Ill.u.s.tration: Fig. 121.--The Cervical and Thoracic Portion of the Sympathetic Nerve and its Main Branches.

A, right pneumogastric; B, spinal accessory; C, glosso-pharyngeal; D, right bronchus; E, right branch of pulmonary artery; F, one of the intercostal nerves; H, great splanchnic nerve; K, solar plexus; L, left pneumogastric; M, stomach branches of right pneumogastric; N, right ventricle; O, right auricle; P, trunk of pulmonary artery; R, aorta; S, cardiac nerves; T, recurrent laryngeal nerve; U, superior laryngeal nerve; V, submaxillary ganglion; W, lingual branch of the 5th nerve; X, ophthalmic ganglion; Y, motor oculi externus.

Hence, all these actions of the internal organs just mentioned that are necessary to the maintenance of the animal life, and of the harmony which must exist between them, are controlled by the sympathetic system. But for this control, the heart would stop beating during sleep, digestion would cease, and breathing would be suspended. Gentle irritation of these nerves, induced by contact of food in the stomach, causes that organ to begin the churning motion needed for digestion. Various mental emotions also have a reflex action upon the sympathetic system. Thus, terror dilates the pupils, fear acts upon the nerves of the small blood-vessels of the face to produce pallor, and the sight of an accident, or even the emotions produced by hearing of one, may excite nausea and vomiting.

The control of the blood-vessels, as has been stated (sec. 195), is one of the special functions of the sympathetic system. Through the nerves distributed to the muscular coats of the arteries, the caliber of these vessels can be varied, so that at one moment they permit a large quant.i.ty of blood to pa.s.s, and at another will contract so as to diminish the supply. This, too, is beyond the control of the will, and is brought about by the vaso-motor nerves of the sympathetic system through a reflex arrangement, the center for which is the medulla oblongata.

284. Need of Rest. The life of the body, as has been emphasized in the preceding chapters, is subject to constant waste going on every moment, from the first breath of infancy to the last hour of old age. We should speedily exhaust our life from this continual loss, but for its constant renewal with fresh material. This exhaustion of life is increased by exertion, and the process of repair is vastly promoted by rest. Thus, while exercise is a duty, rest is equally imperative.

The eye, when exactingly used in fine work, should have frequent intervals of rest in a few moments of darkness by closing the lids. The brain, when urged by strenuous study, should have occasional seasons of rest by a dash of cold water upon the forehead, and a brief walk with slow and deep inspirations of fresh air. The muscles, long cramped in a painful att.i.tude, should be rested as often as may be, by change of posture or by a few steps around the room.

It is not entirely the amount of work done, but the continuity of strain that wears upon the body. Even a brief rest interrupts this strain; it unclogs the wheels of action. Our bodies are not designed for continuous toil. An alternation of labor and rest diminishes the waste of life. The benign process of repair cannot go on, to any extent, during strenuous labor, but by interposing frequent though brief periods of rest, we lessen the amount of exhaustion, refresh the jaded nerves, and the remaining labor is more easily endured.

285. Benefits of Rest. There is too little repose in our American nature and in our modes of life. A sense of fatigue is the mute appeal of the body for a brief respite from labor, and the appeal should, if possible, be heeded. If this appeal be not met, the future exertion exhausts far more than if the body had been even slightly refreshed. If the appeal be met, the brief mid-labor rest eases the friction of toil, and the remaining labor is more easily borne. The feeling that a five-minute rest is so much time lost is quite an error. It is a gain of physical strength, of mental vigor, and of the total amount of work done.

The merchant burdened with the cares of business life, the soldier on the long march, the ambitious student over-anxious to win success in his studies, the housewife wearied with her many hours of exacting toil, each would make the task lighter, and would get through it with less loss of vital force, by occasionally devoting a few minutes to absolute rest in entire relaxation of the strained muscles and overtaxed nerves.

286. The Sabbath as a Day of Physiological Rest. The divine inst.i.tution of a Sabbath of rest, one day in seven, is based upon the highest needs of our nature. Rest, to be most effective, should alternate in brief periods with labor.

It is sound physiology, as well as good morals and manners, to cease from the usual routine of six days of mental or physical work, and rest both the mind and the body on the seventh. Those who have succeeded best in what they have undertaken, and who have enjoyed sound health during a long and useful life, have studiously lived up to the mandates of this great physiological law. It is by no means certain that the tendency nowadays to devote the Sabbath to long trips on the bicycle, tiresome excursions by land and sea, and sight-seeing generally, affords that real rest from a physiological point of view which nature demands after six days of well-directed manual or mental labor.

287. The Significance of Sleep as a Periodical Rest. Of the chief characteristics of all living beings none is so significant as their periodicity. Plants as well as animals exhibit this periodic character. Thus plants have their annual as well as daily periods of activity and inactivity. Hibernating animals pa.s.s the winter in a condition of unconsciousness only to have their functions of activity restored in early spring. Human beings also present many instances of a periodic character, many of which have been mentioned in the preceding pages. Thus we have learned that the heart has its regular alternating periods of work and rest. After every expiration from the lungs there is a pause before the next inspiration begins.

Now sleep is just another manifestation of this periodic and physiological rest by which Nature refreshes us. It is during the periods of sleep that the energy expended in the activities of the waking hours is mainly renewed. In our waking moments the mind is kept incessantly active by the demands made on it through the senses. There is a never-ceasing expenditure of energy and a consequent waste which must be repaired. A time soon comes when the brain cells fail to respond to the demand, and sleep must supervene. However resolutely we may resist this demand, Nature, in her relentless way, puts us to sleep, no matter what objects are brought before the mind with a view to retain its attention.[41]

288. Effect of Sleep upon the Bodily Functions. In all the higher animals, the central nervous system enters once at least in the twenty-four hours into the condition of rest which we call sleep.

Inasmuch as the most important modifications of this function are observed in connection with the cerebro-spinal system, a brief consideration of the subject is properly studied in this chapter. In Chapter IV. we learned that repose was as necessary as exercise to maintain muscular vigor. So after prolonged mental exertion, or in fact any effort which involves an expenditure of what is often called nerve-force, sleep becomes a necessity. The need of such a rest is self-evident, and the loss of it is a common cause of the impairment of health. While we are awake and active, the waste of the body exceeds the repair; but when asleep, the waste is diminished, and the cells are more actively rebuilding the structure for to-morrow's labor. The organic functions, such as are under the direct control of the sympathetic nervous system,--circulation, respiration, and digestion,--are diminished in activity during sleep. The pulsations of the heart and the respiratory movements are less frequent, and the circulation is slower. The bodily temperature is reduced, and the cerebral circulation is diminished. The eyes are turned upward and inward, and the pupils are contracted.

The senses do not all fall to sleep at once, but drop off successively: first the sight, then the smell, the taste, the hearing and lastly the touch. The sleep ended, they awake in an inverse order, touch, hearing, taste, smell, and sight.

289. The Amount of Sleep Required. No precise rule can be laid down concerning the amount of sleep required. It varies with age, occupation, temperament, and climate to a certain extent. An infant whose main business it is to grow spends the greater part of its time in sound sleep.

Adults of average age who work hard with their hands or brain, under perfectly normal physiological conditions, usually require at least eight hours of sleep. Some need less, but few require more. Personal peculiarities, and perhaps habit to a great extent, exert a marked influence. Some of the greatest men, as Napoleon I., have been very sparing sleepers. Throughout his long and active life, Frederick the Great never slept more than five or six hours in the twenty-four. On the other hand, some of the busiest brain-workers who lived to old age, as William Cullen Bryant and Henry Ward Beecher, required and took care to secure at least eight or nine hours of sound sleep every night.

In old age, less sleep is usually required than in adult life, while the aged may pa.s.s much of their time in sleep. In fact, each person learns by experience how much sleep is necessary. There is no one thing which more unfits one for prolonged mental or physical effort than the loss of natural rest.

290. Practical Rules about Sleep. Children should not be played with boisterously just before the bedtime hour, nor their minds excited with weird goblin stories, or a long time may pa.s.s before the wide-open eyes and agitated nerves become composed to slumber. Disturbed or insufficient sleep is a potent factor towards producing a fretful, irritable child.

At all ages the last hour before sleep should, if possible, be spent quietly, to smooth the way towards sound and refreshing rest. The sleep induced by medicine is very often troubled and unsatisfactory. Medicines of this sort should not be taken except on the advice of a physician.

While a hearty meal should not usually be taken just before bedtime, it is not well to go to bed with a sense of positive faintness and hunger.

Rather, one should take a very light lunch of quite simple food as a support for the next eight hours.