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The centers located in the different parts of the nervous system have been discussed in their morphological and physiological relations. It seems advisable, for the sake of clearness, to devote a few pages to a discussion of the nervous control of certain functions whose innervation is unusually complex. The pupilo-constrictor and ciliary muscles are innervated by the short ciliary nerves, gray fibers from the ciliary ganglion (lenticular, or ophthalmic ganglion). This ganglion receives its stimulation by way of the visceral motor fibers of the third cranial nerves. The pupilo-dilator muscles, the non-striated muscles of the levator palpebrarum and the capsule of Tenon, the blood vessels of the eye and the orbit, and the tear glands are innervated from the superior cervical sympathetic ganglion, which is controlled by the upper thoracic centers of the cord. The
eye is subject to the various diseases which are discussed in text-books
dealing with the subject. In addition to the etiological factors
already well recognized, the place of the bony lesion as an etiological
factor must be admitted. Lesions of the upper thoracic vertebrae
are efficient factors in the etiology of disturbances of the circulation
through the eye and the orbital tissues, and in interfering with the normal
innervation of the pupil and the non-striated fibers of the capsule of
Tenon and the levator palpebrarum. These abnormalities of innervation
and circulation are important predisposing causes of certain eye diseases,
infectious and otherwise.
The vagus nerve carries inhibitory impulses to the cardiac ganglia. It carries sensory fibers which may affect the heart’s action, and also the tension of the blood vessels over the body, especially of the splanchnic region. The white rami of the middle upper thoracic region carry impulses, which increase the speed and the force of the heart’s beat, to the superior and the middle cervical sympathetic ganglia. The gray fibers from these ganglia join the vagi and are carried with them to the heart, where they are distributed to the muscle fibers. The same pathway which carries the augmentor and accelerator impulses to the heart carries also, probably, impulses which influence the size of the cardiac blood vessels. This matter is not proved. Also, viscero-sensory nerves are carried from the heart, upward with the vagi, and downward with the cardiac accelerators, to the heart center in the middle upper thoracic segments. Both the vagus center and the middle upper thoracic centers are controlled by a general heart center in the medulla, which may or may not be identical with the vagus center. The heart may be diseased by the following conditions: Abnormal pressure in the blood vessels; Poisonous substances in the blood stream and starvation through poor blood; Local infection; Abnormal positions of the first to the fifth thoracic vertebrae and ribs, and muscular tension in the cervical region, of such a position as to bring the pulsating carotid more closely pressed against the vagus fibers; Abnormal
sensory impulses carried to the centers from other parts of the body, usually
viscera innervated by the vagus.
The pressure of the blood in the vessels depends chiefly upon two factors: the efficiency of the heart’s beat, and the amount of the peripheral resistance. The amount of blood in the vessels becomes a matter of importance in varying the blood pressure only under abnormal conditions. Other factors are to be considered under certain conditions, but are of no interest in this connection. The efficiency of the heart’s beat depends upon two factors: the rate and the force of the contractions. Both of these factors are subject to nervous control, and are discussed in connection with the heart centers. The peripheral resistance depends chiefly upon the contraction of the muscles of the arterioles. The larger vessels, the veins and the lymphatic vessels, are all supplied with nerves, but the arterioles are best supplied with circular muscles, and it is in the field of the arterioles that the most efficient modifications are made in controlling the pressure of the blood through variations in peripheral resistance. The vaso-motor centers in the medulla have been discussed elsewhere. It may be repeated that the vaso-motor centers act in accordance with the algebraic sum of the nerve impulses reaching them, and upon the physiological condition of the nerve cells of the center. This center controls the general circulation of the body. It acts in part by sending impulses to the various lower centers in the medulla, pons, midbrain and spinal cord. These centers also have a certain amount of autonomic power. The general blood pressure may be lowered greatly by loss of tone of the splanchnic centers, for example. The pressure may be greatly increased by an increased activity of the centers controlling any marked area of the circulation, as in the splanchnics or the lungs. Sensory nerves from all parts of the body act upon the subsidiary centers as well as upon the chief vaso-motor centers. The changes in blood pressure which are initiated by the activity of the lower centers may be sufficient to cause disturbance in the general pressure, and the chief vaso-motor centers in the medulla may act to cause a return to the normal conditions. Constantly, there is in action a balancing of the impulses from the different parts of the nervous system in the vaso-motor centers. Thus the circulation is maintained, with the pressure in the vessels remaining fairly constant, and with those variations in the pressure of the blood and in the caliber of the vessels which are needed by the various organs at different times. The practical aspect of this question is of great importance. In the first place, the etiological value of the bony lesion in causing disturbances in the pressure of the blood must be recognized. Slight malpositions of vertebrae and ribs, or other forms of peripheral irritation anywhere, may be a source of streams of abnormal sensory impulses which may affect the chief vaso-motor centers, or may affect the subsidiary centers in the cord or medulla. No part of the body can be well nourished if the blood pressure remains too low. Though the patient may be overfat, he is really poorly nourished, since he is unable to use the fuel with which he is provided. The ordinary neurasthenic shows the effect of the low blood pressure in his tendency to fatigue, loss of mental energy and mucscular weakness, both real and fancied. The bony lesion may act also as a source of excessive vaso-motor impulses. In such cases the pressure may be kept at too high a point, and certain organs of the body, notably the kidneys, liver and heart, are rendered more liable to disease. This condition is not often found existing for any length of time, since the nervous regulation of the heart’s action soon brings the pressure to its proper level. The high blood pressure found in arterio-sclerosis becomes a source of danger. Besides the injury to the vessels and the kidneys, liver and heart, the liability to hemorrhage, particularly of the cerebral vessels, must be remembered. In patients suffering from arterio-sclerosis, even of mild type, the existence of a bony lesion may add to the pressure, and thus to the handicap of the heart, in a manner which is destructive. In the correction of lesions in any part of the body, or in outlining any sort of treatment, or in advising exercises or changes in the habits of living, this question of blood pressure must be considered if the best good is to be accomplished. If the blood pressure is too low, then the movements used in the correction of the bony lesions should be so planned as to send increased sensory impulses into the centers. The movements, which are short and quick, with the effect of stimulation to sensory nerve endings in the joint surfaces, are of most value. Such movements add to the stimulation of the centers innervated from the same segments as the articular surfaces affected by the movements; the blood pressure is correspondingly raised, and the general condition of the patient is improved, not only by the actual corrective measures, but also by the corrective methods. On
the other hand, if the blood pressure is too high, especially if the patient
suffers from any arterio-sclerosis, the use of the same methods may be
responsible for increased injury. The corrective movements indicated
in these cases should be given slowly and gently. No increased sensory
stimulation should be sent into the centers from the articular surfaces.
The lesions may be corrected, but this must be done without increasing
the stimulation of the centers. Directions as to hygienic measures
must be given with due regard to the pressure changes liable to occur.
With care, the blood pressure may be kept within reasonable limits even
while considerable corrective work is being done. Careful watching
of the pulse may prevent undue rise of blood pressure in the aged and arterio-sclerotic.
The same considerations are important in outlining the treatment and the
hygienic advice for patients with cardiac lesions, aneurysms, and all conditions
in which a rise of blood pressure is harmful or dangerous.
The character of the blood depends upon many factors, all of which are more or less under the control of the nervous system. The blood elements are derived from the food. The digestion and absorption of food is controlled by the nervous system. The oxygenation of the blood occurs in the lungs. The circulation through the lungs and the respiratory movements are controlled by the nervous system. Under abnormal circumstances the oxygen supply to the tissues may be deficient because of too low hemoglobin percentage in the blood. The increased respiratory movements initiated by the respiratory center increase the rib movements, increase the circulation of the blood through the red bone marrow, and thus tend to bring about the condition needful for the formation of better erythrocytes. The elimination of the waste products of metabolism occurs in several organs of the body, notably in the liver and the kidneys. The circulation through the kidneys and the liver is controlled through the nervous system, and the rapidity of elimination thus controlled. In the liver and other organs secretory nerves are recognized. The purification of the blood is thus largely a matter of nerve control. A more direct control of the hematopoietic organs is recognized. The nerves to the red bone marrow are of two kinds, at least. One class of these nerves includes the vaso-motors. These terminate upon the walls of the blood vessels of the red bone marrow. Other nerves terminate by fine fibrillae which branch freely among the marrow cells. The function of these is not known. There
is not any exact center for the control of the blood character, but the
centers controlling the circulation through the bones containing red marrow
and the movements of the ribs are efficient in modifying the formation
of the blood cells according to the demands of the body and its changing
environmental conditions. Lesions affecting these centers may affect
the character of the blood.
The nervous control of the somatic muscles is rather complex. First must be recognized the volitional control of specific movements. The centers situated in the precentral convolution of the cortex give this volitional control. The large pyramidal cells of this center send axons downward by way of the internal capsule and the pyramidal tracts. In passing through the medulla fibers are given to the cranial somatic motor centers. Part of the pyramidal fibers continue through the medulla and into the spinal cord of the same side. These terminate at different levels of the cord, almost altogether in the cervical and upper thoracic region, by crossing to the opposite side and forming synapses with the cells of the middle part of the gray crescent. These cells in turn send axons to the anterior horn cells, and the axons of the anterior horn cells pass as the motor nerves to the muscles innervated by these segments—the muscles of the neck, shoulder, arm, hand, and the upper thoracic muscles. A larger part of the pyramidal fibers decussate in the lower part of the medulla. These pass as the crossed pyramidal tracts through the cord, and terminate in the same manner as described above, except that these fibers carry impulses destined for the muscles of the lower part of the body, pelvis, thighs, legs and feet. Injury of the centers of the cortex concerned in the control of any muscle group results in the paralysis of that group, so far as volitional movements are concerned. In certain instances the muscles are not paralyzed for instinctive and emotional reactions, nor for the performance of the autonomic act, as respiration. The reflexes are increased and the emotional and instinctive acts may become excessively abrupt, by the loss of the inhibitions normally sent from the cortex to the lower centers. The coordinate action of the muscle groups is secured by the activity of the cerebellum, and perhaps of the olivary body and other centers in the medulla and pons. The cerebellum is able to act in this way through its incoming impulses; those from the cortical centers enable volitional acts to be performed in a coordinate manner; impulses from the vestibular nuclei, the viscera and skin, and from the muscles themselves, enable the movements concerned in the maintenance of the erect position to be coordinated, and the frequent repetition of the impulses passing to and from the cerebrum during repeated actions enables the cerebellar cells to lower their liminal value sufficiently to assume the control of the act repeated, as in the formation of a habit. The skeletal muscles concerned in the performance of the autonomic acts, as in respiration and the like, are controlled in part by the centers of the medulla and in the spinal cord. The tone of the skeletal muscles is secured in part through the stream of efferent impulses carried over the tracts already described. In addition to these impulses, a constant stream of sensory impulses from joint surfaces, viscera, skin and muscles alternately contracting and relaxing during movements of the body, act upon the cells of the anterior horn and stimulate them to constant, slight, normal activity, so that the muscles innervated therefrom are kept in that condition of slight contraction called muscular tone. Structural conditions which lessen this stream of sensory impulses to the centers for the skeletal muscles lessen their tone, cause muscular weakness, and lessen their nutrition. The control of the muscles concerned in the performance of certain particular acts, such as language, etc., is secured through the activity of certain centers. The movements concerned in the production of language are coordinated in the third frontal convolution. The movements concerned in writing are coordinated in the second frontal convolution of the cerebral cortex. The movements of the eye muscles, both intrinsic and extrinsic, are coordinated in the anterior quadrigeminates. The movements of the ear muscles, both extrinsic and intrinsic, are coordinated in the posterior quadrigeminates. Injury of these centers is followed by a loss of coordination in the performance of that particular act, but by no paralysis in volitional movements, nor in other coordinations of the same muscles. The woman who lost the power of writing through a lesion of the second frontal convolution could knit with no difficulty. The descending impulses are by way of the pyramidal cells and tracts. The control of the movements concerned in the instinctive and emotional acts is secured in part by the centers and pathways already described, since these reactions are, in mankind, subject to volitional initiation and control. Centers in the basal ganglia and the midbrain are concerned in the more direct coordination of these impulses. These centers include the red nucleus and substantia nigra, probably the nuclei of the reticular formation, the sub- and hypothalamic nuclei, and others. The impulses are carried by way of the rubro-spinal tract to the spinal centers, and by the anterior horn cells to the muscles concerned. The lesions of the cortex permit the uncontrolled activity of the emotional centers. Lesions of the centers themselves are not easily recognized, since loss of the emotional reactions is distinguished with difficulty from the loss of sensory coordinations and of mentality. The circulation through the muscles is governed by the vaso-motor impulses. These are derived from the lateral horn cells of the cord from about the second thoracic to about the second lumbar segments. The white rami, axons of the lateral horn cells, form synapses with the cells of sympathetic ganglia, and the gray fibers from these cells (fibers of Remak) enter the cerebro-spinal nerves as their “sympathetic roots,” and are carried with them to their destination in the walls of the blood vessels. The centers for the control of the circulation of the muscles act according to the algebraic sum of the impulses from the higher centers, from the sensory fibers of the particular segment and its neighbors, and from the muscles themselves. The
nutrition of the muscles seems to be controlled also by the cells of the
anterior horns of the cord. Atrophy of these cells in any segments
of the cord, however, produced, is associated with atrophy of the muscles
innervated from those segments. These cells at, in part at least,
through receiving and transmitting the impulses received from the cortex,
the basal ganglia, the pons, medulla, midbrain and neighboring centers,
and from adjacent spinal centers.
The weight of the body depends upon two factors, the income and the outgo. The income includes the foods taken into the body, digested and absorbed, the water taken into the blood stream, and the oxygen breathed in by way of the lungs. The outgo includes the wastes of metabolism eliminated—the carbon dioxid breathed out, the water and other substances excreted by he kidneys, the bile and intestinal secretions which help form the feces, the sweat, and certain other secretions of minor importance in this connection. The influence of the nervous system upon digestion, absorption and elimination have been discussed in connection with the centers controlling these functions. The food intake is largely a matter of appetite and of convenience, both of which are under the influence of the nervous system to a certain degree. The circulating blood carries the products of the digestion and absorption. The outgo depends upon a number of factors. The elimination of carbon dioxid is the most variable factor in the elimination. The excretion of carbon dioxid by the lungs depends upon the amount of oxidation occurring in the body. The oxidation processes most subject to variation are those occurring in the skeletal muscles. During muscular exercise there is an increased amount of oxidation, and for this reason exercise is a method of eliminating superfluous fat. A comparatively great amount of oxidation occurs in the muscles in the maintenance of muscular tone. This is secured through the reflex action of the many sensory impulses, either directly, in the spinal reflexes, or indirectly, through the activities of the higher centers. The tonic condition of the skeletal muscles depends upon the efferent impulses, and these depend in part upon the sensory stimulation, and in part upon the physiological condition of the efferent neurons. Thus it becomes evident that either a lowered liminal value of the efferent neurons or an increase in their stimuli may be responsible for an increase of muscular tone, and thus an increase in this factor of the outgo. This increase of muscular tone is one factor concerned in the emaciation often found in people of the type commonly called “nervous”—that is to say, people whose nervous systems include neurons which are characterized by an abnormally low liminal value. Another loss of weight is found in the character of the movements performed by a certain class of people. A lack of coordination sometimes, a lack of proper balance between the nature of movement required and the character of the movement performed is responsible for much waste of muscular effort and for a correspondingly increased oxidation, increased elimination of carbon dioxid and loss of weight. Certain peripheral irritations may be sources of abnormally increased sensory impulses. Tension upon joint surfaces, the result of slight malpositions of bones, especially vertebrae and ribs, scar tissue in certain parts of the body, eye strain, adenoids, and other irritations of sensory nerves, may initiate such increase of muscular tone as to cause loss of weight. The nervous irritations mentioned are efficient factors in other disturbances also. The outgo may be decreased abnormally and obesity result. This condition may be due to a number of etiological factors discussed in the textbooks devoted to the study of disease. Among these may be mentioned deficient heart action, anemia, constitutional tendency, habits of overeating and underexercising, etc. These factors are partly controlled by the nervous system. The place of the blood-pressure changes in causing obesity is not well recognized. The oxidation processes depend upon the pressure of blood flowing through the muscles. With too low a blood pressure, or in the presence of arterio-sclerosis and a deficient blood supply to the capillaries of the muscles, the metabolism of the muscle cells is rendered abnormal. The oxidation processes are more or less incomplete, the amount of energy expended is less than normal, the accumulated fat fails of oxidation, and the patient, even under what seems to him violent exercise, loses nothing in weight. With normal blood, well oxygenated, flowing at a normal pressure through muscles whose tone is kept at their normal level by the impulses from normal nerve centers, it is probable that neither obesity nor emaciation could occur. |