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In the gray matter of the medulla and pons lie a number of cell groups which may or may not be identical with the centers of the cranial nerves, but which certainly are concerned in the coordination of the nerve impulses which control the organs and systems of organs of certain functions and functional relationships. These cell groups are called centers, not because it is certainly proved that they have in all cases an actual, individual cell grouping, but because they have the functional value of specific cell groups. In other words, any one of these higher viscero-motor “centers” may be formed by a certain and exact group of nerve cells, whose location has not yet been demonstrated, or it may be merely a certain synaptic relationship between the centers of the different cranial nerves, or of the centers to which these are closely related. In the former case, the localization of these centers is merely a matter of further study; in the latter case, localization is impossible, since the same nerve cells, through variations in the physiological conditions of their associated neurons, might be concerned in coordinating different and even antagonistic functions. These matters are to be determined by further study. For
the present we only know that in the medulla and pons somewhere within
certain circumscribed limits lie cells or cell groups whose activity does
secure the coordinate physiological activity of organs or related groups
of organs in the performance of certain duties, and that this coordination
is effected by means of sensory impulses, associational impulses among
the centers, and the descending impulses from the mid-brain, basal ganglia
and cerebral cortex.
This center is concerned in the regulation of the circulation of the blood. It controls the changes in general blood pressure as well as the circulation through specific organs. This center lies near the nucleus of the facial nerve and near the superior olivary nucleus. It begins just caudad to the inferior colliculus and extends downward into the medulla, either as an individual center or through its related nerve cell groups. The center itself is supposed to be about one-tenth of an inch in length, but section of the medulla elsewhere may produce something of the effects of section of the area of the center itself. The center is bilateral, and its two halves are so thoroughly associated in function that under normal conditions the circulation of the blood in the two halves of the body is maintained in a symmetrical manner. The center is one of those whose action is constant—that is, it is in a condition of constant tone. This constant activity is due in part and perhaps altogether to the fact that it is subject to the constant stimulation of the nerve impulses reaching it. The vaso-constrictor center is known to vary in activity according to the stimuli reaching it from a number of different sources. It may be concluded, therefore, that the axons of the nerve cells of the different structures whose stimulation forms an efficient stimulus for the vaso-motor center reach that center in order to perform these duties. It is not known whether these impulses are carried to the vaso-motor center by the axons of the sensory neurons of the first, second or higher orders; it is only known that sensory impulses do affect the activity of this center. The vaso-constrictor center receives impulses which may either increase or decrease its activity. It is evident that in the absence of any tonic activity no conceivable nervous impulses could cause a decrease in its activity. The recognized sources of these impulses are as follows: I. Descending impulses from the basal ganglia affect the action of the vaso-constrictor center both in increasing and in decreasing its activity. In the emotional states, such as shame, pleasure and some forms of anger, in certain instinctive reactions, as sex excitement, and some other conditions, the splanchnic vessels are constricted and the skin arterioles are dilated, so that reddening of the face or of the whole body occurs. Under other emotional states, such as fear, some forms of anger, horror, and in the presence of great excitement, the peripheral arterioles are constricted and the splanchnic vessels dilated. Thus the pallor is caused which is found in these conditions. There is some reason to believe that some at least of these reactions are of functional value in preserving the lives of the individual or his kind; thus, the accumulation of the blood in the central organs may be valuable in preventing the hemorrhages which are likely to follow the attack whose imminence produced the fear. The further discussion of this matter belongs to the consideration of the basal ganglia and their relations. The effects of the emotional and instinctive reactions upon the vaso-constrictor center must be recognized. II. Impulses from the heart itself; may affect the action of the vaso-constrictor center by either increasing or decreasing its activity. The vagus carries a small bundle of fibers called the depressor nerve. Since this nerve is carried in a separate bundle in certain mammals, its individuality is unquestioned. Impulses carried by this nerve seem to be initiated by the pressure of the blood within the heart, or by the pressure of the contracting heart muscle upon its sensory nerve endings. This pressure, itself a source of embarrassment to the heart’s action, becomes the initial cause of the nerve impulses which lead to relief. For the impulses carried to the medulla by the depressor nerve act upon the cells of the vaso-constrictor center in such a way as to lessen the activity of that center, lower the blood pressure, and thus secure the relief of the heart. III. Sensory impulses from any of the somatic structures of the body may affect the action of the vaso-constrictor center. Stimulation of the central end of the divided great sciatica nerve causes rise of blood pressure. Stimulation of the central end of any large nerve trunk has the same effect, usually. Severe pain in any part of the body causes vaso-constriction, not only in the painful area, but in the general arterial system. IV. Association neurons connect this center with the heart, respiratory and other centers of related function. V. Sensory impulses from viscera affect the vaso-constrictor center either by increasing or decreasing its tonic activity. The stimulation of the sensory nerves of the abdomen in surgical operations, the stimulation of the sensory nerves of the urethra in passing a sound, or of the cervic uteri or of the anal tissues in dilatation, may cause so profound a lowering of the blood pressure as to result in shock or in syncope. Stimulation of these areas may cause first a sudden increase of blood pressure, following by the fall, or the lowering may not be preceded by any perceptible rise, or, in the case of less ferocious stimulation, the rise of blood pressure may not be followed by any noticeable fall. The activity of the vaso-constrictor center seems to be affected also by the presence of certain substances in the blood stream. These include the products of muscular fatigue, though not necessarily the excess of carbon dioxide or the lack of oxygen. The presence of small quantities of various acids in the blood causes a fall of blood pressure, evidently due to a partial paralysis of the vaso-constrictor center. Certain drugs seem to have a selective action upon this center also. The fact that the fatigue products lessen the activity of the center, as well as the presence of small amounts of acid, may account for the lowering of the blood pressure in neurasthenics, as well as in the normal degrees of fatigue. The action of the vaso-constrictor center is thus seen to depend upon a number of more or less antagonistic factors. The physiological conditions of its intrinsic neurons must affect the nature of the reaction, since the cells are probably not different from the other nerve cells in their physiological activities. The nature of the impulses sent out from the center at any given time must, then, depend upon the condition of the cells of the center at that time, the character of the blood flowing through it, and the algebraic sum of the stimulating and inhibiting impulses reaching it at that time. The impulses initiated by the activities of the vaso-constrictor center are carried as follows: Descending
impulses are carried, probably in the anterior longitudinal bundle or in
the anterior fasciculus proprius, to the levels of the spinal cord, including
the first or second thoracic to the second or third lumbar segments.
In these levels the descending fibers terminate by entering the gray matter
of the lateral horn, and forming synapses with the cells therein.
The axons of the cells of the lateral horns (lower vaso-constrictor centers)
leave the cord, mostly by way of the anterior roots, but partly by way
of the posterior roots, and enter into the formation of the white rami
communicantes. These fibers enter the sympathetic chain, pass through
one or more ganglia, and terminate by entering into the formation of the
sympathetic pericellular network. The gray axons of the sympathetic
cell pass without interruption to the walls of the blood vessels.
The chief innervation is to the walls of the arterioles, but the muscle
fibers of the larger arteries and the veins also receive nerve fibers.
It has been supposed that, since the existence of the vaso-constrictor center is so certainly demonstrated, a vaso-dilator center should be found in the same region. No evidence of a dilator center has been found, though dilator nerves are recognized. The physiology of dilator nerve impulses remains to be worked out by experimental methods. Dilator fibers have been shown to exist in the following regions: I. The facial nerve carries dilator fibers for the submaxillary and sublingual glands by way of the chorda tympani. II. The glosso-pharyngeal nerve carries dilator fibers for the tongue and pharynx, and by Jacobson’s nerve for the parotid gland. III. Dilator fibers are carried in the splanchnics. IV. Dilator fibers are carried in the sympathetic chain. It is not known whether these are transmitted through sympathetic neurons, or whether the fibers are simply carried with the sympathetic fibers. V.
The nervi erigentes carries dilator fibers to the erectile tissues of the
genitalia.
This center lies very close to the vagus center; it may be identical with it, or it may coincide with the nucleus of the tractus solitarius, or it may be composed of other cell groups in this neighborhood. It is known to be placed in the gray matter of the medulla near the tip of the calamus scriptorius, and to extend through an area of about a quarter of an inch. The center is bilateral, but the two halves are closely related by associational fibers. If medial section is made, the two halves act synchronously so long as the conditions remain normal, or approximately so; but when the movements of either side of the thorax are interfered with, the action of the two sides may become unsymmetrical. It is supposed that an inspiratory center exists, and an expiratory center, and these act independently, but harmoniously. The action of the respiratory center depends largely upon the character of the blood circulating through it. A lack of oxygen causes increased respiratory movements, but an even smaller increase in carbon dioxide causes greater increase in the movements. Acids or the fatigue products, or the products or bacterial action, may increase the activity of this center. Nerve impulses affecting the center are carried to it as follows: I. Descending impulses from the cerebral cortex bring the respiratory movements under a certain amount of volitional control. This control is bilateral. The respiratory muscles are not usually disturbed in paralysis. II. Descending impulses from the basal ganglia cause the respiratory movements to be affected by the emotional and instinctive activities. III. Probably impulses from the cerebellum are concerned in securing coordination of the respiratory muscles, in part, though the respiratory movements are rarely disturbed in cerebellar disease. IV. The vagus carries sensory fibers of two classes which act upon the respiratory center. One class of sensory impulses arises from the pressure conditions in the lungs at the end of inspiration; these impulses either inhibit the action of the inspiratory center or stimulate the expiratory center. Since the act of expiration seems almost or quite purely passive, it seems that the first must be the ordinary relationship. The other class of nerve fibers carries the impulses initiated by the pressure conditions at the end of expiration. These impulses stimulate the inspiratory center. V. Stimulation of the superior laryngeal nerves initiates expiratory movements only. The irritation of the laryngeal membrane under ordinary conditions causes a cough. This respiratory act is modified by the simultaneous occurrence of the reflex actions of the glottis and the respiratory movements of forced expiration. A separate center for coughing is recognized. VI. Stimulation of the glosso-pharyngeal nerve, either at its center, its root, or the area of its distribution, causes the inhibition of the respiratory movements. The value of this reaction is apparent in the swallowing reflex. VII. Stimulation of the area of distribution of the trigeminal nerve may be followed by the sneeze. The physiological action of the pathway of this reflex is not exactly known. VIII. Stimulation of the sensory endings in the skin stimulate inspiration. Excessive stimulation of the skin areas may cause painful forced contractions of the inspiratory muscles, with partial suspension of their respiratory movements. The occurrence of syncope in such cases permits the return of the normal respiratory movements. IX. Stimulation of the splanchnics or the abdominal vagus causes forced expiration. This may be so forcefully produced as to cause death. X. The respiratory center is closely related to other medullary and pontine centers by association neurons. For example, dilatation of the anus affects respiration forcibly. The impulses are carried by the fasciculus gracilis to the nucleus gracilis, and thus to the respiratory center. The impulses from the respiratory center are carried downward through the cord by the fasciculi propril or by the anterior longitudinal bundle to the centers of origin of the phrenic, intercostals and other respiratory muscles, and to the centers of origin of the facial, vagus and accessory nerves. The
activity of the respiratory center at any given time is determined by the
pressure of the blood flowing through it, by the quality of the blood,
and by the algebraic sum of the nerve impulses reaching it from the brain,
basal ganglia, associated medullary and pontine centers, and the sensory
nerves of the body.
Closely related to the respiratory center, though probably quite distinct from it, is a cough center. Direct irritation of the area just above that given as the respiratory center may produce the act of coughing. It receives sensory impulses chiefly from the laryngeal nerves and those of the bronchi, but any branches of the vagus may sometimes be associated with coughing. Irritation of the pleura itself rarely produces cough, unless the liminal value of the cough center is abnormally low. Rarely a persistent cough may be caused by irritation of the auricular branch of the vagus. Abnormally, the cough center may be brought into a condition of excessive irritability through its repeated action. As is the case with all nerve cells, the stimulation of the cells of the cough center lowers their liminal value. Thus, after an attack of any bronchial irritation associated with coughing, the lowering of the liminal value of the neurons concerned in the action may be so pronounced that even after the irritation has disappeared the cough may continue; that is, the neurons are so irritable that the stimuli which are normally brought to the medullary centers initiate the act of coughing. This cough is itself a source of further irritation to the bronchial passages, and there is some tendency for the habit to be perpetuated indefinitely. In persons who are in good health, energetic, with normal habits of living, the habit soon disappears. In
caring for people in whom there is reason to believe that the cough is
a symptom of the persistence of a habit it is necessary to raise the liminal
value of the neurons concerned. This must be planned with regard
to the condition of the patient. The person with courage and persistence
may simply be told to urgently endeavor to prevent the cough. The
descending inhibitory impulses from the cortical centers is sufficient
to decrease the reaction, give the neurons rest, and permit the normal
metabolism to be reestablished. In other cases other methods are
needed, but all must depend primarily upon securing the rest of the neurons
concerned in the coughing reflex, and thus the return of their physiological
activities to the normal condition.
The centers for the control of the rate of the heart’s beat are placed in the medulla, with certain subsidiary centers in the cord. The cardio-inhibitory center is either identical with or very closely associated with the glosso-pharyngeal vago-accessory center in the lower part of the lateral area of the medulla. The stimulation of this location causes a slowing of the pulse rate, but not a decrease of the amount of work done by the heart in any given time. The fibers from this center leave the cord by way of the accessory fibers, but they join the vagus almost immediately. They pass with the other vagus fibers to the cardiac plexus, where they terminate by forming synapses with the cells of the cardiac ganglia. The stimulation of the vagus nerve gives variable results, according to the location of the point of stimulation. Stimulation of the nerve trunk before the sympathetic fibers from the upper, middle or lower cervical sympathetic ganglion reach it causes inhibition of the heart. Stimulation of the trunk below the juncture of the sympathetic fibers with the vagus may give acceleration and augmentation of the heart’s beat. The evidence in favor of a cardio-accelerator center in the medulla is not so conclusive. Such a center must act through the intermediation of the accelerator-augmentor center in the upper thoracic cord. Certainly there is very initimate relationship between the inhibitor center and the accelerator, whether this is secured through the activity of a superior accelerator center or not. Whether the existence of the medullary accelerator center is granted or not, the cardiac centers in the medulla must be considered as exerting the superior control. The cardiac centers are affected by impulses from the following sources: I. Impulses from the heart itself, by way of the sensory fibers of the vagus, may lessen the frequency of the heart’s beat. II. Impulses from the basal ganglia may either increase or decrease the pulse rate and the force of the individual beats. This reaction probably depends in part upon simultaneous activity of the vaso-motor centers. III. Sensory impulses from the nucleus gracilis, nucleus cuneatus and the nuclei of termination of the cranial sensory nerves may affect the heart’s action. The impulses from the sensory fibers of the sacral and lower lumbar nerves and from the abdominal viscera seem to be especially efficient in arousing cardiac disturbances. IV. The impulses from other centers of related function affect the cardiac centers. Thus the changes in heart beat during swallowing, respiratory changes, vomiting, etc., are secured. The impulses from the cardiac centers are carried to the following destinations: I. From the medullary centers to the spinal cardio-accelerator center in the upper thoracic cord. II. From both centers to other medullary and pontine centers of related function. III. From the inhibitor center to the intrinsic cardiac ganglia. Three of these are recognized in the frog—von Bezold’s, Remak’s and Bidder’s. These seem to be represented in higher animals and mammals by a rather variable number of ganglia, placed, in general, at the roots of the great veins, in the auricular septum, and at the auriculo-ventricular junction. Smaller ganglia are scattered through the heart muscle. It is not known whether the inhibitor fibers terminate in these ganglia by forming synapses with the solitary cells of the cardiac muscle, or by inhibiting the activities of the accelerator fibers in their synaptic relations in the ganglia. The
cardiac nerves in general seem to act rather by regulating the rhythm of
the heart in accordance with the needs of the other parts of the body than
by maintaining any particular rhythm of the heart with reference to its
own activity. In other words, the heart muscle seems able to act
rhythmically for a long time in the presence of normal circulatory conditions
without the influence of the nervous system, but it is not able to act
in such a manner as to adapt the heart’s action to changing somatic conditions
without the nerve influence.
This center lies closely associated with the sensory nucleus of the vagus. It receives impulses from the following sources: I. From the gastric division of the sensory fibers of the vagus. II. From the sensory fibers of the vagus distributed to other viscera. III. From the nucleus gracilis and cuneatus, but mostly by way of the gracilis from the sensory impulses from the pelvic organs. Thus the vomiting associated with dilatation of the urethra or the cervix uteri, or with the presence of abnormal conditions in the pelvis generally. IV. From the nuclei of the vestibular nerves. V. From the cerebellum. Probably the viscero-sensory neurons of the indirect sensory pathway are thus transmitted to the vomiting center. VI. From the basal ganglia. The presence of objects of disgust, or even of disgusting thoughts, may cause stimulation of the vomiting center. VII. Rarely, impulses from the cortical centers seem to be efficient in stimulating the vomiting center. Usually the only volitional impulses causing vomiting act indirectly, by increasing the intra-abdominal pressure, or by compelling the thoughts to dwell upon the act, and thus lowering the liminal value of the center through the basal ganglia centers. The vomiting center may be acted upon directly by poisons in the blood. Impulses from the vomiting center are carried as follows: I. To the splanchnic centers, causing reversed peristalsis. This may be observed by stimulating these centers after section of the vagus. Stimulation of the splanchnic segments of the cord produce reversed peristalsis after section of both vagi, and sometimes after section of either vagus. II. To the centers for the phrenic, abdominal and intercostals nerves. III. To the cardio-accelerator and cardio-inhibitor centers, increasing the heart’s beat. IV. To the respiratory center, bringing the respiratory movements to the aid of the act of vomiting. V. To the salivary center, increasing its action. VI. To the deglutition center, inhibiting its action. VII. To the sweat center, increasing sweat. VIII. To the vaso-motor center, first raising, then lowering, blood pressure. The cells of the vomiting center may act abnormally under several conditions. The
center may be overstimulated by recurrent abnormal conditions, so that
even normal stimuli may suffice to initiate vomiting, and severer malnutrition
may occur. The cells may be affected abnormally by poisons in the
blood stream so that more or less constant activity results. This
stimulation may cause frequent vomiting, or it may simply produce enough
of the conditions associated with vomiting to prevent eating, or to cause
the sense of nausea.
These are bilateral. They act in unison, though it is possible to stimulate them experimentally so that one side of the body may be moist, while the other side remains dry. The sweat centers receive impulses as follows: I. Impulses from the heat centers cause the sweat to be increased or decreased according to the temperature requirements of the body. II. Impulses from the centers concerned in the emotional reactions may increase or decrease the sweat. This is evident in the presence of many emotional states, in which the sweat may fairly drip from the face. The so-called “cold sweat” of fright is due to the increase of sweat secretion, together with the vaso-constriction of the blood vessels of the skin. III. Impulses from the other medullary and pontine centers affect the sweat center. The sweating in vomiting is due to this relationship. IV. Impulses from the centers for the sensory nerves may increase or decrease the secretion of sweat. Irritation of the skin over any considerable area, without pain, may cause sweat. Severe pain may cause profuse sweating. Impulses from the orifices of the body may cause profuse sweating without necessarily being associated with much pain. Dilatation of the anus, or urethra, or the vagina, or the cervix uteri may thus arouse increased sweating. The impulses concerned in these reactions are carried by the fasciculus gracilis to the nucleus gracilis, and thence to the sweat centers. The sweat centers are also affected by the character and pressure of the blood flowing through them. Impulses
from the sweat centers are carried to subsidiary centers in the cord, in
the gray matter between the second thoracic and the second lumbar segments.
The axons of the lateral horn cells of these segments pass as white rami
communicantes into the sympathetic ganglia, where they enter into the formation
of the pericellular baskets of the sympathetic ganglia. The gray
fibers of the sympathetic cells pass chiefly with the cerebro-spinal sensory
nerves to be distributed to the sweat glands in the skin.
The evidence for the existence of a pilo-motor center is not conclusive. The facts supporting its existence indicate its place near the sweat center. It must be affected much as the sweat center is affected, and its impulses must pass over about the same path as that followed by the impulses concerned in the nervous control of sweat. The
muscles of the hairs of the body are contracted in cold and under the influence
of fear and certain other emotional states. The phenomenon of “goose
flesh” is due to this contraction of the hair muscles, which is a rather
amusing effort to increase the heat of the body by raising the hairs.
In the case of fright or anger the effort is to increase the apparent size
of the body by raising the hairs, in order that a more ferocious appearance
may affright the enemy, and also in order that the attack of the enemy
may not be permitted to cause such deep wounds because of the protection
of the erect exoskeleton.
This center is located near or with the motor center for the vagus nerve. I. It receives impulses from the centers of the trigeminal, the glosso-pharyngeal, and the superior laryngeal branch of the vagus. These initiate the normal act of deglutition. II. It receives impulses from the other medullary centers. Thus the act of forced inspiration inhibits deglutition. This center sends impulses to the motor centers of the trigeminal, facial, vagus and pypoglossal. The movements concerned in the complex act of deglutition are synchronized and coordinated by the activity of these centers. The deglutition center sends inhibiting impulses to the respiratory center, the heart center, and probably others of the medullary and pontine centers. Dilatation of the pupils follows repeated forced acts of deglutition. The
action of the deglutition center is rarely rendered abnormal except by
actual paralysis of the muscles involved or destruction of the nerve cells
themselves. Rarely, hysterical paralysis or overactivity of this
center may be found.
This center is well developed and functional at birth. It is of interest in the fact that its function seems to be lost, at least in some cases, after infancy. It is stimulated to action in part through sensory impulses, as in the case of the deglutition center, and in part by the physiological conditions of the center itself. It seems to act more in the quality of an automatic center than do most of the other nerve centers. The center which controls the secretion of saliva lies in the neighborhood of the terminal nuclei of the glosso-pharyngeal and the mandibular division of the trigeminal. Its exact location is not known. This center receives impulses from the following sources: I. Sensory impulses from the glosso-pharyngeal, the trigeminal and sensory part of the facial nerves cause the secretion of saliva which is purely reflex. This may be caused by stimulation of the nerve endings concerned in taste, or by the nerves of common sensations. II. Impulses from the basal ganglia may increase or decrease the secretion of saliva. The dryness of the mouth in cases of certain excitements, as in stage fright, or the frothing associated with violent emotional disturbances, is thus produced. III. Impulses from the cortical neurons indirectly affect the activity of the center. The reaction in this case is probably due to the effect upon the lower centers. The optic thalamus has been found concerned in the reaction in some cases, though other tests seem to deny the place of the thalamus in the secretion of saliva. The slight smell or thought of food associated with at least certain degree of desires. The thought or sight of substances known to be sour, as a lemon, or juicy, as an orange, may cause increased secretion through the fact that the thought of the taste of the acid or the feeling of the flowing juice stimulates the memories of past experiences, and thus the reaction occurs through the intermediation of the lower centers, as if the actual taste and feeling were present. This form of psychic secretion is not properly termed reflex. The value of the thorough chewing and tasting of food lies in the psychic stimulation of the salivary center. It is not possible to increase the secretion of the saliva directly by voluntary action, though the use of stimulating movements of the tongue and lips in the effort to initiate the reflex activity of the salivary center is well known. IV. The impulses from other centers in the medulla, pons and midbrain affect the activity of the salivary center. This is noted in several ways. Vomiting stimulates secretion of saliva, as does also the act of chewing. In sucking saliva is increased. The salivary center sends impulses to the following structures: I. Impulses pass to the upper thoracic segments of the cord, thence by way of the lateral horn cells, the white rami, the sympathetic chain, and the superior cervical ganglion to the blood vessels of the salivary glands, and to the gland cells themselves. II.Impulses pass by way of the facial nerve and the chorda tympani nerve through the submaxillary ganglion (sympathetic) and by way of the facial and the lesser superficial petrosal through the otic ganglion (sympathetic) to the gland cells and their blood vessels. The activity of the salivary center may be affected by substances circulating in the blood stream. This is not the normal condition, but may be found in certain intoxications. Certain drugs greatly increase the amount of the saliva. Abnormal positions of the mandible may affect this secretion, probably through the action of the abnormal sensory impulses carried by way of the fifth cranial nerve. The
“paralytic secretion” of bulbar paralysis is of interest in this connection.
A
center for the dilatation of the pupil lies in the upper part of the floor
of the fourth ventricle. This center is subsidiary to the centers
in the superior colliculi, and it relays the fiber paths from the nuclei
of the colliculi to the cilio-spinal center.
Other centers exist in the medulla and pons which have not been well studied. The injury of certain areas, for example, may cause the condition of diabetes mellitus; another injury may cause, or seem to cause, diabetes insipidus. The secretion of bile seems, in some cases, to be affected by injuries of the medulla, though whether it is the secretion itself or the contraction of the bile ducts, which thus seems to depend upon the activity of these more or less hypothetical centers, is uncertain. |