Studies in the Osteopathic Sciences
The Nerve Centers: Volume 2
Louisa Burns, M.S., D.O., D.Sc.O.
            The motor neurons of the first order are those whose axons are distributed to the active structures of the body.  The term is not usually applied to the sympathetic neurons, though these cells are as truly motor neurons of the first order as are the cerebrospinal neurons.  The lack of exact information in regard to the relationships within the sympathetic ganglia renders the use of these exact terms inadvisable at the present time.  So the term “motor neuron of the first order” is usually applied only to the cerebrospinal neurons whose axons are distributed to the skeletal muscles.  Motor neurons of the second order are those neurons whose axons form synapses with the motor neurons of the first order, and which transmit the impulses from the higher centers.  The sensory neuron of the first order might send its axon or collateral to the motor cell; but since it does not transmit impulses from the higher centers, it could not properly be called “motor” neuron at all.  Thus it appears that the distinction between the sensory and the motor neurons is not always easy to make.  Both the sensory neurons of the first order and the motor neurons of the first order are distinguishable by their peculiarities of structure and of relationships, but the neurons of higher orders are not easily classified in all cases.  The motor neurons of the first order are controlled by the impulses reaching them from other parts of the nervous system.  Except for the direct effect of the constituents of the blood and lymph upon the neurons, the ultimate control of the motor neurons is from the streams of sensory impulses.
The Somatic Motor Neurons
            The somatic motor neurons of the first order occupy a column placed in the anterior horn of the spinal cord and extending into the medulla, pons and midbrain.  The part of this column which lies within the cord is fairly regular in outline and in the number and position and appearance of the cells.  In the medulla the column is crowded toward the median line by the spreading of the posterior fissure as it widens out into the fourth ventricle and the consequent displacing of the lateral and posterior funiculi.  The interposition of the masses of gray matter of the medullary  nuclei, the decussation of the pyramids and of the fillet, and the passing of the tracts associated with the cerebellar peduncles, all help to distort the column of the somatic motor neurons in the medulla and pons so greatly that only the study of the phylogenetic and ontogenetic development of the neuron groups makes their homology with the spinal somatic motor neurons recognizable.

            The motor neurons of the first order are of large size and of rugged outline.  They are very closely crowded during embryonic life.  During later development they are crowded apart by the increasing development and growth of their own dendrites and by the richly interlacing pericellular baskets which grow around them.  Their axons are distributed to the skeletal muscles.  Those of the spinal cord pass outward by way of the anterior roots.  The axons, in the adult neuron, become medullated within a few microns of the exit from the cell body, and this medullary sheath is retained throughout the whole length of the axon, except where it is interrupted by the nodes of Ranvier, and within a few microns of the termination of the fiber in the motor nerve ending upon the muscle fiber.  The axons of the cranial motor neurons of the first order are arranged in a similar manner, but display a few variations, due to the requirements of the peculiarities of the cranial structures and their development.

            The pericellular network is very close and complex.  It is composed in part of the dendrites and fibrillae of the motor neurons themselves, and in part of the branching axons and collaterals from other neurons.  Sometimes a collateral from the axon of the motor cell itself enters into this basket.  It does not seem that there is any protoplasmic continuity of the fibrils, though this matter is at present a matter of doubt.  It is certain that the network is extremely dense, and that the transmission of the impulses carried by the axons and collaterals have every opportunity to affect the dendrites and fibrillae of the motor neurons.

            The motor neurons of the first order receive impulses from the following sources:

            I. Axons and collaterals from the sensory neurons of the first order form synapses with the motor neurons of the first order.  By this means the simplest possible reflex actions are made possible.

            II. Axons and collaterals from the sensory neurons of the second order, notably those of the posterior horns of the spinal cord and of homologous cell groups in the higher centers, form synapses with the motor neurons of the first order.  By this means reflex movements requiring the coordinated action of simple muscle groups are secured.  (Fig. 44.)

            III. Axons and collaterals from adjacent segments of the cord, and from adjacent centers in the higher motor cell groups, affect motor neurons of the first order.  By this means the more complex reflex actions are controlled.

            IV. Descending fibers carry impulses from the cerebellum.  These may be axons directly from the cerebellar cells, or they may be axons from the cells of the central cerebellar nuclei, the olive, and perhaps of other similar cell groups very closely connected with the cerebellar centers.  By this means the most complex of reflex actions, such as those concerned in walking and standing, may be secured.

            V. Descending fibers from the red nucleus and certain other cell groups included as basal ganglia make up the rubrospinal tract.  The impulses carried by these fibers stimulate the motor neurons in such a way as to cause the instinctive and emotional reactions.

            VI. Descending fibers from the vestibular nuclei, passing in the vestibulo-spinal tract, carry to the motor neurons certain impulses originating in the semi-circular canals.  These impulses, of much less importance in mammals than in fishes and amphibia, are probably concerned in the maintenance of the symmetrical and the erect position.

            VII. Descending fibers from the quadrigeminate bodies are carried by the tecto-spinal tract (anterior longitudinal bundle).  These, like the vestibulo-spinal fibers, are representative of an ancient arrangement for the coordination of the impulses in the complex actions necessary to the maintenance of life.  Impulses chiefly from the retina and cochlea initiate descending impulses over this tract to the motor neurons of the first order.  It is probable that in this way the tone of the skeletal muscles is partly affected, and that a certain amount of complex coordination is secured, especially in reactions following visual stimuli.

            VIII. The pyramidal tracts carry impulses from the central or kinesthetic area of the brain (Rolandic area).  These fibers form synapses directly with the nerve cells of posterior or base of the anterior horn, and the axons of these in turn enter into the pericellular basket of the motor neurons of the first order.  The impulses carried by this tract are concerned in the control of the movements called volitional, and which are also usually well recognized in consciousness.

            IX. Descending fibers from certain autonomic centers in the medulla, midbrain and the pons carry impulses concerned in the performance of the autonomic functions to those skeletal muscles needful for those duties.  This is exemplified in the action of the respiratory center in the lower part of the calamus scriptorius.  Impulses from the respiratory centers are carried downward in the cord, probably through the fasciculus prorius, to the motor cells controlling the different respiratory muscles.  The vomiting center, the deglutition center, and other of similar action, all act through descending impulses to the skeletal muscles concerned.

            By means of these varying methods of control of the motor neurons of the first order, with their varying degrees of structural complexity and the related variations in the complexity and numbers of the sources of sensory impulses from which the different stimulations are derived, the motor impulses are made of such quality that comparatively simple muscular mechanisms are enabled to serve remarkably complex purposes.  The simple reflexes serve the purposes of simple needs.  They are not especially coordinated, serving the immediate needs of the organism quickly.  The more complex reflexes serve the needs of the body for the complex actions, expressive of the needs of the many individuals of the race, alike for all individuals, the result of racial experiences, of many survivals and of infinite deaths.

            The more delicately coordinated reactions, especially those depending upon the cerebellum and its related ganglia for their control, are of even greater complexity, and are the results of the experience of the individual, chiefly and perhaps exclusively.  These reactions are initiated and modified and controlled, largely, through educational methods.  The volitional impulses, again, while probably controlled by simpler structures than those coordinated by the cerebellum and the related centers, are of a more complex nature in that they are the result of present sensory impulses, modified by the results of the history of each individual, plus the past of his neighbors with whose experiences the individual is familiar, plus the results of the stimulation of the association cells, whereby the action and its results are foreseen, even though it may be, as a whole, totally apart from the experience either of the individual or his race.  This complexity of coordination is possible only because the cells of the central convolutions, as well as of other parts of the cerebral cortex, are affected by inhibiting impulses.

Control of the Viscero-motor Neurons

            The visceral muscles are innervated through the sympathetic nervous system, as it is illogically termed.  This part of the nervous system includes a number of ganglia placed in various parts of the body cavities, and the fibers which relate them to the spinal cord, pons, medulla and midbrain.

            The nerve cells in the sympathetic ganglia are controlled by the impulses from the cells in the lateral  horns of the cord (the lateral group of the anterior horns, according to certain writers) and in homologous centers in the medulla, pons and midbrain.  These autonomic centers are themselves controlled by the impulses from higher centers and from the sensory neurons.  The sympathetic ganglia do not, under any but experimental conditions, act independently.

            The nerve fibers which transmit impulses from the autonomic centers in the central system are of finer calaiber than are the fibers from the somato-motor neurons.  The autonomic fibers make up most of the bands called the white rami communicant es, which leave the spinal cord in the dorsal region—that is, from the first or second thoracic segment to the second or third, or sometimes the fourth lumbar segments.  These white rami enter the sympathetic ganglion nearest their origin, usually, but rarely terminate until they have passed through one or more ganglia.  Then they break up into fibers which form a part of the pericellular net around the bodies of the nerve cells of the sympathetic ganglia.  Each axon of the white rami may send collaterals to several different sympathetic cells in a single ganglion, and it seems probable that collaterals from the white rami fibers may pass from one ganglion to another, thus bringing the cells of two or more ganglia under the control of a single white rami fiber.  On the other hand, each sympathetic cell may receive fibers from two or more white rami fibers.  Thus the activity of a single sympathetic cell may be modified by the impulses from several cells within the central nervous system, and the impulses form a single cell within the central nervous system may modify the activity of many sympathetic cells.  The complexity of these structural relationships accounts for the well-known complexity of the functional relationship between the sympathetic and the central cells.

            The vagus, the third cranial, the seventh and perhaps the ninth cranial nerves also send fibers, comparable in function, to the sympathetic ganglia.  In the pelvis, the nervus erigens sends fibers to the hypogastric ganglion.

            The autonomic cells in the lateral horns of the cord and the homologous centers in the medulla, pons and midbrain are somewhat smaller than are the cells of the somato-motor centers.  They are not quite so rugged in outline; the dendrites are shorter and less profusely branching.  They are surrounded by a network which is similar to that already described as surrounding the somatic motor cell bodies.  This network includes the fibrillae derived from the spongioplasm of the cell body, and also the axons and collaterals from the following cells in other parts of the nervous system:

            I. Collaterals from the axons of the entering posterior roots of the cord bring to the lateral horn cells impulses from the sensory neurons of the first order.  (Fig. 45.)  These include (a) viscero-sensory neurons, by means of which the visceral conditions initiate or modify the autonomic impulses to the sympathetic ganglia and thus to the muscles, glands and blood vessels of the viscera, (b) somatic sensory neurons, by means of which impulses from skin, muscles and joint surfaces may modify visceral action.

            II. Axons and collaterals from the cells of the posterior horns of the cord probably have functions similar to those just mentioned.

            III. Fibers from the cells of the opposite side of the cord bring the two halves of the body into functional relationship.

            IV. Terminals and collaterals from the descending rubro-spinal tract bring impulses from the red nucleus, the substantia nigra, and probably others of the basal ganglia, to the autonomic cells.  Thus the emotional reactions include visceral as well as somatic manifestations.

            V. Descending impulses are carried from the various centers in the medulla, midbrain and pons to the viscero-motor neurons in the cord.  The vaso-motor center, the heart centers, and other viscero-motor centers act in this manner.

            Fig. 40.  Cilio –spinal center. – Gray fiber; Sympathetic ganglion; Sympathetic cell; Anterior root; Rubro-spinal; White ramus; Lateral horn; Sensory fiber; Sensory cell; Posterior root; Dividing fiber.

            According to their termination, the viscero-motor neurons have the following functions:

            I. They contract the walls of the blood vessels, especially of the arterioles, thus decreasing the blood supply of that certain area and raising the general blood pressure in corresponding degree.

            II. They cause contractions in several manners of the walls of the alimentary canal, the heart, the urinary and the gall bladders, the uterus, and other organs and ducts of the body.  In this way the contents of these hollow viscera are variously propelled.

            III. They contract the pupilo-dilator, the pupilo-constrictor, the ciliary muscles, and the other non-striated muscle fibers of the orbit.

            IV. They contract the pilo-motor muscles, by means of which the hair, feathers, quills, and other varieties of the exo-skeleton are made erect during cold, fear, anger, etc.  In this way the loss of heat from the body is lessened, the danger of wounds in battle is lessened, and the animal is caused to assume a more ferocious appearance.

            V. They increase the secretions of glands in all parts of the body.

            VI. Certain of these fibers seem to have the power of inhibiting the action of those just mentioned.  The manner in which the inhibiting fibers act is one of the greatest puzzles of physiology.

            It seems that the inhibitory function is exercised only by neurons of the central nervous system upon other neurons, either of the central system or of the sympathetic ganglia.  The existence of inhibitor neurons of the first order is not demonstrated, and their existence is extremely improbable.

            The stimulation of the sympathetic nerves to the salivary glands increases the secretion of these glands, but the secretion is very thick and rich in organic matters.  The blood vessels are greatly constricted.  The stimulation of the cerebrospinal nerves to the same gland, as the chorda tympani, on the other hand, also increases the secretion, but in this case the fluid formed is very thin and watery, containing a certain quantity of inorganic salts, but very little organic matter.  The blood vessels are tremendously dilated.  Just what the real relation is between these nerves and their action is not yet known.  The circulatory changes must modify the character of the secretion also.  If all nerve impulses are identical in quality, as seems indicated by many of the phenomena of nerve activity, then there must be some structural difference in the relations of the two classes of nerve with the glands they supply.  On the other hand, if nerve impulses are not identical in quality, then we have before us the more complex, but not more explicable, problem of differentiating between the almost infinite varieties of classes of impulses needed for the determination of the almost infinite variety of physiological activities controlled by the nervous system.

            The viscero-motor neurons are not directly influenced by the volitional impulses from the somesthetic area.  But they may be influenced indirectly in either of two ways.

            If one remembers distinctly the events which are associated with emotional reactions in his own past, or if he imagine distinctly, in such a way as to present vividly before himself a series of incidents which bring to him any emotional state, the muscles concerned in such emotional state become contracted involuntarily, and the viscera whose activity is usually associated with such emotions become active.  The reactions thus produced are not ordinarily so strenuous as those produced by the actual presence of the emotion-producing circumstances, but at times they seem even to be increased in memory or imagination beyond that characteristic of the actual occurrence.  This is noted with disastrous circumstances in the effects of certain mental shocks.  In these cases the memory of the fright is often more unendurable than the occurrence itself seemed to be.  The phenomena of hysteria and of certain insanities illustrate this reaction.  Probably no rational use can be made of this relationship in therapy.

            The lateral and anterior horns are so closely associated in the gray matter that the stimulation of one group of cells is practically certain to affect the action of other groups of the same segments.  Thus the stimulation of the skeletal muscles of any segment, by means of volitional impulses, affords a certain amount of normal stimulation to the visceral muscles and glands also.  In those cases of cardiac lesions in which hypertrophy is desired, it is very good to cause the gentle use of the arm muscles and the intercostals.  In cases of visceroptosis, dilated stomach, intestinal atony, etc., much good can be accomplished by the patient himself if he will use conscientiously those exercises which increase the tone of the skeletal muscles innervated from the same spinal segment.

            In any forms of exercise, if the elements of enjoyment and desire can be added to the volitional impulses, if the patient can be made to enjoy the exercises, or to feel some emulation, then his progress in strength is more rapid.  For this reason useful exercises are best, other things being equal.  In all these cases the additional stimulation due to the emotional state is associated with a stream of impulses from the red nucleus and related basal ganglia.

The Motor Cranial Nerves

            It is not possible to divide the cranial nerves into exactly two classes, somatic and visceral, because in the case of these nerves the original distribution and function has been so greatly modified through both ontogenetic and phylogenetic development.  It becomes necessary, then, to consider their relations separately, and to view them in the light of their present functions and relations, noting their ontogenetic and phylogenetic relationships only with such care as will serve to explain in part their irregularities.

            The hypoglossus or twelfth cranial nerve is, in the adult, purely motor.  In the embryo it has one or two sensory ganglia, with corresponding embryonic sensory roots.  These become lost in development.  The hypoglossus contains fibers corresponding to about five nerve segments.  It arises from a genetic nucleus in the floor of the fourth ventricle, in the trigonum hypoglossi.  Its nucleus is in direct line with the anterior horns of the cord, and it is a somatic motor nerve in development as in present function.  The fibers pass through and between the inferior and the accessory olives and emerge from the anterior sulcus of the medulla.  It is distributed to the muscles of the tongue and the depressors of the hyoid bone.

            The hypoglossal nucleus receives association fibers from the other motor cranial nerve nuclei, notably the fifth and seventh, and from the sensory cranial nerve nuclei, notably the fifth, seventh, and eighth.

            Descending fibers from the pyramidal cells of the lower part of the precentral convolutions of the cerebral cortex form synapses with the cells of this nucleus, and by this pathway the volitional movements of the tongue and probably the movements of the tongue in speech are effected.

            Impulses from the deglutition center in the medulla affect this nucleus.

            Impulses from the red nucleus and other basal ganglia reach the hypoglossal nucleus, and it is in part because of this connection that in emotional disturbances the tongue becomes stiff—in other words, the emotional effects upon the tongue are, directly, inhibiting.  It is scarcely needful to state that this inhibition is frequently less potent than the stimulation afforded by the descending impulses from the volitional centers in times of emotional stress of certain types.

The Accessory Nerve

            The accessory, or spinal accessory, nerve is to be considered in two parts.  The cerebral root originates in the nucleus ambiguous.  It is morphologically and physiologically a part of the vagus, and the fibers derived from this nucleus join the vagus after their exit from the cranium.  The fibers of the spinal root originate in a column of cells at the lateral portion of the upper five segments of the cord.  These fibers pass upward in the canal, join the cerebral root, and leave the cranium by the jugular foramen.  It is distributed to the trapezius and the sterno-mastoid muscles. One or two rudimentary sensory ganglia have been found in embryos upon the spinal accessory nerve.

            The origin of this nerve from a nucleus in the column of the viscero-motor nerves appears at first sight an anomaly.  In its phylogenetic development the peculiarity disappears.  The trapezius and sterno-mastoid muscles are derived from the old branchial musculature, and are, therefore, phylogenetically, visceral muscles.  Their innervation from the column in line with the other visceral motor nuclei is thus appropriate, though the muscles are at this time skeletal, striated, and as thoroughly under the control of the volitional impulses as any other voluntary muscles.

            The spinal nucleus of the spinal accessory receives impulses from the sources already mentioned as influencing the activities of the spinal motor neurons.

The Vagus Nerve

            The motor fibers of the vagus arise chiefly from the nucleus ambiguous, but some fibers arise also from the neighboring gray masses in the medulla.  This nucleus belongs to the column of the viscero-motor neurons.  The cells are, like those of the spinal lateral horn, rather small, multipolar cells, with rather fine axons.  The vagus is, phylogenetically and physiologically, a viscero-motor nerve.  Its motor fibers are homologous with the white rami fibers.  They terminate by forming synapses with sympathetic cells.

            The motor nucleus of the vagus receives impulses from the following sources:

            I. The sensory fibers of the vagus send collaterals and probably terminals to the nucleus ambiguous.

            II. Axons and collaterals from the sensory nuclei of the fifth, seventh and eighth cranial nerves, the nucleus gracilis and nucleus cuneatus, form synapses with the cells of the nucleus ambiguous.

            III. The various centers of the medulla, the cardiac, the vaso-motor, the respiratory, etc., either are identical with the nucleus ambiguous and the neighboring gray matter from which the vagus motor fibers arise, or they are very intimately related to these by association neurons and by collaterals from the afferent and efferent axons.

            IV. The rubro-spinal and tecto-spinal tracts, the descending root of the fifth, the vestibular nuclei, the olives, and the pontine nuclei, send axons and collaterals to the nucleus ambiguous.  By this means the various activities of the vagus are modified by sensory impulses from practically every part of the body.

            Pyramidal fibers do not seem to affect the action of the vagus, and are not described as entering the nucleus ambiguous.

The Glosso-Pharyngeal Nerve

            The motor fibers of the glosso-pharyngeal nerve arise chiefly from the nucleus ambiguous, and also from the nucleus of the alae cinerae and certain small cell groups in the immediate neighborhood.  This is phylogenetically a viscero-motor nerve, and its nucleus is a part of the viscero-motor column.  Its function is essentially visceral, since it is concerned with deglutition.  The constrictors of the pharynx are remnants of the branchial musculature.  The nucleus ambiguous receives associational fibers from the other cranial nerve nuclei, as has been given in the case of the vagus.  The reflexes with which the glosso-pharyngeal nerve are concerned are chiefly those of deglutition.

            The glosso-pharyngeal motor neurons receive fibers from the pyramidal cells of the inferior part of the precentral convolutions.  By means of this relation it becomes possible for the act of swallowing to be voluntarily performed or inhibited, to a certain extent.

            Fibers carrying inhibitory impulses seem to reach the glosso-pharyngeal nuclei from certain autonomic centers in the medulla, since the act of swallowing is urgently inhibited during coughing, mastication, etc.  The descending impulses from the red nucleus and related centers appear to be chiefly inhibitory, since emotional disturbances inhibit swallowing.  The dryness of the mouth associated with certain emotions, however, initiates the swallowing reflex in many emotional conditions.

            The fibers emerge from the upper end of the posterior sulcus of the medulla, join the sensory roots, and pass through the jugular foramen to reach their area of distribution, the pharyngeal constrictors and the stylo-pharyngeus muscle.

The Facial Nerve

            The motor fibers of the facial nerve are the axons of the nerve cells of a single nucleus in the pons, under the superior fovea.  This nucleus is a part of the continuation of the lateral horn of the cord, and the nerve is phylogenetically a viscero-motor nerve.  Originally its motor fibers innervated the gill muscles.  It is specifically the nerve of the hyoid arch, and only secondarily is it a nerve of expression.  Its nucleus does not give fibers to the sixth, as is sometimes stated, but some fibers of the sixth run in the same path for a certain distance in the neighborhood of the genu of the facial.

            The fibers leave the groove between the pons and medulla and pass with the sensory root of the facial to the muscles of expression—that is, to the muscles of the face, but not those of mastication.

            The motor nucleus of the seventh has certain peculiarities which affect its liability to disease.  In the first place, its almost  constant use renders its constituent neurons irritable beyond the common habit of neurons.  The liminal values thus being lower than is usual, excessive stimulation, the presence of the fatigue products in the blood stream, reflex irritations originating almost anywhere in the body, and, indeed, the abnormal conditions associated with very many diseases, affect the neurons of the seventh with especial severity.  For this reason the expression of the face is of considerable value in diagnosis; the facial muscles are especially liable to the various tics and spasms, and the habitual uses of certain muscle groups gives rise to the permanent and characteristic position of the facial tissues which results in the sum of what is called “expression” or character in appearance.  The nucleus of the facial nerve receives association fibers from many sources.  Its most conspicuous control is that by the red nucleus and related basal ganglia.  It is by this relationship that the various emotional states so greatly affect the facial expressions.

            The pyramidal fibers pass to the nucleus directly, though the volitional control of the facial is less absolute than is the volitional control of certain other nerves.  The facial muscles may be brought under almost absolute control through constant education.  This control is manifest more as a repression of emotional expression than as increased or modified expressions.  This is shown most clearly in the case of those who wish to imitate an expression, as in acting.  It is commonly recognized that the volitional imitation of an expression is practically impossible, and that the only way to imitate the expression of an emotion is to imitate the sensation—that is, to actually feel the emotion whose expression is desired.  In this way the red nucleus and associated ganglia are brought into control in such a way as to initiate the very stimulation of the structures needed as to bring about the real expression of the emotion sought.  This condition must be remembered when one is dealing with those patients in whom the existence of habitual expressions of ill feelings are manifestly a cause of depression and ill health.

            Fig. 41.  Control of the brachial muscles.  Direct pyramidal; Anterior root; Anterior horn; Rubro-spinal; Mixed nerve; Sensory; Posterior root; Gra;y fiber; Sympathetic; Cell; White fiber.

            The motor facial nucleus receives fibers from the sensory nuclei of the facial, the trigeminus, the auditory and the vagus nerves.

            Axons and collaterals from the fillet carry impulses from the nucleus gracilis and nucleus cuneatus.  Axons and collaterals from the posterior longitudinal fasciculus carry impulses from the midbrain and probably from the pontine nuclei.  Through these very complex relations the facial nerve is constantly under varying degrees of stimulation and inhibition.

Motor Root of the Trigeminal Nerve
            The nucleus of the motor part of the fifth nerve lies in the upper part of the pons and under the floor of the cerebral aqueduct (aqueduct of Sylvius).  Its fibers leave the anterior face of the pons, join the larger sensory root of the fifth, and pass with its mandibular division to the muscles of mastication.

            This nucleus is also of the viscero-motor series, and its function is now of this order, though the muscles which it innervates are skeletal, striated, and largely under volutional control.

            The nucleus of the trigeminus receives fibers from the sensory nuclei of the fifth, and it is through this relationship that the masticatory reflexes are intermediated.

            The nucleus of the trigeminal receives fibers of association from the other sensory nuclei of cranial nerves, from the nucleus gracilis and nucleus cuneatus, and from the pyramidal tracts.  The red nucleus and other basal ganglia send fibers to this nucleus also.  Though this control is not so conspicuous as is the similar control of the facial, yet the forcible tension of the temporal and masseter muscles in certain emotional conditions is indicative of the intensity of the stimulation sent from the basal ganglia centers to the motor nucleus of the fifth.

            The constant use of these neurons, as in the case of the facial, renders them susceptible to the ill effects of abnormal reflexes, as in case of trismus, and to the action of certain poisons, as in the case of tetanus and eclampsia.

The Ocular Motor Nerves

            The nuclei of the third, fourth and sixth cranial nerves occupy a column extending from the floor of the aqueduct under the superior colliculus to the abducent nucleus in the pons.  This column is in the line of the anterior horns of the cord and the hypoglossal nucleus.  It is composed of the somatic motor neurons, and it is chiefly somatic motor both in its phylogenetic development and its present function.  The different nuclei have  certain peculiarities in common.  Each of these nuclei exchanges association fiber with every other; each receives fibers from the pyramidal tracts; each receives many fibers from the superior colliculus and the cerebellum; each receives a few fibers from the red nucleus.  Each receives also a few fibers from the sensory cranial nerve nuclei and from the nucleus gracilis and the nucleus cuneatus.  The different nuclei have certain individual peculiarities.

            The nucleus of the abducens lies within the bend of the genu of the facial, and a group of cells near the facial nucleus is included as part of the abducens nucleus.  It was originally supposed that these fibers, really from the accessory nucleus of the abducens nucleus, were from the facial nucleus.  The very close morphological relationship renders the existence of close associational neurons very probable.

            The trochlear nerve  lies beneath the inferior colliculus.  Its fibers decussate, almost or quite completely, so that the fibers from each nucleus innervate the trochlear muscles of the opposite side.  No explanation has been offered for this phenomenon.

            The oculo-motor nerve includes several groups of nerve cells, each of which performs its own function.  Besides the somatic motor nuclei, of which a variable number have been recognized, there is a viscero-motor nucleus which requires some attention.  The viscero-motor nucleus of the third nerve includes a number of small nerve cells which are homologous with the lateral columns of the cord and the cranial viscero-motor nerve nuclei.  Its fibers leave the interpeduncular fossa of the midbrain with the somatic motor fibers of the third, and pass with these to the ciliary ganglion.  Here they terminate by forming synapses with the sympathetic cells of this ganglion, and it is the non-medullated fibers of these, passing in the short ciliary nerves, which innervate the ciliary muscle and the constrictor of the pupil.

            The viscero-motor nucleus, like the somatic-motor nucleus, receives fibers from the superior colliculus, the other sensory cranial nerve nuclei, the red nucleus, and perhaps the other nuclei of the ocular muscles.  It does not receive fibers from the pyramidal tract.  This nucleus is of considerable interest in being involved in certain paralyses.  It is rather infrequent to find any other viscero-motor nucleus the seat of paralysis.  This may be due to the fact that a paralysis of other viscero-motor nuclei would either result in a death so speedy as to preclude the possibility of diagnosis, or the symptoms would be too vague for the correct diagnosis to be made, or it may be that  the position of the third nerve viscero-motor nucleus renders it more susceptible to the action of disease conditions than are other nuclei of the visceral column.