|
The cerebral cortex is associated with the other parts of the nervous system by means of systems of fiber tracts which transmit impulses from the lower centers to the cortex, and from the cortex to the lower centers. Other fiber tracts associate the different parts of the cortex with one another. The cerebral cortex is not directly related to the external world. Sensory impulses reaching the cortex have been transmitted by means of at least three different neurons, and it seems probable that most or all of the sensory impulses have been transmitted by a great many more than three neurons. It is not possible to say how much the stimulation received by the sensory neuron of the first order may be changed in thus being handed from one neuron to another on its way to the cerebral cortex. The phenomena of color vision, the temperature sense, pain, etc., seem to indicate that there is not necessarily any qualitative relationship between the characteristics of the external world and the sensations these characteristics initiate in consciousness. On the other hand, the cortical neurons do not enter into any direct relationship with the motor organs of the body. All impulses concerned in controlling the movements of the body are transmitted by at least three different neurons on their way to the muscles, and it is probable that a great many more than three neurons are concerned in the transmission of any motor impulses from the cortex to the muscle concerned. The isolation of the cortex is complete. Nothing but nerve impulses can bring it into relationship either with the environmental variations or with the instruments of its own activity. In the cortex are found the layers of neurons thus
imprisoned, probably more than a thousand millions in number, each of which
lives its own life, performs its own duties, acts in accordance with the
algebraic sum of all the impulses reaching it, and as its activity is modified
by the variations in the pressure and quality of the blood which supplies
its nutrient lymph. Under abnormal conditions the effects of bacterial
invasion and various toxic or pressure conditions also affect the neuronic
activity. These millions of cortical neurons receive impulses by way of
the afferent tracts directly, or from one another by way of the cortical
association tracts. The association tracts transmit impulses which must
ultimately have been carried to some cortical area by way of the afferent
tracts. The cortical areas send impulses by way of the efferent tracts
directly, or to other cortical areas by way of the association tracts.
The impulses transmitted from one cortical area to another must have their
ultimate function in the modification of the impulses transmitted by way
of the efferent tracts. The ultimate origin of the impulses concerned in
cortical activity must be from the sensory neurons of the first order,
and the ultimate destination of the impulses initiated by cortical activity
must be the motor neurons of the first order. There are thus to be considered
with the cortical relations the tracts which are ascending, tracts which
are descending, and association tracts.
The neuron systems of the ascending pathways carry all the impulses reaching the cortex. Upon the impulses carried in this manner rests all the complicated mechanism we are pleased to call the intellectual faculties. These few tracts and nuclei, with their few fibers of infinitesimal delicacy, their few cells of apparently simple structure, transmit all of the impulses which initiate the associational and motor impulses of mankind. All the tremendous superstructure of science, and art, and philosophy, all the notable achievements of mankind through the ages, all the ingenuity which has preserved and translated the history of the past, even through the geological ages, which has weighed and analyzed the stars, outlined the paths of the comets, and subjugated the most powerful forces of nature, has for its foundation only the impulses carried by this apparently inefficient complex of thin white strands of fibers, with a few groups of almost invisible ganglia and nuclei scattered among them. By means of impulses carried by these paths the history
of an individual may be modified, and his activities may, in turn, affect
the history of the race for all time. By means of the impulses carried
by these tracts to his own brain, the physician is able to determine the
needs of his patients, and by means of the motor tracts he may affect the
cortical activity of his patient in such a manner as to help him to increased
strength and usefulness in the world.
The cortical fillet carries the impulses from the common sensory nerves of the entire body to the cortex. It apparently emerges from the thalamus and passes to the cortex, chiefly of the central region. Lesions of the cortical fillet cause anesthesia of the opposite side of the body if complete, or the corresponding part of the body if partial. Hemiataxia occurs at the same time. The parietal stalk appears the more important. Flechsig’s study of this tract has not been modified by later tests. The parietal stalk issues from the optic thalamus upon its lateral aspect. It terminates chiefly by forming synapses with the cells of the cortex in the postcentral gyrus, but some of the fibers pass to the precentral gyrus, and perhaps in part the anterior frontal gyrus and the gyrus cinguli. Flechsig give the different bundles, arranged according to their time of medullation, as follows: 1. The alpha bundle runs from the globus pallidus to the upper third of the central gyri, chiefly the anterior gyrus. Its function is not known. 2. The beta bundle is a large one. It originates in the posterior part of the lateral nucleus of the optic thalamus and terminates in the upper third of the postcentral gyrus and the adjacent cortex. 3. The gamma bundle is also a large one. It seems to originate in the globus pallidus, but its actual origin is not known. It passes to the upper part of the postcentral gyrus. 4.The delta bundle runs from the lateral nucleus of the thalamus to the middle third of the postcentral gyrus. 5. The eta bundle runs from the anterior part of the lateral nucleus of the thalamus to the lower third of the postcentral gyrus. 6. The zeta bundle runs from the superior part of the lateral nucleus of the thalamus and to the foot of the superior frontal gyrus and the neighboring part of the cingulus. It seems, from the comparative times of the medullation
of these fibers, that the transmission of the impulses from the lower part
of the body precedes the transmission of the impulses from the upper part.
The impulses to the lateral nucleus of the thalamus,
and possibly to the globus pallidus, are carried by different pathways.
The most direct sensory path is that which includes the sensory neurons
of the first order, the fasciculus gracilis or fasciculus cuneatus, the
nucleus gracilis or nucleus cuneatus, and the medial fillet to the lateral
nucleus of the thalamus.
The indirect paths include the sensory neurons of
the first order, the dorsal nucleus, the spino-thalamic tract to the lateral
nucleus of the thalamus, and the even more complex paths through the cerebellum.
These tracts have not been exactly determined, but they are known to include
the cerebellar hemispheres, the nucleus dentatus and the red nucleus. Probably
the inferior olive and the arcuate nucleus should be included. The brachium
conjunctivum carries the impulses from the nucleus dentatus to the red
nucleus, and the fibers from the red nucleus join the fillet on their way
to the lateral nucleus of the thalamus. The impulses concerned in the sense
of touch and the sense of muscular effort are carried by the more direct
paths, as well as by the indirect paths. The sense of temperature and the
sense of pain are probably carried only by the indirect paths. A certain
amount of time is required for the transmission of a nerve impulse from
one neuron to another, and this is one reason that a thing may be felt
as touching the skin first, then it may be felt as being warm or cold at
a time appreciably later.
The temporo-thalamic, or auditory, or acustic radiation, as it is variously called, originates in the medial geniculate body, passes posteriorly to the lenticular nucleus, and terminates in the cortex of the superior temporal gyrus. This tract carries auditory impulses. They are transmitted from one neuron to another from the ganglion spirale, the nuclei of insertion of the acustic nerve, the nuclei of the trapezoid body, the superior olive, the lateral fillet and the nucleus of the lateral fillet, the posterior quadrigeminates, the medial geniculate body, and thus to the cortex. Lesions affecting the acustic radiation cause a partial
deafness, which may be more pronounced in the opposite ear, but which involves
both ears to a certain extent. This is due to the fact that the acustic
pathway is partially crossed.
The occipital, or optic, or visual radiation, as it is variously called, is composed of fibers which are the axons of cells in the lateral geniculate body and the pulvinar of the thalamus. They pass in the posterior limb of the internal capsule to the lingual and cuneate gyri. This tract carries the impulses which originate in the homolateral halves of both retinae and the macula lutea of both retinae. Injury of this tract is followed by loss of vision in the homolateral halves of both retinae, with a corresponding loss of the contralateral field of vision. Injury of either tract interferes very little with vision in the direct line, since the macula lutes has the double representation in the cortex. Visual impulses are certainly transmitted by five, and probably by many more, neurons before reaching the cortex. This tract includes some descending fibers, probably
the axons of the large pyramidal and stelate cells of the occipital cortex.
The pathways of the impulses concerned in the sensation of taste have not been determined. Even the sensory neurons of the first order concerned in taste are not certainly demonstrated. The clinical evidence is so contradictory that it appears probable that there are considerable individual variations in the manner in which the impulses of taste are carried. Probably the impulses follow the following paths: Sensory neurons of the first order of the fifth, ninth and seventh cranial nerves, the nuclei of insertion of these nerves in the medulla and the solitary nucleus, the lateral fillet, the lateral nucleus of the thalamus, and the thalamic radiations to the cortex of the inferior aspect of the third temporal convolution, the gyrus cinguli, and the fusiform gyrus. Lesions of the cortical gyri mentioned have been found in cases of epilepsy with gustatory aurae, and in cases of parageusia. The determination of the pathway of the impulses concerned in taste is made the more difficult because the taste sensations depend to so great an extent upon the activity of other nerves. The sensibility to taste sensations is modified by the temperature sense, and variations in the vascular conditions of the tongue affect the sensations of taste both quantitatively and, to a certain extent, qualitatively. The fact that olfactory impulses may be interpreted in consciousness as taste sensations affects the validity of the clinic histories as evidence in some cases. Lesions affecting the vaso-motor nerves to the tongue,
or of the sensory impulses upon which the vaso-motor reflexes to the tongue
depend, are thus indirectly causes of parageusia and perhaps of ageusia.
The term “ascending” seems subject to criticism when
applied to the olfactory tracts, which pass almost horizontally into the
brain. The olfactory pathway begins with the olfactory neurons of the first
order, which are placed in the olfactory region of the nasal mucous membrane.
The olfactory nerves are the axons of these cells, and they pass upward
through the cribriform plate and into the olfactory lobes, where they enter
into synaptic relationships with the mitral cells. The axons of the mitral
cells make up the olfactory tracts, which pass backward to the cerebral
hemispheres. They pass by complex paths to the gray matter of the anterior
perforated space, septum pellucidum, subcallosal gyrus, gyrus cinguli,
fasciola cinerea, fascia dentate, subiculum, hippocampal convolution, emygdala,
and uncinate gyrus. Consciousness of olfactory images probably occurs as
a result of the activity of the cells of the cortex of the gyrus cinguli
and hippocampal gyrus with the neighboring cortical areas, while the other
neuron groups mentioned are chiefly concerned with relating the somatic
and visceral activities of the body in answer to the olfactory impulses.
The impulses carried by the neuron systems of the sensory conduction paths reach the cortex usually about midway of its thickness. In the auditory area the radiating fibers (radiations of Meynert) reach the external layer of the cortex. Elsewhere they terminate in the line of Bailarger, which occupies the area of external large pyramids. These radiating fibers form synapses with the various types of cells of the layers of the parts of the cortex traversed. These include the Golgi Type II cells, the inverted pyramids and small multipolar cells, besides the small, large and medium pyramids. (Figs. 5, 6.) The inverted pyramids send their axons into the external layer of the cortex, the stratum zonale. Here they enter into synaptic relations with the dendrites of the cells of the pyramids of all layers. The cells of the external layer of the cortex, the stratum zonale, are small, stellate or fusiform in outline, and of a structure which has yet to be more fully studied. Cells of this layer are described as having two or three axons which branch freely among the fiber elements of the stratum zonale. If these cells have more than one axon, it is evident that they are capable of transmitting impulses in more than one direction. In the stratum zonale are found also Golgi Type II cells, whose axons branch very freely among the fiber elements of their immediate neighborhood. Amacrine cells are also described for the stratum zonale. These cells all unite in having as their function the coordination of the impulses reaching the cortex. By means of these complex relationships the impulses are subjected to various modifying influences, which have the effect of making the reaction following any given stimulation correspondingly efficient. Fig. 5. Diagram of the
layers of the typical cerebral cortex. The neuroglia appears at the external
surface. The first layer of the cortex contains the spindle and polymorphic
cells. (See Fig. 3.). Among these cells the dendrites
of the other cells, and the axons and collaterals of the inverted pyramids
of Martinetti branch freely. The layer of small pyramids lies net. The
dendrites of these reach the first layer; the axons exhaust themselves
branching among the deeper layers. The third layer is characterized by
the medium pyramids. The relations of these are as the small pyramids.
The fourth layer is characterized by the large pyramids. The axons of these
may enter the white matter and pass to other parts of the nervous system.
The fifth layer includes small pyramids and polymorphic cells. The sixth
layer contains large pyramidal cells, and the axons of these may enter
the white matter. The seventh layer contains spindle and polymorphic cells,
whose axons also may reach the white matter and pass to other parts of
the nervous system. Small pyramidala cells, multipolar cells, Golgi Type
II cells, and inverted pyramids may be found through all except the first
layer. The line of Bailarger coincides with the external layer of large
pyramids.
The second layer of the cortex, the layer of small
pyramids, sends dendrite into the stratum zonale, and thus these cells
are capable of being stimulated by the cells of that layer. The small pyramids
of the second layer are rather short and broad, with basal dendrites which
branch rather near their origin, and are not very long. The apical dendrites
also are short, since they need no great length to reach the stratum zonale.
The small pyramids send axons into the deeper layers, where they give off
collaterals, which form synapses with the cells of the lower levels. The
axons of these cells do not reach the underlying white matter. Among these
cells also lie Golgi Type II cells, whose activity probably coordinates
the activities of the small pyramids. Collaterals from the axons of the
inverted pyramids also branch among the small pyramids.
The third cortical layer is composed of pyramids
somewhat larger. It is called the layer of medium-sized pyramids. There
are small pyramids and Golgi Type II cells found among the mediuim-sized
pyramids also. The apical dendrites of this layer reach the stratum zonale
and branch therein. The basal dendrites branch freely among the fiber elements
of this layer, which includes t he axons and collaterals from the small
pyramids and the collaterals from the axons of the inverted pyramids. The
axons of the medium-sized pyramids pass into the deeper layers, but do
not seem to enter the white matter. They give off collaterals which branch
among the basal dendrites of the deeper layers of pyramids, and which may
also pass outward toward the stratum zonale. The medium-sized pyramids
thus may receive impulses from the cells of the stratum zonale, from the
small pyramids, from the inverted pyramids, and from the Golgi Type II
cells. They send impulses to the deeper pyramids by their axons, to the
Golgi Type II cells, and to the stratum zonale by the recurrent collaterals.
The fourth layer of the cortex is characterized by the large pyramids. This layer is coincident with the line of Bailarger in most cortical areas. The large pyramids send apical dendrites to the stratum zonale. Their basal dendrites are extremely long and branch very freely. The axons of these cells pass toward the deeper layers, and they enter the white matter. It has not yet been possible to separate them from the other descending fibers in studying the cortical relationships. In passing through the gray matter these axons give off collaterals which branch among the deeper layers. Some of these collaterals turn backward toward the cortex, and ultimately terminate in the stratum zonale. The external layer of large pyramids includes also some small pyramids, some medium pyramids, some Golgi Type II cells and the inverted pyramids of Martinotti. The radiations of Meynert, including the axons of cells in the lower centers and in other parts of the cortex, branch freely among the large pyramids. The line of Bailarger is made up of these branching axons, the basal dendrites of the large pyramids, the collaterals and axons of the small and medium pyramids as they form synapses with the large pyramids, and other transverse fibers which have not yet been traced to their origin. The external layer of large pyramids is thus capable
of receiving impulses from the cells of the stratum zonale, from the small
pyramids, from the medium pyramids, from the inverted pyramids, from the
Golgi Type II cells, and from the incoming axons of the radiations. They
send impulses by their axons to other parts of the nervous system by way
of the centrum ovale and the fiber tracts, to the inverted pyramids, to
the deeper layers of the cortex, and to the stratum zonale by their recurrent
collaterals.
The fifth layer includes stellate and polymorphous cells. These have many dendrites which branch freely in a very irregular and eccentric manner in the same layer. This layer varies greatly in different parts of the cortex. The axons of these cells pass horizontally in the same layer, giving off collaterals which form synapses with other cells of the same layer, and which may turn toward the stratum zonale. The line of Bailarger includes these axons. The cells of the polymorphic layer include GolgiType
II cells, pyramidal cells, and probably amacrine cells. They receive impulses
from the small, medium and large pyramidal cells, from the incoming fibers
of the radiations, and from the other cells of the same and adjacent layers.
They send impulses by their axons to the deeper layers and to the more
superficial layers.
The sixth layer includes the internal large pyramids. These are the largest cells of the typical cortex. In this layer are found, besides the typical large pyramids, polymorphic cells, small pyramids, medium pyramids, Golgi Type II cells and inverted pyramids. The apical dendrites of these large pyramids pass to the stratum zonale, where they branch very freely among the cells of that layer. The basal dendrites branch freely and are very long. The axons of these pyramids pass into the white matter, giving off collaterals within the gray matter. These collaterals form synapses with the cells of the seventh layer, and some of them turn toward the external layers. They may reach the stratum zonale, and they give off branches in passing to the other layers. The internal layer of large pyramids is capable of
receiving impulses from the cells of the stratum zonale, from the small
pyramids, the medium pyramids, the external large pyramids, the polymorphic
cells of the fifth layer, the cells of the seventh layer, the Golgi Type
II cells, and the incoming radiating fibers. They send impulses by their
axons to other parts of the nervous system, and by the recurrent collaterals
to other layers of the same cortical area.
The seventh layer of the cortex includes a very rich fiber plexus, which makes up part of the feltwork of Kaes. Within the external part of this feltwork lie the fusiform cells characteristic of this layer, together with polymorphic cells and inverted pyramids. The axons of the polymorphic cells and the fusiform cells may enter the white matter, but seem to be rather short. The axons of the inverted pyramids pass to the stratum zonale, giving off collaterals to the different layers in passing. The cells of this layer may receive impulses from the incoming radiating fibers, from the collaterals of the large pyramids of both layers, from the Golgi Type II cells, and from other cells of the layer. They send impulses to adjacent cortical areas and to the other layers of the same cortical layer. (Figs. 5, 6.) In different areas of the cortex certain variations
from the typical structure are found. These are mentioned in connection
with the physiology of the special areas.
The different areas within the cortex are related
to one another in function by means of fiber tracts. Adjacent gyri are
connected by fibers which are short and are not to be classified as tracts,
except in a general way. They are called fibrae propriae. Rather longer
bundles of these fibers are called fasciculi propriae. Longer and better
marked bundles make up the short association tracts, while still larger
and longer masses of fibers are classed as long association tracts. It
is not possible to draw any exact line between these various classes of
association tracts. All are concerned in unifying the parts of the cortex,
and it is by means of these tracts that related activity of the different
neuron systems becomes possible. Because of the relative liminal values
of the cortical areas so associated in functions, varying cortical activities
are related in function in different individuals, and at different times
in the same individual.
The stratum calcarinum includes two groups of fibers, the longer of which lies rather more deeply placed than the shorter. The shorter fibers pass from the upper lip of the calcarine fissure to the lower lip. The longer fibers are immediately beneath these; they pass from the medial portion of the cuneus to the medial and inferior portion of the lingual gyrus. The fasciculus occipitalis transverses cunei passes from the upper lip of the calcarine fissure first lateralward and then upward to enter the cortex of almost all parts of the occipital lobe. Fig. 6. Diagram illustrating the relations of the various elements of the cortex. The arrows show the direction of the nerve impulses. The fasciculus occipitalis transverses gyri lingualis is similar to that just mentioned. It passes from the lower lip of the calcarine fissure to the occipital lobe through almost its entire extent. The stratum proprium cunei passes from the upper lip of the calcarine fissure vertically upward, to be distributed to the cortex near the junction of the convex and the medial surfaces of the occipital cortex, and the adjoining area of the parietal lobe. All of these shorter tracts are probably concerned in transmitting visual impulses from the primary visual area to the visual overflow, and from one part of the overflow to other regions, both of the visual overflow and of the intermediate areas. Fasciculi propriae are found in all of the intermediate
areas. These tracts unite in function adjacent cortical areas. They vary
in different brains, and are more pronounced in the brains of man than
in animals, and in older people than in children. They receive their medullary
sheaths later than the longer tracts or the projection fibers. They are
thus probably concerned in the more complex coordinations.
The long association tracts of the cortex include
two groups, those which relate the hemispheres, and those which relate
the different parts of the same hemisphere.
This body is one of the most conspicuous factors in the human brain. It is less marked in the other mammals, is found only as a few fibers passing with the anterior commissure in monotremes, and is represented not at all in non-mammals. It is, in a way, a measure of cerebral development, but its exact place in the association of the nervous impulses is not known. The fibers of the corpus callosum are the axons of
almost or quite all of the cortical areas. These fibers are distributed
to the contra-lateral hemispheres; to the corresponding area, and also
to other areas of the cortex. It thus unites in function many parts of
each hemisphere with many parts of the other hemispheres. It includes,
besides the axons of the cells of association, collaterals from the descending
tracts, notably the pyramidal tracts. By means of this relationship the
two hemispheres act as a unit in function. Lesions of the corpus callosum
are followed by symptoms which vary greatly, and are not to be explained
at present.
This tract is phylogenetically very old. It is composed
of two parts, one of which transmits the olfactory fibers of each side
to the limbic lobe and the olfactory area of the other side; the other
part transmits fibers from each temporal and occipital lobe to the contralateral
temporal and occipital lobes. It is a part of the olfactory apparatus.
Lesions of this tract are not to be localized ante mortem.
This tract unites in function the hippocampus and
adjacent areas of each side with corresponding areas of the opposite side.
It also is concerned in the transmission of the impulses concerned in the
reactions initiated by olfactory impulses. Injuries to this tract are not
to be recognized ante mortem.
The cingulum, the fornix and the uncinate fasciculus
are all concerned in the transmission of olfactory impulses, or of the
impulses initiated by these. These tracts are probably not intimately related
to consciousness, yet they are often concerned in modifying the reactions
which occur, and thus indirectly they modify the conscious life of the
individual.
This bundle arises from the cortex of the frontal
lobe and passes to the occipital lobe, chiefly, and also to the temporal
lobe at its posterior portion. The tract makes up the tapetum. No symptoms
referable to its injury are described in the authorities consulted in the
preparation of this volume, and the function of the tract is unknown.
This tract includes fibers passing from the temporal
lobe to the occipital, and from the occipital lobe to the temporal. It
seems to be especially concerned in the transmission of the impulses from
the visual speech center to the auditory speech center, and contrariwise.
It also transmits impulses concerned in the naming of things seen and in
the memories of things named from the visual overflow to the auditory overflow.
Lesion of this tract is followed by the loss of the power of naming things
seen, or reading aloud, or of remembering a thing named.
This tract extends from the superior lobule of the
occipital lobe to the inferior gyrus of the occipital lobe and the middle
and inferior temporal lobes, and to the fusiform gyrus. It includes some
fibers from the adjacent parietal lobe. Its function is unknown, except
that it must be concerned in the transmission of associational impulses.
This is included with the short tracts by some authors.
After all the passing to and fro of the nerve impulses, there results ultimately a series of efferent impulses which relate the actions of the individual to his environment. In the broadest sense, nerve impulses which are totally unrelated to motor reactions are valueless, and they may be harmful. The physiological relations of the cortical activities are thus, in a sense, essentially pragmatic. The ultimate end of all this tremendous structure
of neuronic activity is the initiation of motor reactions. This is secured,
finally, by means of the cortical efferent neurons. These pass to lower
centers, which, by various coordinations and interrelations, affect the
motor neurons of the first order, and the active structures of the body
stimulated.
Two classes of efferent neuron systems carry the impulses concerned in the transmission of these motor impulses. Of these, the direct pathway represents the more highly developed relationship. The indirect pathway represents the older phylogenetic plan, and is built upon the centers which were active and fairly efficient long before the cortical centers had attained any degree of activity worth mentioning. The direct pathway is composed, first, of the large pyramidal and probably the large polymorphic cells of the precentral gyrus. In this area the various muscle groups are represented within fairly well marked limits. The evidence in favor of this cortical representation of muscle groups is convincing. Extirpation of the areas in animals is followed by the paralysis of the muscles stimulated by that area. Stimulation of any given area is followed by the movements of the muscles represented by that area. Cases of paralysis are found to depend upon lesion of the area in which the lost muscular activities are represented. Experiments upon human beings whose brains are subject to surgical procedures verify the experimental stimulation of the animal brain. In amyotrophic lateral sclerosis the giant pyramidal
cells and large polymorphic cells of the motor area are found degenerated.
The fibers, axons of the giant pyramidal cells and
the large polymorphic cells, pass through the internal capsule and occupy
the central three-fifths of the basis pedunculi. They pass through the
anterior region of the pons and medulla into the spinal cord. In passing
through the basal part of the brain, they give off certain collaterals,
but very few of the fibers themselves are lost. Some few fibers enter the
median nucleus of the thalamus, and perhaps a few center the corpus striatum.
The fibers from the lower part of the precentral gyrus terminate in the nuclei of the cranial nerves of the opposite side from that of their origin. The fibers from the middle and upper part of the precentral gyrus pass onward into the spinal cord. The fibers which arise from the upper part of the precentral gyrus, and from the neighboring region on the median aspect of the cortex, decussate in the lower anterior part of the medulla. The decussation of the pyramids is seen from the anterior external aspect of the medulla. These fibers carry the impulses concerned in the movements of the lower part of the body and the legs and feet. The fibers from the middle part of the precentral gyrus remain upon the same side of the cord until they reach the segment of their termination. At that place the fibers decussate and enter the gray matter. These fibers, the axons of the large pyramidal cells of the cortex of the precentral gyrus, are among the longest nerve fibers of the body. They form synapses with the cells of the central part of the gray crescent of the cord. The short axons of these cells enter into the formation of the pericellular baskets of the large multipolar cells of the anterior horns, and these, in turn, send their axons to the skeletal muscles. The impulses sent by this direct pathway are concerned
in the volitional control of the skeletal muscles. The cortical area is
not so large nor so well developed in animals. It reaches its most complex
and efficient development among those races who have made the most complex
and most efficient reaction to the demands of life.
The indirect pathways are much more complex, and the exact relationships have not been determined with any degree of accuracy. Almost all of the primary sensory areas of the cortex send descending fibers to the lower centers associated with the organs concerned in the specific energy of that area. The descending impulses from the visual area are carried to the anterior quadrigemina and the lateral geniculate body; the descending impulses from the auditory area are carried to the posterior quadrigeminates and the lower auditory centers; the olfactory tracts carry axons passing in both directions, etc. Now, it seems probable that these impulses are concerned in the maintenance of the nutritive relationships of the sensory structures, though they may be concerned to a certain extent in the motor phenomena of attention. The intermediate and overflow areas send axons downward
through the internal capsule, chiefly to the lower centers, and from the
cells of these centers axons are carried to others, and so on through a
number of interposed centers, until finally the muscles, both striated
and non-striated, and the glands of the body may be affected. These are
called the indirect pathways of efferent impulses.
The fronto-pontal tract is that described in older books as the fronto-cerebellar tract. It arises in the cortex of the frontal lobes, anterior to the precentral sulcus—that is, anterior to the motor area. This part of the brain, in the left hemisphere, seems to be concerned with the coordination of those nerve impulses which relate the individual to his environment. The impulses arising in this region are carried to certain lower centers, and thus the bodily activities are governed in accordance with the results of the frontal coordinations. The fronto-pontal tract passes downward through the internal capsule, giving off fibers to the median nucleus of the optic thalamus and probably the globus pallildus, then it occupies the medial one-fifth of the basis pedunculi and passes onward to the nucleus pontis. It gives off either fibers or collaterals to the red nucleus and substantia nigra, and perhaps to the sub-thalamic nuclei. The tract terminates in the nucleus pontis. The basal ganglia mentioned, which are certainly of considerable importance in this connection, send fibers onward to still lower centers. The globus pallidus sends fibers by way of the olivary bundle to the inferior olivary body; from this body the olivo-spinal tract transmits the impulses to the spinal centers, and perhaps also to the cerebellum. The median nucleus of the thalamus sends fibers in the thalamo-spinal tract to the spinal centers and to the motor nuclei of the cranial nerves. The red nucleus and substantia nigra, and probably the sub-thalamic centers, send fibers to the motor nuclei of the cranial nerves and to the spinal centers by way of the rubro-spinal tract. Fibers from the red nucleus also may send fibers to the dentate nucleus of the cerebellum, though this relationship has been doubted. The thalamus and the corpus striatum of each side exchange fibers of association and send fibers to the contra-lateral bodies and to the other related centers. From the nucleus pontis and the olivary body fibers
pass to the contra-lateral cerebellar hemispheres. From the cortex of these
hemispheres the axons of the Purkinje cells carry the nerve impulses, either
directly downward to the cranial and spinal motor nuclei, or to the nucleus
dentatus or the olivary body, and the axons of the cells in these centers
carry the efferent impulses to the lower centers. Ultimately, the centers
controlling both the somatic motor and the visceral motor structures are
affected by the impulses from the frontal cortex and from the lower centers
controlled by that part of the cortex.
The temporo-pontal tract is that which has been described as the temporo-cerebellar tract. It arises in the cortex of the temporal lobe, probably in all three of its convolutions. The fibers, axons of the large pyramidal and polymorphic cells, pass by way of the internal capsule, the outer one-fifth of the basis pedunculi, to the nucleus pontis. It gives off fibers to the substantia nigra, and probably also to the thalamus, globus pallidus, red nucleus, and others of the centers around the base of the brain. The tract terminates in the nucleus pontis. The impulses carried by this tract are transmitted from the globus pallidus by way of the intermediate bundle to the substantia nigra, and probably neighboring centers, and to the inferior olive by way of the olivary bundle. The red nucleus, etc., send impulses partly by way of the rubro-spinal tract and partly by way of the brachium conjunctivum, the dentate nucleus and the cerebellar centers to the spinal and cranial centers. The nucleus pontis sends fibers to the contra-lateral cerebellar hemispheres, and these transmit the impulses to the cranial and spinal motor centers. The nature of the impulses carried by the temporo-pontal
tracts is very uncertain. It seems probable that a certain part of these
impulses is concerned in relating the bodily activities to the sensory
impulses from the auditory tracts. The stimulation of this cortex in the
brain of the cat or the dog causes movements of the ears, and sometimes
of the eyes; less often the head is moved.
Descending fibers, axons of the large pyramidal and stellate cells of the occipital lobes, are carried by way of the optic radiations to the lateral geniculate body, the anterior quadrigeminates and the pulvinar. The impulses carried by these fibers seem to be concerned in the control of the orbital tissues, and especially as these are concerned in vision. From all areas of the cortex descending fibers seem to pass to the centers of the thalamus and the striatum. The cortical activities are thus very intimately associated with the activities of these lower centers. The structural basis for the relationship of the cortical centers concerned with the reactions called intellectual are thus able to control and to be modified by the basal centers, whose activities are concerned in the emotional and instinctive reactions. |
