Studies in the Osteopathic
The Physiology of Consciousness:
Louisa Burns, M.S., D.O., D.Sc.O.
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.
Isolation of the Cortex
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.
Common Sensory Tracts
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,
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
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
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
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
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
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
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
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
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.
Short Association Tracts
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
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.
Long Association Tracts
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
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
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.
Superior Longitudinal Fasciculus
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.
The Perpendicular Fasciculus (Wernicke)
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
Direct Motor Paths
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
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.
Many collaterals enter the median nucleus of the
thalamus, and a few perhaps enter the striatum. Quite large numbers of
collaterals enter the red nucleus, substantia nigra, and the sub-thalamic
region. These collaterals are finer than the axons from which they arise.
They terminate within the centers named by forming synapses with the cells
of those centers.
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
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
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.
Indirect Motor Paths
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
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
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.
Other Descending Fibers
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
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