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The special senses of smell and taste are widely represented upon the cortex, but the limits of the areas are not known. The two senses seem to occupy practically identical areas, yet the difficulties in the way of exact localization are so great that this matter can not be regarded as settled by any means. The anterior part of the gyrus hippocampi and the cygus cinguli and most of the uncinate gyrus seem to be concerned in the senses of taste and smell. Other parts of the rhinencephalon appear to be more efficient in initiating reflex actions than in arousing sensations in consciousness. Lesions of the areas named are associated with paragensia and parosmia rather than with anosmia and ageusia, since the great extent of the olfactory cortex renders its complete injury almost impossible without death. In certain cases of epilepsy with aurae of smell or of taste lesions have been found in the region of the hippocampus, the uncinate gyrus and the gyrus cinguli. A few cases are recorded in which anosmia has been associated with cerebral lesions. Probably there are individual peculiarities in the cortical areas associated with the consciousness of smell. No records are found of total agensia associated
with cortical lesions alone.
The olfactory apparatus is unique in many respects. The olfactory neurons of the first order lie within the mucous membrane. No nerve cells are found upon the surface of the body in any vertebrate, except in the case of the olfactory cells. The fine fibrillae, which are the dendrites of these nerve cells, are directly affected by the odorous substances. No apparatus is found which has the evident function of modifying in any way the nature of the impulses initiated by odorous substances. Thus it seems possible that at least a certain amount of relationship may exist between the real nature of odors and the olfactory sensations. Yet the odors of substances of similar chemical structure do not necessarily resemble one another. The olfactory area in the nasal passages is small,
even in animals whose sense of smell is acute—the hyperosmatics. In man,
hyposmatic, the olfactory region occupies only a few square millimeters
that the impulses originating in such a small area, apparently without
any arrangement for increasing or modifying the nature of the impulses
thus initiated, should be of so great importance in controlling the activities
of the body, and should arouse even as much of consciousness as olfactory
impulses do, probably depends upon the wide cortical representation and
upon the greatly developed stratum zonale of the olfactory cortex.
The olfactory cortex throughout differs from the typical cortical structure. In the uncus and hippocampus the stratum zonale is unusually thick. In the place of the layers of small and medium pyramids are found nests of cells. These nests include two classes, the rather large multipolar cells, with masses of tigroid substance not very closely placed, and the nests of very small pyramids, very closely placed, and which display unusual affinity for the nuclear strains. These nests of cells have very dense pericellular baskets. (Figs. 8, 9, 10.) The large pyramids of this area are replaced by cells of peculiar appearance, which Cajal called “tassel cells.” These cells have many apical dendrites somewhat resembling those of the large pyramids, but the basal dendrites trend toward the ventricular surface, and the whole cell assumes a symmetrical appearance, with bundles of dendrites attached like tassels to each extremity of the spindle-shaped body. The axons of these cells seem to pass into the white matter, and probably are carried to other parts of the rhinencephalon. The layers of cells deeper than the tassel cells include the multipolar and fusiform cells, like those found in the typical cortex. Fig. 12. Diagram of
the sensory areas.
The subicullum is the lower lip of the hippocampal fissure. Its structure is characterized by the thickness of the stratum zonale and the great length of the radiating fibers, which reach the stratum zonale. The nests of cells are present in the subiculum also. The Golgi Type II cells of the region of the subiculum are characterized by the very great richness of the branches of their axons. The gyrus cinguli differs from other parts of the olfactory area in having a narrow stratum zonale. The pyramids are small and not to be separated into layers. They are irregular in form and in position. The deeper layers of the gyrus ciguli contain numbers of triangular, polymorphic and spindle cells, which have a most remarkable affinity for stains. With the exception of the gyrus cinguli, the olfactory
cortex is characterized by the nestlike arrangement of the cells, by the
great thickness of the stratum zonale, and by the great complexity of association
fibers relating the cells to one another. These characteristics are evidently
associated with the characteristics of olfactory impulses in consciousness—the
appreciation of odors is vague and indefinite; not very much knowledge
of things smelled is possible; and the direction from which smells come
and the locality of their origin are not to be determined by the olfactory
sense at all. Primarily, things are smelled as if merely present without
any ideas of form, distance, locality or origin, and with usually very
slight vividness of perception. Only very strong stimulation of the olfactory
apparatus is able to initiate vivid consciousness of things smelled in
the normal person.
The olfactory cortex is rather freely associated
with other parts of the cortex, but no olfactory overflow area has been
found. Thus there is lacking the structural relationships necessary for
the development of exact memories and coordinations of olfactory impulses.
We have no such exact memories of smells or tastes as we have of the sights,
sounds, pain, the temperature sense, sensations of muscular effort, and
the other sensations upon which the structure called mentality can be built.
Olfactory and gustatory sensations do not form any direct foundation for
the processes in the so-called higher intellectual faculties, probably
because of this lack of overflow areas. There is not recognized any logical
classification of smells; there are no classifications of the objects of
the external world according to their odoriferous qualities. We are not
able even to classify the primary olfactory sensations in any satisfactory
manner. Among normal people odors have very little place in the intellectual
life, however much they may affect this indirectly.
People whose nervous systems are more irritable than
normal, and those who are of the neurasthenic type, with unbalanced nervous
systems, are sometimes capable of appreciating smells with quite remarkable
vividness. A patient being treated in the clinic of The Pacific College
of Osteopathy, a woman of most unstable reactions and decidedly neurasthenic,
with a few of the stigmata of degeneracy, displayed very remarkable acuteness
in the sense of smell. She was able to recognize the presence of persons
of her acquaintance in the next room, and she could tell whether an acquaintance
had passed through the room within the hour or two. The very fact of such
abnormal acuteness is evidence of atavistic tendencies, and such persons
are liable to those defects of the nervous system associated with deficient
development. They are neurasthenic or hysterical, under suitable conditions,
and are usually more or less unbalanced nervously throughout life.
The intimate relationship of the primitive olfactory
cortex to the rest of the cerebrum has never been altogether abrogated
There are found fibers associating the different parts of the rhinencephalon
with almost or quite all of the primary and overflow areas of the cortex.
As a result of this intimacy of associational relationships the stimulation
of the olfactory cortex is very apt to initiate the activity of other cortical
areas. This, together with the fact that the olfactory images are not kept
in memory (because of the lack of olfactory overflow areas), accounts for
the fact that smells are usually efficient for the reproduction in consciousness
of visual and auditory and somesthetic images with particular vividness.
Somesthetic images may recall visual images, and visual images may recall
auditory images, but the presence of odors recalls with peculiar vividness
and distinctness of detail the auditory and visual and somesthetic images.
The lack of olfactory memories and the fact of the intimacy and complexity
of the primary olfactory cortex accounts for these peculiarities.
The lack of the importance of olfactory impulses in the more complex and conscious coodinations is not indicative of a lack of importance of olfactory impulses in governing the reactions of life. The olfactory areas are especially intimately related to the ganglionar centers of the cortex. Impulses from the olfactory cortex are carried by way of the fornix and the corpora mammillaria to the thalamic center, and from these to the gray matter around the cerebral aqueduct. From this gray matter the visceral centers of the midbrain, pons, medulla and cord are affected. Thus, the olfactory impulses are efficient in modifying the visceral activities. From the nucleus habernulae, impulses are carried by way of the fasciculus retroflexus to the interpeduncular ganglion, and from this center impulses are sent to the somatic centers of the midbrain, pons, medulla and cord. Thus the olfactosomatic reflexes are controlled. These relationships of the olfactory centers to the ganglionar centers are indicative of the important place of the olfactory impulses in modifying the emotional and instinctive reactions. This relationship is not often recognized. The odors not recognized may yet be efficient as guiding factors during life. The “instinctive distrust” which people sometimes feel for one another is often due to the presence of unrecognized odors. The odor may affect the body so as to produce a slight shrinking due to the activity of certain muscle groups, and a slight visceral disturbance, not recognized as nausea in consciousness; but the sum of the visceral disturbance, plus the muscular reaction, may be sufficient to initiate the unrecognized and “instinctive” feelings of repugnance. These feelings may be pronounced enough to affect the relationships of individuals. On the other hand, similar conditions may account in part for the attraction which one person has for another. Odors have a certain important place, not very often recognized, in modifying the social relations. Sometimes the choice of one pathway rather than another, the modification of one’s position in life, depends upon the antipathies and attractions and repulsions associated with unrecognized odors. These may sometimes govern the reactions which an individual makes to his environment without his really being aware of the fact. It is in life’s pleasures and unpleasantness, however, that the most pronounced effects of olfactory impulses are felt. Pleasant odors may cause pleasant memories, and foul odors may initiate such disgust for even beautiful surroundings that the memories may be associated with feelings of dislike for a long time, and thus may affect one’s whole future history. Among the lower animals the sense of smell is of tremendous importance in the sexual life. Among mankind the place of smell during courtship varies inversely to civilization. The passionate delight in odors is probably of a pseudo-sexual nature among neurasthenic or hysterical persons. The odors of certain secretions seem to increase sexual excitement, especially with some persons. Many other people find the odors of the body at all times most repugnant. The more uncivilized races, and the more uncultured among the races ordinarily called civilized, attempt to add to the body odors, and to increase their effectiveness. The attempt to conceal body odor under sweet scents is of later origin, and is superseded among enlightened people by the lack of odor due to exquisite cleanliness and the use of the substances which exhale rather a fragrant freshness than a real perfume. Even among civilized people there is much difference
of taste regarding odors. This is probably due to the fact that the olfactory
centers are practically incapable of associative education, that their
value in modifying the reactions of the individual is becoming decreased,
and also to the fact that the pleasures of odors are more due to the factors
associated with the odors than to the primary sensations. For all these
reasons the olfactory impulses play a decreasing part in human life.
Olfactory stimuli seem to increase the tone of certain muscle groups, sometimes apparently without arousing consciousness. This perhaps is the reason why certain individuals find the presence of even pleasant odors associated with great fatigue. The effects of olfactory impulses in lowering the blood pressure increase this sense of fatigue. It is not possible to make much adequate use of the
olfactory impulses in the treatment of disease. An enjoyment of the fragrance
of fresh air, of the fragrance of foods suitable to the individual, may
be of value in securing better hygienic and dietetic conditions. For the
most part, flowers with marked fragrance are out of place in the sick room,
unless they are perfectly fresh and the room is extremely well ventilated.
Even then flowers of strong odors should not be permitted to remain long
in the room.
The relationships of the taste impulses are equally general. The sense of taste is vivid enough under excessive stimulation, but in ordinary life the taste impulses are not, usually, very vividly present in consciousness. The teachings of Fletcher in this connection are of interest. The digestion of foods which tastes well is certainly more complete than is the digestion of foods of equal potential nutritive power which have no taste. This is recognized to be a fact. It follows, then, that the very act of attention to the taste of food should increase its digestion—provided, of course, that the taste is agreeable. Food which is not agreeable to the taste would naturally be omitted from the diet more frequently when the attention is devoted to the taste than when tastes are not consciously perceived. The value of attending to the tastes of foods, then, depends in part upon the increased digestibility of good foods, and in part upon the increased tendency to the omission of foods which are really not fit to eat anyway. The discussion of tastes in this connection includes
the flavors also, which are really classed properly as odors. Since the
relationships of the gustatory cortex and the olfactory cortex are probably
identical, the separation of the two is of academic rather than of practical
interest.
Touch and muscular effort are fairly well represented upon the cerebral cortex. The position which these sensations occupy indicates the relations which must exist during the phylogenetic development of the cortical areas. The areas for the sensations of the body in general are located on the post-central gyrus, and the area for the common sensations of any part of the body is placed just posterior to the area in which are located the neuron groups governing the movements of that part of the body. Just posterior and continuous to the area devoted to the common sensations lie the areas concerned in the sensations of the muscular sense. The overflow areas for both these sensations are continuous posteriorly with the overflow of the visual area. The stereognostic centers lie upon the boundary between the two. The fact that the muscular sense is represented rather farther from the primary motor areas than are the common sensations indicates the later development of the muscular sense as a factor in controlling the motor reactions. The impulses of muscular effort and of common sensations are of considerable importance in the coordination of the motor impulses. The destruction of the common sensory area or of the tracts transmitting them to the cortex is associated with a certain form of ataxia. In consciousness the bodily sensations are associated with the idea of personality. This is true of the viscero-sensory impulses as well as of those of the common sensations and of muscular effort. The sensations of touch and of muscular effort add comparatively little to actual knowledge of the environment, acting alone. By means of their effects upon the intermediate areas and through their importance in the coordination of the muscular movements, however, they play a part in the determination of the nature of consciousness which is not to be overestimated. There is present in consciousness constantly a sort of dim and unnoticed background which is made up in great part of the bodily sensations. These are, normally, not vividly appreciated, but under abnormal conditions they may become the most vivid factor in consciousness. Whether there is any cortical representation for the visceral sensations, or whether the fibers passing generally from practically all of the sub-cortical centers carry the viscero-sensory impulses, can not now be determined. It is possible that there is no cortical representation of the viscera. This would account, in part, for the fact that visceral disturbances are interpreted in consciousness as disturbances of somatic sensations. Head’s Law is concerned here—that the areas of low sensibility are projected in consciousness upon areas of high sensibility innervated from the same spinal segment. This is not exactly true, in all cases. But it is true that, in the cases in which Head’s Law seems to fail, a consideration of the cortical, or medullary, or midbrain relations shows that there is a central connection, either spinal or cerebral. The fact that lesions of the cortex are rarely associated
with visceral pain indicates that probably there is not any exact visceral
representation in the cortical areas.
The somesthetic overflow areas are not concerned
in what is commonly known as intellectual processes, since the coordination
of those impulses which add value and efficiency to the auditory and visual
overflows, which are considered of more intellectual worth, occurs in the
somesthetic overflow. The interpretation of visual and auditory images
is secured in great part by the coordinations occurring in the somesthetic
and motor overflows.
The temperature sense seems to be associated upon the cortex with the sensations of touch. A few clinical records are found of persons in whom disturbances of the temperature sense preceded the disturbances of the sense of touch in sensory paralysis of cortical origin. It is not recorded in the papers and books consulted in the preparation of this volume that disturbances of the sense of touch have preceded disturbances of the temperature sense in the purely cortical lesions. It is supposed by certain neurologists that the temperature senses are represented in the cortex by the cells of the outer layers, for this reason. The evidence is rather insufficient as a foundation for such conclusions. The temperature sense is of value chiefly for the
avoidance of danger. The effects produced in consciousness by temperature
changes are rather more vivid than in the case with other common sensations.
The sense of heat or of cold is largely due to the consciousness of the
manner in which the body is reacting to the temperature changes than to
a direct effect of temperature changes. The consciousness of the chill,
for example, is due to the shivering, the erection of the pilo-motor muscles
and the constriction of the peripheral vessels, rather than to the actual
temperature of the body, which is usually above the normal at that time.
The temperature senses seem to vary. The temperature changes within the
body itself may be interpreted in consciousness truly, but very often the
temperature of the body itself is either unrecognized at all or is associated
with sensations of different character. The sense of chilliness in the
beginning of fever, the sense of heat in shock, are instances of this inversion
of the temperature sensations. The temperature of the skin is associated
with more exact knowledge in consciousness, though this is far from being
an exact sensation The physiological relationships of the various heat
areas and cold areas of the skin are yet to be determined.
The impulses arising from the vestibular neurons are of a certain importance. These impulses are carried by the sensory neurons of the first order, whose cell bodies lie within the vestibular or Scarpa’s ganglion. The axons of these cells pass with the acustic part of the auditory nerve, and are called the vestibular part of that nerve. These axons terminate in the nulei of insertion of the vestibular nerve. These include several masses of gray matter in the floor of the fourth ventricle, which have not been exactly described. It is known that a number of these cell groups are related to the vestibular nerves. The axons of these vestibular nuclei are sent to the spinal centers, to the cerebellum, and to the olivary bodies, chiefly. These relationships are functional controlling the movements of the skeletal muscles in accordance with the impulses arising from the vestibular structures. These paths are of importance biologically in the fishes and birds, which are compelled to coordinate their movements with such great delicacy in order to maintain their equilibrium in a medium of about the same specific gravity as their own bodies. In man and in those animals which walk upon the surface of the earth, or which swim upon the surface of water, such delicate coordinations are not needed, and the tract is thus of lessened biological value in these classes of vertebrates. Impulses from the vestibular nuclei may be carried by the lateral fillet, but it seems more probable that they are carried by the medial fillet, or that the fibers join the spino-thalamic tract as it passes through the pons on its way to the lateral nucleus of the thalamus. At any rate, the impulses finally seem to reach the cortex in the neighborhood of the areas concerned in the body sensations for the head and face. The impulses carried by the vestibular nerves are efficient in modifying the muscular reactions of the body, chiefly by way of the reflex paths already mentioned, and also by the activity of the cortical motor area. The place of these impulses in controlling motor reactions is noted in Meniere’s disease, in which the injury or disease of the vestibule causes a loss or an abnormal condition of the vestibular nerves. The lack of coordination of the muscles of the body is very evident in this disease. The sensations produced in consciousness by Meniere’s disease include the symptoms of the disease apart from its neurological aspect, and also a sense of the lack of reality of things present in consciousness, or remembered as having been present in the past. Lesions of the cortex producing similar symptoms are described for the stereognostic area of the left hemisphere, and also for the same area upon the right hemisphere. Lesions affecting the pressure conditions of the brain as a whole also are associated with similar symptoms. The place of the vestibular impulses in consciousness is very small. It is possible that in part the ideas of the position of the body and the head may be associated with impusles form the vestibule, but this part must be of comparatively little importance. The lack of the normal impulses from the skin, joint surfaces or muscles interferes with the position-sense much more seriously than does the occurrence of Meniere’s disease. These clinical considerations seem to indicate that the vestibular impulses give very little consciousness of position. The vestibular sensations appear to be homologous with the sensations received from the lateral line organs in fishes, and the central connections of the vestibular centers are similar to those of the nuclei of the lateral line sensory nerves in those lower vertebrates. The lateral line organs were well on the road to extinction before the cortex had attained any marked functional development. Thus the projection of the vestibular and the lateral line impulses upon the cerebral cortex never assumed any particular importance. The vestibular nuclei retain their primitive relationships with the cerebellum, and are still efficient in modifying the movements of the muscles of the body. The effects of the vestibular impulses in modifying reactions may be found illustrated in the manner in which people place themselves on a moving car or train. Even in the darkness there is a tendency for people to try to place themselves so that the motion of the stopping and starting of the car may affect the body symmetrically. While it may not be consciously uncomfortable, yet the reflexes aroused by the unequal stimulation of the vestibular nerve endings produces the series of movements which place the body symmetrically in regard to the direction of movement. The nuclei of the vestibular nerves are closely associated with the nucleus of insertion of the vagus. The sensory impulses from the two nerves, different in distribution as they are, are not readily interpreted in consciousness, and they are not localized. The sensations, such as they are, usually include rather a sense of dizziness and discomfort, rather than a sense of real pain. The impulses carried by the vagus give the same sensations in consciousness. Under abnormal conditions these sensations may be extremely uncomfortable. It is thus not rare that the early stages of Meniere’s disease may be mistaken for some digestive disorder. The place of the vestibular neve impulses, like the functions of the other viscero-sensory nerves of the body, must be found in their action upon the lower centers. The impulses carried by these nerves, the vagus and the vestibular, should not be permitted to affect consciousness in any degree avoidable. The sense of pain and discomfort should arouse the reactions which tend to the relief of the pain, but after the sensation of pain has initiated the appropriate motor reaction, the only thing to do is to inhibit the further stimulation of the neurons concerned in carrying the impulses of discomfort. This is done most efficiently by causing the increased activity of other neuron groups, not concerned in the appreciation of discomfort. In planning this increased activity, it must be remembered that the neurons of the cortex, whose activity is essential to conscious phenomena, are governed by physiological laws, and that their increased or modified activities must be based upon the same laws which govern the activities of the spinal centers, or any other neuron groups. Thus it is necessary, in dealing with persons who because of nervous abnormalities feel pain in the absence of efficient causes of pain, to provide, first, those circulatory and nutritive conditions needful to normal neuron activity, then to cause those parts of the cortex which have been left rather undeveloped to be stimulated by streams of normal nerve impulses. This increased stimulation of the cortical areas, not associated with the pain and discomfort, lessens the liminal value of those other neurons; the constant streams of stimulation associated with normal living do not find the excessively irritable areas concerned in pain sensations, and the consciousness of the individual is unaffected, ultimately, by any pain sense not originating from really injured parts of the body. Stimulation of other cortical areas is easily done in persons of anything like normal nervous systems. Any form of new activity, the recrudescence of any old fad, or the training of unused muscles may be effective in lowering the liminal value of new areas, and in giving the hyper-irritable neurons opportunity for recovery. |
