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The metabolism of the multicellular animals differs from that of the simpler animals and from all plants in the need for some method for unifying the functions of the body. Unicellular organisms or the cells of undifferentiated tissues react to their environment each for itself, with no apparent regard for the conditions of their neighbors. Plants, even of considerable complexity of structure, exhibit no correlation of activity such as is displayed by even quite simple animals with nervous systems. Plants make no speedy reply to changes in their environment; their adaptation also is very slow; they live the life prepared for them by their ages of inheritance. The same conditions are true of those simple animals which have not attained the dignity of a nervous system. These do not lack for the unifying powers of the nervous system, because their cells are not differentiated and so act all alike, each in answer to its own environment. Such simple animals do not quickly adapt themselves to changes in their environment. They display very remarkable instances of regeneration of lost parts, but make no very evident efforts toward compensation. In animals which possess simple nervous systems these seem to exercise some control over the processes of regeneration. Among the higher animals, with their well organized and efficient nervous systems, the phenomena of regeneration disappear, and are superseded by the phenomena of compensation. Regeneration is not altogether lacking in our own bodies, as, for instance, the epithelial cells are continually being regenerated and continually being lost. The large and complex bodies of the higher animals and of man could scarcely survive were it not for the unity of action secured by the action of the nerve centers.
Loeb refers to the nerves as “bridges of protoplasm uniting the sense organs with the muscles.” The nerves are bridges which unite the various structures with their various functions into an integral whole. The nerve cells, with their unstable protoplasm and their long unbroken filaments so well adapted to the transmission of the metabolic conditions of one part of their substance to all other parts, with their intimate relationship with skin, muscle, gland, and sense organs, with their rapid metabolism and with their long life, are surely well adapted to their function of unification. They unify the many activities of the body in health as well as in disease; they unify the individual of today with the individual of yesterday; they unify the individual with the other members of his race; by the action of the higher association centers, the race is unified in power, in needs, in aims, in attainments and in inheritance. The whole function of the nervous system is comprehended in the statement that it unifies the individual in his reply to his environment.
The cells of the body are characterized by a certain individuality of metabolism. They are in no case absolutely independent, yet they are in great degree individual. The adherents of the myogenic theory of the cardiac and gastro-intestinal movements have demonstrated the very great degree of independent activity that is possible to the cells of muscles of even very complex bodies. In the absence or the disease of the nerves the muscle cells may act with a certain amount of rhythm for remarkably long periods, but in such cases there is no adaptation of the body as a unit to the environment as a unit. The heart kept beating without nerves, nourished by warm, defibrinated blood, may preserve its rhythm unchanged in any serious degree, but there can be no change in its force or rhythm in answer to the demands of active structures elsewhere in the body, nor does any fatigue of its own initiate vascular changes looking to its relief. The other parts of the body display analogous phenomena. Every cell and tissue leads its own individual life, and each influences and is influenced by almost every other organ of the body for the common good.
Inasmuch as the cells of the body are adapted to the receipt of these streams of nerve impulses, they are not long able to maintain any degree of efficiency after the section of their nerves. The nutritional condition of almost all of the body tissues is dependent upon the maintenance of its normal relations with the central nervous system. In olden times the effects produced by the section of the nerves to any part of the body was thought to be due to the lack of the “trophic” impulses which were supposed to be carried for the most part with the sensory nerves. Later authors doubt the existence of these impulses as such, but recognize the value of the sensory nerves in initiating the normal stimulating impulses to the organ concerned, and the more important protective movements. It is probably the lack of the sensory impulses which is responsible for the abnormal nutritive conditions observed after section of the sensory nerves to any tissue. In the case of the ulcers sometimes found upon the feet of the person with tabes dorsalis, for example, it is evident that it is the lack of the normal sensory impulses which is responsible for the condition. Here the sensory impulses aroused by slight injuries to the feet do not attract attention until the wound has become infected and the deeper tissues are seriously involved. Not only is it true that consciousness is not affected, but it is also true that the vascular reactions in answer to the variations of heat and cold and muscular effort either fail or become very inefficient in the absence of the normal sensory impulses from any organ. The presence of the trophic nerves has not been demonstrated, nor have the problems which occasioned their suggestion been satisfactorily solved. However these facts may be interpreted, it is indisputable that the section or disease of the sensory nerves to any part of the body result s in its malnutrition and disease.
All efferent impulses depend ultimately upon afferent impulses. Even the impulses called volitional are derived from sensory impulses which may have been a long time retained within the nerve cells. The character of the metabolism of the nucron is somewhat modified by demands made upon it,--every time that a neuron is affected in any manner its powers of reacting to its environment are affected. Under normal conditions, whatever stimulus reaches a neuron lowers its liminal value. Under abnormal conditions, the liminal value may be increased, or it may be lowered in an abnormal degree. The effect produced upon the neurons by education is simply a lowering of the liminal values of the areas affected. If the education be wisely planned, the neurons are developed in a coordinated manner, not any one system at the expense of another; and the systems which associate the different cortical areas are not neglected. In the unwise systems of education of children, and in the unwise methods adopted by some physicians for the mental treatment of patients, the neurons of certain systems are developed at the expense of a lack of development of others. The result of such a procedure is not all that might be desired in the way of mental or physical development.
The phenomena of memory depend upon the fact that the activity of a neuron or a neuron system lowers its liminal value. Because of the phenomena associated with memory, the individual of today is practically identical with the individual of the past. The neurons are the structures which retain the records of past experience, and they do this by means of the changes in their metabolism resulting from their activity.
The basal ganglia coordinate the emotional reactions. These are a needful and essential part of life, a source of strength and of normal pleasure in both work and recreation. The normal activity of the nerve centers depends upon their normal relationship with the other neuron systems. The irrational methods of education which eliminate the consideration of these centers fails in its most important ends. Many a nervous collapse is due primarily to the effort to compel a conduct of life based upon the action of the cortical neurons alone.
If
the body is to act as a unit, the neuron systems concerned in every reaction
must be developed in equivalent degree. This is a problem for both
educator and physician, it is true, but it is the failure of the educator
which sends the abnormal nervous systems to the physician. It is
here, again, that the physician fails in his duty if he neglect the instruction
of his patient in the rules of hygienic living.
Tabes Dorsalis, in “Mental and Nervous Diseases” by Church and Peterson. The Physiology of the Brain, by Jacques Loeb. |