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Various forms of malfunction are prevented by what, for want of better term, we are calling “the habit of health.” By this expression is simply meant that property of living cells which enables them to retain their characteristic metabolism for a time in the presence of abnormal conditions. Through centuries of progressive reactions to similar environal changes, the cell has acquired a certain conservatism in the rhythm of the metabolic changes which have persisted for so long a time. This conservatism, this fixity, this persistence in the straight and narrow path of inherited rectitude is what we are calling “the habit of health.” The term is not altogether satisfactory, yet it is the best which has been suggested. The habit of health persists in the simplest types of cells. If we may employ the terms of Ehrlich’s side chain theory in this discussion, we may say that the habits of the cell, whether of health or disease, are the functions of the affinities of the side chains of its molecules. The side chains of the living molecule have affinities for certain food stuffs, etc., which are always found in the normal environment of the cell. The molecules or radicals which are attached to the side chains satisfy the affinities of the chains for a time, and are variously used by the cell either as a source of energy, or as material for building or rebuilding cell structure. If the substances for which the side chains have especial affinities are lacking in the environment, these affinities may be satisfied by other compounds and radicals chemically similar to the normal foods. If these related substances are essentially like the normal foods, no harm is done by the change. If there has been present in the environment more than one element of possible food value, a choice is proposed to the side chains. Let it be granted, for sake of clearness, that there are two slightly different substances present in the environment of a cell, neither of which is the food stuff normal to the cell, which is lacking. Some of the side chains of the molecules of the cell will unite with each of these compounds, in all probability.
One side chain unites with a radical or molecule which we will call “A,” the other with the radical or molecule which we will call “B.” The side chains whose affinities have been satisfied by “A” and “B” have certain duties to perform in the cell metabolism. These duties or functions are somewhat modified by the presence of “A” and “B” instead of the normal food stuffs. If this modification is such as maintains or increases the rapidity or the efficiency of the functions of the side chain which has united with the radical “A” and is such as decreases the efficiency or the rapidity of the action of the side chain which has united with “B,” then “A” is more quickly used in energy formation or tissue building than “B.” The side chain which has grasped “A” has then free valencies again. The same quality or position which rendered this side chain more liable to unite with “A” in the first instance is very apt to facilitate its second union with a molecule of the “A” type. The use of “A” instead of the normal food stuffs exerts an influence upon the side chain with which it is united. This change in the function of the side chain affects the rapidity and the efficiency and the quality of the katabolism and the katabolic products of the entire living molecule, and, through these, of the cell of which these molecules are a part. This change in the cellular metabolism is the first step toward adaptation. During the time of the series of changing periods of free valencies and of satisfaction on the part of the side chain, the affinities are still most strong for the food stuffs originally normal to the cell, but after a time, when the series of chemical changes which make up metabolism have been many times repeated the affinities of the side chain are for the molecule or radical “A.” The food stuffs originally normal are then foreign substances. The side chain which has united with “B,” the one whose function in the metabolism of the cell was somewhat retarded by its unlucky union, is rendered more or less inert by the presence of “B,” or it may be that “B” is more or less toxic to the chain or to the cell, and destruction or death results. But there is some reason to believe that side chains whose affinities have been satisfied in a manner poorly adapted to the ultimate good of the cell are simply discarded, and are eliminated from the body of the cell as waste matter. These discarded side chains are sometimes of value to complex organisms in the presence of infection. The living molecule persists in the rhythm and the affinities to which it has been accustomed, and this persistency is the habit of health which often preserves life in the presence of abnormal surroundings. Cells have a habit of health which is the sum of the habits of its many and various constituent molecules. Unicellular organisms are affected by certain changes in their environment which usually make for the preservation of the life of the individual. For example, an increased degree of heat increases the motion of certain motile animals and plants. Lately many scientists are investigating the various forms of taxis and tropisms. These reactions all make for the preservation of life, in the long run, else would they not be.
The cells of complex bodies, such as our own, enjoy a habit of health in superlative degree. The factors already mentioned are effective in these cells also, and other factors which are dependent upon the many generations of united living add to the conservatism of cell metabolism, and to the perpetuation of the rhythm of functional changes which are characteristic of these cells. The periodicity of hunger, thirst, sleep, of the increase in the amylolytic power of the saliva, of increased and decreased temperature during the day, with its concomitant increase and decrease of muscular power, the occurrence of the growth changes, of puberty and the climacteric, are all indicative of the power of this inherited rhythm of metabolic changes. The fact that these rhythms are all susceptible to variations in answer to environal changes is indicative of the origin of habit in the repetition of certain recurrent environal changes, and also of the power of living things to adapt themselves to further variations in their environment. The mental aspect of habit is somewhat aside from this discussion, yet it is rather closely related with it.
Habits, in the accepted sense of the word, are almost altogether psychical in their origin. They are perpetuated, often, without the intervention of consciousness. The physiology of habit is a very interesting and inexhaustible subject. In the beginning, a habit is a conscious reaction to certain bodily or external factors. The sensory impulses, whether somatic or visceral, whether originating in the body or in the environment, are carried to consciousness and there correlated with one another and with remembered experience. The motor reaction is carried to the appropriate muscles, and the first action is performed. Now, when a nerve impulse is carried over a system of neurons, the threshold value of that system of neurons is lowered. This renders these neurons more easily stimulated than they were before, and hence they react to slighter stimulations than before. If the first reaction is frequently repeated, the lower centers become so easily irritated that stimuli utterly inefficient in the beginning are able to initiate the whole series of motor reactions. The habit is formed when the motor reaction occurs independent of consciousness. It is not needful to assume that the reaction is performed unconsciously, but only that consciousness is not involved in deciding the nature of the reaction. From the purely psychical standpoint, the same factors are concerned. Those centers upon the cerebral cortex and in the basal ganglia which are most frequently used are those which have the lowest liminal value. These are most easily stimulated by external changes, and these therefore affect most strongly the nature of the efferent impulses aroused by the incoming sensory impulse. It frequently occurs that in the presence of poor nutrition, auto-intoxication, peripheral irritations, and some other abnormal conditions, the neurons of the cerebral cortex are unable to preserve their normal metabolism. In such cases, the mentality of the patient is affected in some degree. There is liable to be an abnormal lowering of the liminal value of the centers which are phylogenetically the oldest, and the motor reactions to sensory stimulation are not modified by the considerations of altruism, delicacy and unselfishness characteristic of the normal mentality of civilized and cultured people. NOTE A.—“Our habits make ourselves. What a difference is there between individuals that is not measured by habit,--habit of speech, of manner, of thought? To every change in our surroundings we give an answer back, an answer which may be speech, deed or silence, but which is always determined, or at least modified, by our habits of thought and action. The manner of this answer is invariably characteristic of ourselves, and is usually very little more than the manifestation of a habit. By means of habit, the thought of yesterday governs the action of today, the decisions of the child modify the gait and the speech of the man, the habits of our savage grand-parents are shown in the clinching of fists and showing of teeth in our own anger, the use of the ring in the marriage service, and the offering of food to our friends, without any regard for their hunger. “The persistence of habits through the life of the individual, the family, the race, even through changing environments which overlay the original habits with a thousand modifications, renders it extremely probable that there is some structural basis for their development and perpetuation. In order to consider a suggested explanation of the formation of habits, it will be needful to consider for a moment some of the facts already demonstrated with regard to the structure of nerves and their actions. “The brain and other parts of the nervous system are made up of small gray bodies, irregular in shape, known as neurons, or nerve cells, together with the tissues which nourish and support them. Each neuron has at least one long, fine fiber growing from its body, and not more than two. The strong, white cords called nerves are made up of bundles of these fibers, each with its own sheath, and all bound firmly together. The neurons vary greatly in size. It would take about twenty-five thousand of the bodies of the smaller to make a row an inch long, but others have a diameter fifty times as great. The fibers growing from the larger cells may be thirty or even forty inches long. There are hundreds of millions of neurons within the body. Each of these, like the other cells of the body, leads its own life, maintaining its own individuality, yet in a manner dependent upon the rest of the body, as, in a city, the baker depends upon the miller, the tailor, the teacher, and each of these depends upon every other. The blood and lymph bring food and oxygen to the neurons and carry their waste materials away. From this food the cell builds up its own body, and stores energy for future needs. “Certain granules, first discovered by Nissl, are found within the bodies of neurons. These represent the storehouse of energy. These granules are of very complex and unstable composition. Their disintegration liberates the stored energy very much as the disintegration of gunpowder liberates energy. The granules are built up by the activities of the neurons just as the green coloring matter of leaves is built up by the activities of plant cells. The Nissl granules are far more complex than the coloring matter of plants, however. So unstable are these that a ray of light breaks down the granules of the neurons of the retina, a faint sound causes the disintegration of the granules of the neurons within the ear, the most delicate touch upon the end of the fiber growing from the neurons near the spinal cord to the tip of the finger breaks down the granules within the body of these cells. The energy liberated by the disintegration of these granules is called a ‘nerve impulse.’ Nerve impulses pass from one neuron to another through the brain, the spinal cord, and other neuron systems according to their structural relations. “The granules in different parts of the nervous system vary greatly in stability. In neurons rarely used the granules are relatively stable. The frequent passage of impulses over neurons and neuron systems causes them to build up granules which are more unstable. That is, the granules of neurons, like nearly all other complex organic structures, are more easily broken down when more rapidly built up. All mental development and all training depend upon this progressive decrease in the stability of the granules within the neurons. “The neurons which receive sensations are arranged in little masses just outside of the brain and spinal cord. These cells send fibers to all parts of the body, and by means of these we receive sensations of heat, cold, pain, touch, weight, sound, smell, taste,--indeed, it is by means of these cells that we receive knowledge of our own bodies and of the world about us. Each of these cells sends a second fiber into the spinal cord or into the lower part of the brain. The fiber branches within the cord, or the lower part of the brain, sending one division toward the higher centers and others to the neurons which immediately control the movements of the body. “The neurons which control the movements of the body are called motor cells. They send fibers to the muscles, and the nerve impulse from a motor cell causes the shortening of that muscle cell with which its fiber is connected. These motor cells are found in the spinal cord and in the lower part of the brain. They are induced to send out nerve impulses to the muscles by the receipt of impulses from the sensory nerves or from the higher brain centers. The sensory nerve cells from any part of the body are connected with the motor cells sending fibers to the muscles moving that part of the body. “Now, when an impulse passes over a certain sensory nerve, it reaches both the motor nerve cells controlling the muscles of it s own area of the body, and the higher brain centers where consciousness is affected. At first, the impulse reaching the motor cell is not sufficient to cause the liberation of its energy. The impulse carried to the higher brain centers affects consciousness, i.e., gives the person a knowledge of the source of the impulse. As a result of this knowledge he sends impulses through the motor cells which result in appropriate action. The granules of the motor cells concerned are disintegrated and their energy set free as a nerve impulse which travels along the nerve fibers to the muscles whose motion is desired. The motor cells must then build up another set of granules, must store another fund of potential energy. These new granules are just a little more rapidly built up than were the old ones, and are therefore just a little more unstable. “Every time the original sensation is repeated a part of the impulse from the sensory cells reaches the corresponding motor cells. If this sensation is always, or is frequently followed by the passage of impulses from the higher brain centers to the motor cells, the granules formed by these cells will become progressively more unstable, until a time will come when the impulses reaching them from the sensory cells will be sufficient to cause the liberation of their energy. This energy, or nerve impulse, travels over the nerve fibers to the muscles, and the movements which result are those which already have been so often repeated. The original sensation is carried to the higher brain centers, as before, but since the required movements have already been performed, attention is less and less vividly aroused until presently the whole series of events becomes mechanical,--the habit is formed. “These ‘short circuits,’ if we may so call them, are formed in many of the lower nerve centers, but never altogether within the sympathetic system. The short-circuits which are formed through the spinal cord, or the medulla, or the mid-brain, are called “reflex actions.” These are inherited habits,--the short circuits are established at or before birth. Other short circuits are formed through the cerebellum. By means of these nerve by-paths we are able to perform very complex coordinated movements without thought. Walking, dancing, knitting and such handiwork and dozens of other such complicated actions, at first learned with difficulty, at last seem almost to do themselves. The Island of Reil, or “speech center,” affords another opportunity for short circuits. By means of this by-path, language becomes easy and vigorous. “As the result of all these short-circuits, the higher faculties of the brain, freed from the necessity of attending to the minutiae of routine tasks, are able to attend the more fully to matters requiring decision. “The
possibility of the inheritance of acquired habits is one of the puzzles
of our day. At present, there seems to be evidence to justify at
least the tentative supposition that individuals inherit increased or decreased
stability of nerve cells or systems, rather than any mental traits as such.”—From
the Osteopathic World, December, 1905.
The Associative processes in the Guinea Pig. Jessie Allen, in The Journal of Comparative Neurology and Psychology, Vol XIV, No. 4. Instinct in Man and Animals, J. P. Morat, The Physiology of the Nervous System, p. 421, Edition of 1906. |