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So far as the life history of the unicellular organisms is concerned, their only function seems to be to eat, to grow until the ratio between the assimilative surface and the mass of the food-demanding protoplasm renders further growth impossible, then to divide into smaller cells which in turn eat, grow, and divide, and so on. In the economy of nature, there may be functions of races, as in our own bodies there are functions of parts, but this aspect of the subject does not concern us in this connection. Among these unicellular organisms, and among the simpler types of the multicellular animals, any lack of function is very quickly followed by a loss or change of structure. Under ordinary conditions, the normal cell in its normal environment does not fail to perform its normal function, nor does it exceed the normal limits of its metabolism. But under experimental conditions, factors which increase, decrease or change the activities of a normal cell may be employed, and the effects noted. Increased rest may be forced in the absence of light, or by lowering the temperature, or by decreasing the amount of food, water or air in a degree not incompatible with the maintenance of a fairly normal life. Increased activity may be secured by increasing the temperature slightly, by giving artificial light at night, by the use of electricity, by increasing the supply of food, and by increasing the oxygen content of the air. Other methods of especial application are employed also. Changed forms of activity, triangular cell division, abnormal forms of growth, abnormal metabolic products, may be forced by the use of more abnormal methods of stimulation or of restraining cell activity. The use of various salt solutions for this purpose has resulted in increasing the general knowledge concerning cell activity in many directions. The experiments of Loeb upon sea-urchins are of interest in this connection, as are also the experiments demonstrating the myogenic factors in the contraction of the heart and the non-striated muscles.
Triangular cell division, and other abnormalities of karyokinesis, result from the use of abnormal stimulants to increase or change the processes of cell division. If a culture containing fish eggs which are actively growing is violently shaken, many of the eggs will be killed outright, some will remain uninjured, and there will be some which will continue to grow, but because of their injuries will undergo the subsequent stages of growth in an abnormal manner,--the cells divide in a triangular or even a quadrangular manner, a second mitosis will be initiated before the completion of the first. Deformed cells result form these conditions, there are changes in the staining reactions, and other evidences of serious malfunction. The
cell division of the malignant growths, carcinoma, sarcoma, etc., display
abnormalities of karyokinesis similar to those found present in the cells
of the simpler animals which have been subjected to various forms of abnormal
stimulation, or to the action of toxins, or to physical changes which injure
without killing them.
The changes in the structure of the cells which are affected by adverse influences are perhaps in part due to actual physical injury, but in other instances, where the stimulant has been very slightly different from those normally active,--as, for example, a very slight increase in the oxygen supply, or of sunlight, or of very dilute non-toxic salts,--the abnormal effect seems to be at first one of function, which itself is a change of molecular structure of the living protoplasm, and the series of chemical changes which occur therein. The chemical configuration of the molecule determine the nature of its chemical reactions, the quality of the end products of its metabolism, and its staining reactions. Whether the phenomena of karyokinesis are essentially chemical or not remains to be determined by further investigations. It is evident, from the work done, that these phenomena are rendered abnormal by both chemical and physical stimulation, and under other conditions wherein the etiology is unknown.
The metabolism of the cell may be rendered abnormal by a lack of the normal quantity of food. In this case, the unicellular organism suffers merely progressive starvation and death. The cell which is a part of an organ of the body suffers also from starvation, but it does not suffer alone. The relations of the different cell groups of the body have been somewhat fully discussed in another chapter. In starvation the cell fails in growth, and therefore fails in normal reproduction. The starvation may itself initiate reproduction, and the daughter cells are then smaller than normal. In some of the unicellular organisms, the lack of food or water initiates a condition of rest. The rest period may be spent within a membranous envelope, by which evaporation from the protoplasm is hindered. These facts indicate that under normal conditions changes in the function of the cell which answer environal changes may themselves be a cause of changing structure.
If a cell is stimulated to increased metabolism by methods which are not very different from their normal condition, they usually undergo a series of changes which vary very little for all forms of life. The deutoplasmic granules disappear, the protoplasm becomes clear, the vacuoles increase in size if any are present, or are formed, if they were not present already, the nucleus becomes pushed to one side, the protoplasm swells, and then becomes shrunken. These changes occur with some variations in almost all cells subjected to over stimulation. The changes which occur in the environment of any cell are its only source of energy. The cell is able to avail itself of this energy only under certain conditions, which vary with cells of different forms of structure. For the most part, animal cells derive energy chiefly from the oxidation of their food substances. If their environment offers food but no oxygen, they are unable to use the food. If their environment offers oxygen but no food, or if water is lacking, or if a proper amount of heat be not present, the cell is unable to avail itself of those favorable factors which are present. In these cases, the cell structure suffers a more or less rapid disintegration and death. Whatever may be the cause of any malfunction on the part of any cell, the ultimate effect is a structural injury to the cell.
Among the cells which make up the organs of the body, the same principles apply. In bodies so complex as these, each organ depends more or less completely upon the normal activity of all other organs. It thus occurs that organs which are themselves normal, so far as our present methods of investigation determine, as subjected to the influences of other organs which are not quite normal, and the normal organ is thereby forced to act in a manner somewhat different from that to which it is accustomed. This condition occurs in efforts at compensation and adaptation. The series of events as a part of the life history of the individual or the race is on the whole beneficial, else would it never have persisted. The over work which is forced upon any organ, produces certain changes in the structure of its cells. These changes are characteristic of the cells affected, as long as the over work does not occasion actual exhaustion. In this case, the structural changes in the tissue cells are those already mentioned as following the fatigue of the unicellular organism. If a nerve cell be stimulated somewhat in excess of the normal degree, the substance which represents the potential energy of the cell (probably the precursors of Nissl’s substance) become partially disintegrated. The exhaustion of the cell is accompanied by their complete disappearance. If the over work is continued for a time, the substances which are used up by the cell are replaced from the nutrient lymph with more and more celerity. These quickly formed substances are the more unstable, and the neuron threshold is proportionately lowered. The molecular structure of the cell is thus changed by stimulation which represents an amount scarcely beyond the bounds of the normal. The progressive differentiation of the neuron systems in development and education is a manifestation of the principle, that the structure of an organ or cell varies in accordance with the demands made upon its function. The muscle cell which is caused to act in a somewhat increased manner endures also characteristic structural changes. Its cells increase in size, and in many instances also in number. The efficiency of the muscle also increases, as is the case with the nerve cell. This increase in size and function is a normal characteristic of muscle, though under circumstances which are clearly abnormal the hypertrophy may attain a degree which is pathological.
Other cells of the body react to unusual demands in a characteristic manner. It is evident that the structural changes which occur as a result of functional changes are a factor in differentiation and development, in the adaptation of the cell and the body to changing environment, in the manifestations of disease symptoms as well as in recovery from abnormal conditions.
Note A. The lack of function of the organs of the human body may be due to failure of the normal mental stimulation. The education of an individual may be so much at fault that he will fail to make efficient use of the energy which is offered him by his environment. The normal brain cells in their normal function incite the individual to rational endeavor, but because of irrational training, not every one replies to the impulses of the normal brain in a normal manner. The impulses which incite to normal endeavor are answered by compelled inactivity, or by forced efforts at pleasure; the impulses which incite to normal rest are answered by further stimulation. These abnormal reactions are the result of faulty education. They produce structural changes in the cells of the bodies, and these structural changes may be a fairly successful effort at adaptation to the abnormal environment, or they may be unsuccessful and so result in a pathological condition. The mind which derives less than its full complement of energy from its environment fails in the work and the joy of life; the body which derives less than its full complement of energy from its environment fails in its possibilities of development. |