The problem of aging is so immense that I should like to take a few moments for orientation. In order to comprehend any complicated situation involving human affairs or involving natural affairs in which human beings are concerned, such as floods, wars, volcanoes, divorce, marriage, aging, and other potential catastrophies, we need to look at it from three perspectives. If we look at it only from one or two of these perspectives, we assume a very asymmetric viewpoint. The three perspectives can be easily defined by using magnification as our basic simile. First, we must examine the problem with the naked eye. The individual is indivisible; body and mind are one. The individual is the unit. Second, with a microscope, we can take this person apart with blood counts, kidney function tests, blood pressure observations, basal metabolism determinations, and various other procedures. When we study the individual microscopically and dissect him into his biological components, the cell becomes the unit of thought, rather than the individual. Lastly, we must stand back far enough so that we may look at this individual with a telescope and see him in relation to the total environment, social and physical, economic, biological, and historical.
The science of aging, or gerontology, includes all three of these facets or perspectives. Man is the core. Our motivation for study is concern with ourselves. . . .
The second division of gerontology is concerned with the biology of senescence. Here the cell is the unit of thought. Each of us is composed of approximately two billion cells. The world population, in contrast, consists of about two billion people. The third division of gerontology is concerned with the sociologic problems of the aging in this crowded world. . . .
Aging may be defined as the element of time in living. Aging is part of living. Aging begins with conception and terminates with death. It cannot be arrested unless we arrest life. There is no elixir of eternal youth, thank goodness! It would be dreadful to remain infants all our lives. We may retard aging or accelerate it, but we cannot arrest it while life goes on, because it is essentially an element of living. Living is a continuous process, variable in its rates. Aging slows as we grow older. This is one of the compensations for later years. Aging change is rapid in youth and even more rapid prenatally in the period between conception and birth.
Aging involves two simultaneous processes which operate continuously in spite of the fact that they are contradictory to one another. On the one hand growth or evolution occurs, on the other atrophy (which means shrinkage) or involution. These processes continue throughout life, though at varying rates. We can observe illustrations of atrophy even before the infant is born in the disappearance of the gill clefts which first develop and then atrophy in the early mammalian embryo. At the time of birth, when the child begins to breathe and get its oxygen from the lungs instead of from the mother's circulation, the atrophy of certain arterial structures is indistinguishable under the microscope from the involutionary changes which we see late in life. The atrophic process is the same in the newborn infant and in the senile grandparent. A very interesting phenomenon occurs in the placenta or afterbirth. It becomes atrophic or "old" when its functional life is near termination. At nine months of pregnancy, these exists an intimate proximity and interdependence in a very young baby, a middle-aged mother, and a senile placenta. Biologically adjacent and functioning together are three widely divergent biological ages. Here is an area of study which has by no means been explored adequately.
On the other hand, growth continues late in life. The hair continues to grow throughout life, and certain hairs grow more vigorously than ever. The eyebrows become shaggy, and often about the ears there is a growth of bristly hairs that were not there in middle age and youth. The whiskers become heavier, more brittle, and thicker. The lower jaw continues to grow throughout life, according to reports of several anthropologists. These students of anatomy, however, do not tell whether the rate of this growth in later years varies with the amount of wagging that the lower jaw has done during the lifetime. It is quite possible that such functional activity is a modifying factor, as we shall see later. I am not being facetious.
Cells are the ultimate units of life. What can we learn about the aging of cells? Here the biologist in search of information is blocked, because individual cells do not age. They grow to maturity, and then divide to create two young daughter cells. Individual cells simply do not age in the sense that an individual ages. With the exception of a very few cells in our central nervous system, the brain, and spinal cord, none of the cells of our bodies is as old as we are. They are being replaced constantly with young cells. You and I are constantly rubbing off our hides and developing new skin underneath. The same process, without the friction, occurs in all other structures. So we come up against an insurmountable difficulty in studying the aging of the individual cell, because the individual cell does not age.
The limiting proviso is a fundamental clue to the mechanism of aging. Continuation of a culture of cells is possible only if the culture medium in which it grows is sterile and refreshed every forty-eight hours. If the medium is permitted to become depleted of necessary food elements and allowed to accumulate the toxic garbage of the living, growing cells, the tissue culture quickly degenerates and dies. The essence of senescence lies not in the cellular structure, but in the matrix fluid in which they live. This intercellular medium is often spoken of as the internal environment.
Perhaps some of you do not realize that we live in two environments-an external environment of social strife, stale air, tobacco smoke, grime, grit, wind, snow, competition, sunshine, love, and hate, and an internal environment which is extraordinarily stable. The internal environment varies very little chemically and very little physically. We are by no means healthy if our temperature varies more than a degree, or more than 1 per cent of the optimum. If the concentration of sugar in the blood rises above a certain level we are no longer biologically healthy. Or if the glucose content of the blood falls below a certain level we become ill. The same applies to the many other chemical elements included in the composition of the organism. There are many elaborate mechanisms for maintaining the constancy of this internal environment. If this internal environment is not maintained within a relatively narrow range, health is impaired. We may say that an optimum internal environment is synonymous with health; that an internal environment deviating from the desirable in some respects but still within tolerable limits, is equivalent to disease, but that any intolerable deviation of the internal environment leads to death. The constants, and by that I mean such factors as body temperature, pulse, blood pressure, chemical concentrations, and many other things which are relatively constant, are not actually changed by aging. These things are not absolutely constant, but vary within narrow limits. Normal body temperature is the same at eighty as it is at eight. Pulse rate varies relatively little with age under comparable conditions of rest. Of course, the child of eight is much more active and energetic than the individual of eighty, and thus reveals a more rapid pulse when active, partly because he is more active.