Senescence, the Last Half of Life

by G. Stanley Hall

Life is growth; the retardation of growth is old age; and its cessation is death. “Senescence is at its maximum in the very young stages, and the rate of senescence diminishes with age” (p. 250). The embryo in its earliest stages rushes toward old age at an almost inconceivable velocity, the new-born infant runs, the child walks rapidly, youth saunters, the adult mopes, and old age only crawls on toward death. In other words, the momentum of life given by impregnation at the age of zero is retarded—most at first and with a diminishing rate at every stage.

Something like this same paradoxical law holds, Minot believed, for the human brain and mind. In one of his Harvey lectures he tells us that the brain of a child at birth is but little differentiated. During the first year it learns all the great adaptations in the physical and human world: time, space, ego, etc. “It learns more during the first year than in all the subsequent years of life” and from birth on the power of learning is rapidly diminished. It declines very fast during infancy, more slowly in childhood, etc.

Accepting Metchnikoff’s dictum that senility is atrophy and that toxins of intestinal origin poison and debilitate tissues so that they succumb to, if they do not actually attract, the predaceous phagocytes (though not proposing his substitute of sour milk for religion and philosophy), Minot points out that we are always throwing off dead cells. Blood corpuscles collapse and are utilized by the liver; the skin is incessantly shedding dead cells, as is the whole intestinal tract and each organ; stature declines some 13 cm.; the brain loses some 19 gms. in weight; the rate and depth of respiration sink; the heart, although growing larger and from the age of prime to senility beating some eight times per minute faster, is nevertheless obstructed in its action by rigidifying arteries; the bones grow spongy and their hard outer part becomes a thin shell; the muscle fibers decline both in size and number, exercise being able to increase only the former and not the latter; both structure and function go on to rigidity and inflexibility after sufficient firmness and size have been attained, till the part becomes too hard and inflexible to function and then is shed as the ripened leaf falls in autumn. But none of these processes are abnormal and hence death is in no sense a disease. Indeed, the power of repair and even recuperation persists far more in the old than has been generally recognized.

The more specific cause of what is generally called old age he finds in the increase of the quantity and the hyperdifferentiation of the structure of the protoplasmic envelope of the nucleus. This protoplasm constitutes the body of the cell. In the earliest stages of cytomorphosis, which follow impregnation, the total amount of nuclear material increases fastest, while later and especially in the senescent cells it is the protoplasm that does so. In the early stages of their embryonic development, too, the cells differ relatively little; but those that constitute the adult body differ so greatly that any skilled observer can tell from which organ they came, whether from the brain, muscle, skin, stomach, liver, etc.; that is, they differentiate more and more as these organs mature. This differentiation is, however, all on the way to death and is never reversible; that is, old body cells never grow young. Nuclei change but it is the protoplasm that changes the most and acquires a new structure, while the composition of the nucleus not only changes less but always retains certain fundamental traits. “The increase of the protoplasm, together with its differentiation, is to be regarded as the explanation (or should we say cause?) of senescence” (p. 134). This is necrobiosis. All old cells, from whatever organ, are thus as recognizable as old faces. “Growth and differentiation of protoplasm are the cause of the loss of the power of growth” (p. 161). He even holds that the first stages of the segmentation of the ovum must be called rejuvenation. On page 167 he says:

The life of the cell has two phases—an early brief one during which the young material is produced and the later and prolonged one in which the process of differentiation goes on; and that which was young, through a prolonged senescence becomes old. I believe these are the alternating phases of life, and that as we define senescence as an increase and differentiation of the protoplasm, so we must define rejuvenation as an increase of the nuclear material. The alternation of phases is due to the alternation in the proportions of nucleus and protoplasm.

In adults, and even in the old, there are always young cells in reserve, often grouped in certain foci, for example, the marrow of the bones, which can in emergencies come forward, take up the function of growth, regenerate lost tissues or, in lower animals, even lost organs. At and even after the death of the aged there are always cells and even parts that are relatively young and growing. There are also, of course, the cells and their matrix, which are very early set apart for the purpose of reproduction, and these, of course, are least of all differentiated. Most cells of the body, however, follow the law of genetic restriction. This means that as differentiation proceeds, the possible directions in which cells can develop become more and more limited till finally they cannot divide at all and lose even the power of nourishing themselves, and so die. The cell and all of it represents life, and Minot has no use for any of the smaller metamicroscopic vital units, gemules, plastidules, plasomes, ideosomes, granules, etc., but thinks that if we wish to accept any kind of ultimate elements of this sort, Weismann’s scheme of them is perhaps, on the whole, the best.

As to the practical questions, how we can help rejuvenation and delay senescence, he states that he has nothing to suggest, although he believes it possible that some time in the future a means may be found of increasing the activities and volume of the nucleus and restricting the growth and differentiation of the protoplasm, which would mean a prolongation of youth.

Minot concludes his volume with a glance at paidology in order to stress the great relative importance for both the bodily and mental development of the early stages of life. The baby develops faster than the child; the child, than the youth, etc., and the rate of psychic unfoldment declines very rapidly from the first, as does that of the body. Week by week, from birth, there is a remarkable expansion of life. Each one of the senses learns how to function effectively and most of them learn to attract the attention, the power of correlating movements and making voluntary ones, and the rudiments of memory and association are laid down, as are the bases of disposition. The infant from the earliest months of its life knows much of the persons and objects in its environment and perhaps has even discovered its own ego. It touches, handles, tastes everything; is an inveterate investigator in an ever widening field of research; has at least a sense of intercourse and companionship; is already at home with time, space, cause, and relation; its feelings, will, and even intellect are developed, and in this order; and the foundations for knowledge and achievement are laid. Thus the child of school age is already senile so far as its infancy is concerned and the boy’s psychic processes are retarded, hard, and unspontaneous. Learning begins to be difficult. Nature no longer shoots the mind up the phyletic ladder but it must climb and grow henceforth by work as well as playwise. Thus man’s mental powers show the same law of progressive retardation as does his physical growth. Instead of drawing the dead line at forty, as Osler did, Minot draws it at twenty-five. Had he been versed in paidology or even known the Freudian conceptions of infancy, he might have greatly amplified his treatment of this stage of the psychic life with which his volume closes. But as it is, there are certain definite criticisms of his conclusions concerning gerontology.

First, as I have said, he only attempts to show the cause and has nothing to say as to the cure of senescence. But he was not in quest of a panacea and was too true to the limitations of his science to pretend to have found one. This will be a disappointment only to those laymen who read him in furtherance of this pragmatic quest.

More serious is the objection that, according to his criterion and curves of declining growth rate, we are really old when we stop growing, for the mature young man and the very old one both are living but a very little above the deadline. On this view, the extinct saurians that grew all their lives were far more vital than creatures that attain a relatively fixed and constant size early and then stop growing. Growth is one measure of vitality, but surely function is another. The dynamic curve of energy and the power of work rises rapidly as that of growth declines and the curve of brain work reaches its apex somewhat later. Determining the increment of pounds or even of foot pounds of energy is not the sole measure of vitality.