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often amongst vertebrates there are some animals the organisms of which withstand the ravages of time much better than that of man. I think it a fair inference that senility, the precocious senescence which is one of the greatest sorrows of humanity, is not so profoundly seated in the constitution of the higher animals as has generally been supposed. It is not necessary, therefore, to discuss at

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to propagate Infusoria to the 700th generation without any sign of senility being displayed. Here we are far from the condition in the human race.

1

R. Hertwig, one of the best observers of the lower animals, has recently attempted to show that the very simple animalculæ of the genus Actinosphaerium are subject to true physiological degeneration. He has several times seen cultures of this Rhizopod degenerate, until all the individuals had died, notwithstanding the presence of abundant food. Prof. Hertwig attributed this to the "constitution of the Actinosphaerium having been weakened by too great vital activity at an earlier stage." I should have thought that it was a much more natural explanation to suppose that the culture had undergone infection by one of the contagious diseases which so often destroy cultures of different kinds of lower animals and plants. As this idea had not occurred to the observer, he had not searched for parasitic microbes amongst the granulations which are always present in the body of an Actinosphaerium. However this may be, I cannot accept the facts brought forward by this distinguished German as a valid proof of the existence of senile degeneration in these lowly creatures.

The facts that I have brought together in this chapter justify the conclusion that human beings who reach extreme old age may preserve their mental qualities notwithstanding serious physical decay. Moreover, it is equally plain that the organism of some vertebrates is able to resist the influence of time much longer than is the case with man under present conditions.

1 Ueb. d. physiologische Degeneration bei Actinosphaerium eichhornii. Jena, 1904.

II

THEORIES OF CAUSATION OF SENILITY

Hypothesis of the causation of senility-Senility cannot be attributed to the cessation of the power of reproduction of the cells of the body-Growth of the hair and the nails in old age-Inner mechanism of the senescence of the tissuesNotwithstanding the criticisms of M. Marinesco, the neuronophags are true phagocytes-The whitening of hair and the destruction of nerve cells, as arguments against a theory of old age based on the failure of the reproductive powers of the cells

ALTHOUGH it has not been proved that living matter must inevitably undergo senile decrepitude, it is none the less true that man and his nearest allies generally exhibit such degeneration. It is therefore extremely important to recognise the real causes of our senescence. There have been many hypotheses on the subject, but there are comparatively few definite facts known.

Bütschli has supposed that the life of cells is maintained by a specific vital ferment which becomes feebler in proportion to the extent of cellular reproduction, but I cannot regard this as more than a pious opinion. The ferment has never been seen, and we do not know of its actual existence. According to the better-known theory of Prof. Weismann, old age depends on a limitation in the power of cells to reproduce, so that a time comes when the body can no longer replace the wastage of cells which is an

inevitable accompaniment of life. As old age appears at different times in different species and different individuals, Weismann has concluded that the possible number of cell generations differs in different cases. He has not found, however, a solution of the problem as to why multiplication of cells should cease in one individual, whereas it proceeds much further in other individuals. Prof. Minot, the American zoologist, has developed a similar theory, and has employed an exact method to determine the gradual diminution in the rate of growth of an animal from its birth onwards. According to him, the power of reproduction of the cells weakens progressively during life, until a point is necessarily reached at which the organism, no longer capable of repairing itself, begins to atrophy and degenerate. Dr. Buehler 2 has recently laid stress upon this theory.

There is no doubt that cells reproduce much more actively during the embryonic period. The process becomes slower later on, but, none the less, continues to display itself throughout the whole period of life. Buehler attributes the difficulty with which certain wounds heal in the case of old people to the insufficiency of cellular reproduction. He thinks in particular that the proliferation of the cells of the skin, to replace those which are worn off from the surface, becomes less active with age. According to him, it is theoretically obvious that a time must come when the replacement of the epidermic cells completely ceases. As the superficial layers of the skin continue to dry up and be cast off, it is plain that the epidermis must disappear completely. Buehler thinks that there must be

1 "Senescence and Rejuvenation," Journal of Physiology, 1891,

t. xii.

2 Biologisches Centralblatt, 1904, pp. 65, 81, 113.

a similar fate for the genital glands, the muscles, and all the other organs.

These theoretical considerations, however, are not compatible with certain well-known facts indicating that there is no general cessation of the power of cell reproduction in old age. The hairs and the nails, which are epidermic outgrowths, continue to grow throughout life, their growth being due to the proliferation of their constituent cells. There is no sign of any arrest in the development of these structures, even in the most advanced old age. The reverse is true. It is well known that the hairs on some parts of the body increase in number and in length in old people. In some lower races, for instance in the Mongols, the moustache and the beard grow vigorously in old age, whilst young people of the same race have only very small moustaches and practically no trace of beard. So also in white women the fine and almost invisible down which covers the upper lip, the chin, and the cheeks in the young may become replaced by long hairs which form a moustache or beard.

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Dr. Pohl, a specialist in the growth of hair, has measured the rate of growth in different circumstances. He has shown that in an old man of 61 the hair on the temple grew 11 mm. in a month; on the other hand, the hair on the same region in boys of 11 to 15 years old grew in the same time only from 11 to 12 mm. Plainly, there is no case here of a progressive diminution of cell-proliferation with age. The same observer, it is true, has shown that the hair of young men of between 21 and 24 years grew at the rate of 15 mm. a month, whilst in the same individuals, at the age of 61 years, the rate of growth was only 11 mm.; but this diminution in the rate of growth is only apparent. The first figure concerned the hair taken from different

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