The Prolongation of Life: Optimistic Studies. Élie Metchnikoff
the skull; the meningeal artery becomes sinuous and atheromatous, and the grooves on the inner side of the bones of the skull in which it runs, flatten out, and become larger because of other malformations.18
There is no disharmony in the nature of old people so striking as this transference of the lime salts from the skeleton to the blood-vessels, producing as it does a dangerous softening of the former, and a hardening of the latter that interferes with their function of carrying nutrition to the organs. It is the manifestation of an extraordinary disturbance of the properties of the cells that compose the body. The atheromatous condition of the arteries is closely linked with arterial sclerosis, an affection which is very common, although not constant, in the aged. The whole question of these vascular alterations is extremely complex, and before it can be cleared up, a number of special investigations must be made.
Probably diseases of the arteries of different kinds, and arising from different causes, are grouped under the terms atheroma and sclerosis. In some cases the lesions are inflammatory and are due to the poisons of microbes. An example of such an origin is the case of syphilitic sclerosis, in which the specific microbes (spirilla of Schaudinn) lead to precocious senescence. In other cases the arteries show phenomena of degeneration resulting in the formation of calcareous platelets which interfere with the circulation of the blood.
Investigations which have been made in recent years have led to very interesting results concerning the origin of atheroma of the arteries. In most cases, attempts to produce such lesions of the arteries by experimental methods have not succeeded, but M. Josué19 has been able to produce true arterial atheroma in rabbits by injecting into them adrenaline, the secretion of the supra-renal capsules.
This experiment has been repeated many times and is now well known. Later on, M. Boveri20 obtained a similar result by injecting nicotine, the poison of tobacco. It is obvious, therefore, that amongst the arterial diseases which play so great a part in senescence, some are chronic inflammations produced by microbes, whilst others are brought about by poisons introduced from without.
It is easy to understand, therefore, why these diseases of the arteries are not always present in old age, although they are very common.
The part played by the secretion of the supra-renal glands in the production of arterial disease has brought renewed attention to a theory which supposed that certain glandular organs in the body play a preponderating part in senile degeneration. Dr. Lorand21 in particular has argued that “senility is a morbid process due to the degeneration of the thyroid gland and of other ductless glands which normally regulate the nutrition of the body.” It has long been noticed that persons affected with myxodema, as a result of the degeneration of the thyroid gland, look like very old people. Everyone who has seen the cretins in Savoy, Switzerland, or the Tyrol, must have noticed the aged appearance of these victims, although very often they are quite young. The condition of cretinism, with its profound bodily changes, is the result of degeneration of the thyroid gland. On the other hand, it is well known that in old people the thyroid and the supra-renals frequently show cystic degeneration. It is quite probable, therefore, that these so-called vascular glands have their share in producing senility. Many facts show that they destroy certain poisons which have entered the body, and it is easy to see that, if they have become functionless, the tissues are threatened with poisoning. It does not follow, however, that their action in producing senility is exclusive, or even preponderating. M. Weinberg, at the Pasteur Institute, made special investigations on this point, and found that the thyroid gland and the supra-renal capsules were almost invariably normal in old animals (cat, dog, horse), although the latter showed unmistakable signs of senility. Similarly in an old man of 80 years, who died from pneumonia, the thyroid gland was quite normal.
It must not be forgotten that the aged very often die from infectious diseases such as pneumonia, tuberculosis, and erysipelas. In these diseases the vascular glands generally, and the thyroid gland in particular, are very often affected, with the result that what is due to infection has been set down as a symptom of old age.22
Although the appearance of patients from whom the thyroid gland has been removed, or in whom it has degenerated spontaneously, recalls that of old people, it is possible to exaggerate the similarity. In the masterly accounts of such unfortunates, recently compiled by the well-known surgeon Kocher23 there are many points which are characteristic, without being typical, of old people.
Oedema of the skin which characterises thyroid patients is by no means usual in old age. The loss of hair, normal in the patients, is not a character of old age. In myxedematous women, menstruation is very active; it ceases in old women. The great muscular development of myxedematous patients distinguishes them from old people.
Physiological investigation does not support the existence of any strong affinity between old age and affection of the thyroid gland. It is known that removal of the thyroid is followed by cachexia only in young subjects, MM. Bourneville and Bricon24 having shown that the tendency to cachexia after extirpation of the thyroid ceases almost abruptly at the age of thirty. That age may be taken as the limit of youth, of the time when growth is vigorous and the function of the thyroid most active. Cases of cachexia, where the thyroid gland has been removed in old persons from fifty to seventy, are very rare.
Rodents (rats, rabbits) support the removal of the thyroid extremely well, without signs of cachexia, although these are normally short-lived creatures. According to Horsley25 extirpation of the thyroid is not followed by cachexia in birds or rodents and is followed by it only very slowly in ruminants and horses; it produces the condition invariably but slightly in man and monkeys and extremely seriously in carnivora. If this series be compared with the information given in the next section of this volume on the relative ages which the animals in question attain, it will be seen that there is no correspondence.
In short, whilst I do not deny that the vascular glands may take a share in the causation of senility, in so far as they are destroyers of poisons, I cannot agree with the theory of Dr. Lorand.
I think it indubitable that in senescence the most active factor is some alteration in the higher cells of the body, accompanied by a destruction of these by macrophags which gradually usurp the places of the higher elements and replace them by fibrous tissue. Such a process affects the organs of secretion (kidneys), the reproductive organs, and in a modified form the skin, the mucous membranes, and the skeleton. The testes are amongst the organs which resist invasion by macrophags.
Fig. 12.—An old dog, aged eighteen years.
It is manifest that the tissues do not invariably degenerate in old age, nor do all the organs that are modified in old age show destruction by phagocytes and replacement by connective tissue. Organs which produce phagocytes, such as the spleen, the spinal marrow and the lymphatic glands, certainly show traces in old age of fibrous degeneration but remain sufficiently active to produce macrophags which destroy the higher cellular elements