properly speaking chromophags), become migratory, and, quitting the hair, either find their way under the skin or leave the body (Fig. 16). Th colouring-matter of the hair is removed in this way

by chromophags, leaving the hair colourless.

The process by which hair becomes white is of importance, because it shows that the activity of macrophags is a dominant factor in bringing about senile decay. The brittleness of old people's bones is probably due to a similar cause, i.e., to the absorption and destruction of the framework brought about by macrophags invading the layers of bone. There is still much that remains unknown in this subject, which is well worthy of special research.

FIG. 16. Hair about to become grey. Chromophags transporting the pigment granules.

The activity acquired by macrophags during old age is closely connected with the phenomena that are characteristic of certain chronic complaints. Sclerosis in old persons belongs to the same category as organic sclerosis, which may be set up by various morbid influences. The analogy between senile decay of the kidneys and chronic nephritis, commonly called interstitial nephritis, is incontestable. The destruction of nervous cells

through the agency of macrophags, which we have already mentioned as occurring in old age, is equally a symptom of several diseases of the nervous centres, such as general paralysis of the insane. Arterial sclerosis in old persons is actually an inflammatory disease, similar to the inflammation of the arteries set up by other maladies.

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The similarity between senility and disease has long been recognised, and partly accounts for the repugnance we all experience at the approach of old age. In childhood and early youth people regard themselves as older than they really are, and long to be grown-up," but having once arrived at man's estate, they do not wish to grow old. An instinctive feeling tells us that there is something abnormal in old age. It cannot be regarded as a part of healthy physiological function. No doubt, because old age is the inevitable lot of mankind, it may be termed normal, in the same fashion as we call the pains of childbirth normal, since few women escape them. In both cases, however, we have to deal with pathological rather than physiological conditions. Just as every effort is made to relieve the sufferings of a woman in labour, so it is natural to try to suppress the evils accompanying old age, but whereas in childbirth an anæsthetic affords relief, old age is a chronic malady, a remedy for which is much harder to find. We have seen that in old age a struggle takes place between the higher elements and the phagocytes, the end being usually a weakening in vitality of the former, while the activity of the latter is enormously increased. It would appear, arguing from this, that one means of fighting against old age, pathologically speaking, would be to strengthen the higher elements of the organism, and to weaken the aggressive capacities of the phagocytes. Let me at once warn the reader that this is not presented as a definite, but as a

possible solution of the problem, and is offered for consideration like many other hypotheses on scientific questions. The properties of cellular elements are easily changed when subjected to various influences, and it is therefore not irrational to seek some means of strengthening the blood corpuscles, nerve cells, liver cells, muscular fibres of the heart, and so forth. The task has become easier since the discovery of serums that have specific actions on the tissues.

In the third chapter I stated that serums were known which give precipitates only with the blood of man and of his near relatives the anthropoid apes. serum of this kind has a definite specific action. Serums may be prepared that dissolve only the red corpuscles of particular species of animals, and that are without action on the other organic elements. It has been found possible, even, to prepare a serum that arrests instantaneously the movements of human spermatozoa, and that is neutral to the similar cells of other animals.

These serums are all prepared in the same way. The cellular elements in question, spermatozoa or red corpuscles, cells of the liver or of the kidney, taken from one animal, are injected into an animal of another species. After several injections have been made, the serum of the animal operated on becomes active with respect to the cells introduced into its body. These serums were discovered by J. Bordet of the Pasteur Institute, but the results have been confirmed by investigators in other countries. The serums are specifically cytotoxic, that is to say, they poison particular kinds of cells.

Now it has been shown that such serums, employed in small doses, do not kill or dissolve the specific tissue elements, but actually strengthen them.* Here the case is

* See the "Annales de l'Institut Pasteur," vol. XIV., pp. 369,

analogous with the action of poisons, such as digitalis, which kill in strong doses, but which in weak doses improve or strengthen the action of certain tissues. In accordance with this indication, experiment has shown that small doses of a serum which is capable of dissolving the red corpuscles of human blood, actually increase the number of those in the body of a patient treated by injections. In the same way, in the case of a serum large doses of which destroy the red corpuscles of a rabbit, small doses increase the number of these elements in the blood.

Here there seems to be a rational method by which we may strive to strengthen the higher elements of the human body, and so prevent them from growing old. The task. at first sight indeed, seems an easy one, only necessitating the injection of a horse (or other animal) with finely minced atoms of human organs, such as brain, heart, liver, kidney, &c., when serums could be drawn off in the course of a few weeks, capable of acting upon those organs. In reality the process would be a very difficult one to carry out, as human organs are rarely obtainable in a condition suitable for injecting into animals. Post mortems can only be legally made twenty-four hours after death, and there are many other obstacles in the way of removing organs from dead bodies. Even if all these difficulties were overcome, another difficulty that would present itself would be the experimenting with various doses of cytotoxic serums of various strength. It is not therefore to be wondered at that the 378, 390, 402. 1900. The results described therein have been confirmed by Bélonovsky (“Sur l'Influence de l'Injection de Diverses Doses de Sérum Hémolytique sur le nombre des Eléments du Sang." Saint Petersbourg, 1902), who has found that there is an increase in the amount of hæmoglobin and of red-blood corpuscles in the blood of anæmic patients that have been treated with minute doses of hæmolitic serum.

attempt to reinforce the higher elements of the human organism will require much time. If it be necessary to strengthen the higher elements (nervous, hepatic, renal, and cardiac cells), it is plain that they undergo a progressively weakening process. It would be of the highest importance to ascertain the cause of this, for the knowledge would be a guide to future action.

The similarity between senile decay and the diseases entailing atrophy in the more important human organs suggests a similitude in cause. Scleroses of the brain, kidneys, and liver frequently originate in intoxication by poisons such as alcohol, lead, mercury, and so forth, or the disease may be induced by some virus the virus of syphilis being a common cause.

The immense importance of venereal disease as a malevolent factor in the phenomena of old age, is especially manifested in arterial sclerosis. According to the careful investigations of a Swedish doctor, Edgren,* published in his "Monograph on Arterial Sclerosis," one case in every five of this disease is caused by syphilis, and he shows that chronic alcoholism is an even more frequent cause (25 per cent.). These two factors when united are responsible for nearly half (45 per cent.) the cases of arterial sclerosis that occur. Syphilitic virus and alcohol act as poisons which bring about first degeneration and brittleness of the arterial walls, and eventually a weakening of the higher elements of the organism. The phagocytes, being cells of an inferior order, are less sensitive to these poisons, which accounts for their victory over the poisoned elements.

Rheumatism, gout, and infectious diseases only play a secondary part in setting up arterial sclerosis. Edgren asserts, as the result of very careful calculation, that in ** "Die Arteriosclerosis." Leipzig, 1898.