One very peculiar difficulty in treating old patients is their prejudice and obstinacy in matters of diet and hygiene; because they have lived so long they naturally think they know better than anyone else what is good for them and, with a certain irritability, resist interference. As a rule, they are relatively very susceptible to the action of drugs and two-thirds of the ordinary dosage for adults generally suffices. Very often the very aged suffer from the neglect of personal cleanliness. There are many diseases common from the sixth to the ninth decades of life that are very rare later, for example, tuberculosis. Suicide is very rare, the census of 1910 recording only one of a centenarian and nine among nonagenarians. The latter generally have a long-lived ancestry and many families are remarkable for this trait. But this is by no means essential. The aged often exhibit no predominant symptom of any one disease to which their death is attributable, and hence “senility” is so often given as the cause of death. Tranquillity, moderation, and regularity seem to be the chief factors in securing a long life and a peaceful death. * * * * * M. L. Price, M.D.,[158] attacks Metchnikoff’s theory of old age by assuming that in addition to all toxins and inflammations there is always an old-age exhaustion of a vital principle that he calls bioplasmine, that may, to be sure, be mainly caused by Metchnikoff’s agencies but is equally affected by effort, exposure, growth, and reproduction. He sagely predicts that the solution of the problem of old age, which he thinks is the central theme of all medicine in the sense in which he conceives it, because every disease brings senile phenomena to some part of the organism or to the whole of it, will be solved by biochemistry, although he admits that it may take many generations of investigation to achieve this final solution. * * * * *