The existing pathological character of old age vitiates all conclusions as to natural death, and it is still impossible to be exact in speaking of that subject. It is known that certain organs and tissues remain alive for some time after death. In the case of certain infectious diseases, the heart may be removed from a human body more than thirty hours after death, and if placed under proper conditions will renew its life, and beat for several hours. The white corpuscles, the spermatozoa and the cilia of a corpse, may retain their power of movement. Does this also happen in the rare cases of natural death? That question must be answered in the future. The most important question relating to natural death is the following: Is the appearance of natural death in man accompanied by the disappearance of one instinct, the instinct of self preservation, and by the appearance of another instinct, the instinct of death? Do the phenomena of the ephemerids give us any indication as to this? An exact answer is not to be expected. As old age is generally what may be called an unnatural phenomenon, it is extremely rare for persons to approach the age of natural death with their faculties unclouded. I have had under observation a centenarian old woman, who still remembered some incidents of her youth; in her the desire to live was still strong, but her intellectual faculties were partially dim. Moreover, her brain, of which I have already spoken (p. 241), showed a marked degeneration of the nerve cells due to the activity of macrophages. I have obtained much information about a centenarian who was alive in Rouen in 1900, but a single glance at her photograph was enough to show that she no longer was in full possession of intelligence.[328] She was infirm in many ways. So also, Chevreul, the celebrated chemist, who died at the age of one hundred and three years, showed not the faintest wish for death; he clung to life, but his mental powers had grown weak. The cases to which I have referred are typical, but there are exceptions worthy of close attention. Tokarski, in the essay on the fear of death, to which I referred in the sixth chapter, quoted the case of a female centenarian who stated as follows: “If you come to live as long as I have lived, you will understand not only that it is possible not to fear death, but to feel the same need for death as for sleep.” A new feeling had come into existence in the very old person, a feeling incomprehensible to those less old. Apparently this was a case in which the instinct of natural death had appeared in a centenarian whose mental faculties had been retained in a sufficiently perfect state.