_Necrobiosis._ _Atrophy._ _Degeneration._—The process according to the circumstances may be shortened or prolonged. When death is the result of violence events are precipitated. The physical and chemical transformations of the living matter constitute a kind of acute alteration called by Schultze and Virchow _necrobiosis_. According to the pathologists, there are two kinds of _necrobiosis_:—that by _destruction_, by _simple atrophy_, which causes the anatomical elements to disappear gradually without undergoing appreciable modifications; and _necrobiosis by degeneration_, which transforms the protoplasm into fatty matter into calcareous matter, into granulations (fatty degeneration, calcification, granulous degeneration). There is no disagreement as to the causes of this necrobiosis. They are always accidental; they originate in external circumstances:—the insufficiency of the alimentary materials, of water, of oxygen; the presence in the medium of real poisons destroying the organized matter; the violent intervention of physical agents, heat, electricity; the reflex on the composition of the cellular atmosphere of a violent attack on some essential organ, the heart, the lungs, the kidneys. _Senescence._ _Old Age._—In a second category we must place the mortal processes, slow in their movement, in which we cannot see the intervention of clearly accidental and abnormal disturbing agents. Death appears to be the termination of a breaking-up proceeding by insensible degrees in consequence of the progressive accumulation of very small inappreciable perturbations. This slow breaking up is adequately expressed by the term—growing old, or senescence. The alterations by which it is betrayed in the cell are especially _atrophic_, but they are also accompanied, however, by different forms of degeneration. An extremely important question arises on this subject, and that is whether the phenomena of senility have their cause in the cell itself, if they are inevitably found in its organization, and therefore if old age and death are natural and necessary phenomena. Or, on the other hand, should we consider them as due to a progressive alteration of the medium, the character of which would be accidental although frequent or habitual? This, in a word, is the problem which has so often engaged the attention of philosophical biologists. Are old age and death natural and inevitable phenomena? The recent experiments of Loeb and Calkins, and all similar observations, tend to attribute to the phenomenon of growing old the character of a remediable accident. But the remedy has not been found, and the animal finally succumbs to these slow transformations of its anatomical elements. We then say that it _dies of old age_.