when the equilibrium is lost and when the subject under observation has passed the point where senilization has gone through several changes, and proceeds rapidly, and can not be checked.
Life will flow on with normal energy so long as the noble elements, the more highly differentiated cells, are in excess both in processes and activity. The noble elements are those cells which take upon themselves the preponderating role in accomplishing the function of an organ, in contradistinction to those which play but a secondary part as forming the mechanical support of the organ. Since these can not be replaced in due proportion, function will be interfered with and senescence will begin. Connective tissue now tends to fill all gaps and gradually to invade the tissues, and scleroses will arise, placing obstacles in the way of functional discharge; this constitutes disease. "Inasmuch as the individual is merely an aggregation of special organs adapted to a common existence, the increasing deterioration of these functional activities leads toward gradual deterioration of the individual himself, who will gently fade away out of existence" (Tessier).
The progress of atrophic changes is not regular, either in the general system or in the special organ. All the elements of the mass do not live to the same age. The constituent elements undergo a perpetual restoration, the older disappearing and being replaced by others which have been long maintained in a state of less differentiation, hence of less specialization. As the completed elements disappear the younger ones are matured, hence the compensation is established between atrophy and repair. This movement of partial renovation in tissues is a picture in little of life, the birth of each element, its functional life, senility and death. The explanation of why irregularity should occur in the nutritive activities in the tissues of each organism, and equally in the whole of some organisms, causing individual and constitutional variations, is not so clear. Chemical processes, presumably similar to the small modifications in the cellular arrangements, and the forces that work, must be recognized. In time we may—indeed we must—know what these dynamic features are; then we shall have reached the first step in controlling these variations from a sound working basis. It is certain that these dynamic modifications can not progress indefinitely without producing tangible modifications and alterations in molecular activities; this constitutes disease. Atrophy is an anatomical phase of senility, whose irregular distribution is explained by the inequality of cellular existence, and this is again dependent upon the initial impulse of contraction and upon varying states in the medium, and this by the introduction into the tissues of matters foreign to its normal structure. When this occurs it is degeneration. The study of pathologic changes, by which most of the observations have been
