As _Johannes Müller_ already clearly emphasizes, it is here not the question of a law confined to the sense substance, but one that applies to all living substances. Every living substance has its “specific energy,” that is, its characteristic vital phenomena and this is produced by stimuli of the most varied kind. This doctrine received an extension of inestimable value for its future development by the great discovery of _Schleiden_, that the cell is the elementary building stone of the plant organism. Subsequently _Schwann_ at the instigation of _Schleiden_ made further investigations and found that this discovery applied also to the animal organism. Irritability having been recognized as a general property of living substance, it followed that, after the foundation of the cell doctrine, every cell must possess irritability and have its own specific energy. It now became necessary to study the manifestations of irritability of the cells in their specific form. Strange to say, this was done at an earlier date in pathology than in physiology. Indeed, since the time of _Brown_ the study of irritability was furthered far more by pathology than by physiology. The chief reason for this is probably the great practical interest that the investigation of disease possesses, _Brown_ having already quite correctly ascribed the existence of disease to the relations of the organism or its parts to stimuli. _Rudolph Virchow_ then, after the establishment of the cell doctrine, arrived at the momentous conclusion, that disease must be considered as reactions of the body cells to stimuli. In his epoch-making “Cellular pathologie,”[7] he has carried out this idea in a classical manner. By irritability _Virchow_ understands “a property of the cells, by virtue of which they are set into activity, when affected by external influences.” There are, however, _various_ kinds of actions which can be brought about by external influences. But essentially there are three kinds. The effects produced are functional, nutritive, formative. The result of excitation, or if one will, of stimulation of a living part, can, therefore, according to circumstances, be either merely a functional process, or there can be a more or less intense nutritive activity produced without the function being necessarily at the same time activated, or finally, it is possible that a process of formative change may occur which produces new elements in greater or less numbers. _Virchow_ touches here for the first time upon a question of extraordinary moment, the important bearings of which have only now begun to be recognized and seriously considered. We now know, for example, that the functional excitation can be separated to a certain degree from the cytoplastic excitation of the muscle. If the muscle is acted upon by functional stimuli, the excitation takes place mainly in the form of functional metabolism, nitrogen-free substances are broken