[40] _Nernst und Barratt_: “Ueber electrische Nervenreizung durch Wechselströme.” Zeitschrift für Electrochemie 1904. This exhausts the small number of elementary factors concerned in the course of the stimulation, and which are of importance in considering its effect. The combination of the different varieties of these single factors, that is, the nature, the direction, the intensity, the rapidity, the duration and number of alterations in the external vital conditions of the organism produce the enormous variety of effects of stimulation which we observe in the living world. CHAPTER IV THE GENERAL EFFECT OF STIMULATION _Contents_: Various examples of the effects of stimulation. Metabolism of rest and metabolism of stimulation. Metabolic equilibrium. Disturbances of equilibrium by stimuli. Quantitative and qualitative alterations of the metabolism of rest under the influence of stimuli. Excitation and depression. Specific energy of living substance. Qualitative alterations of the specific metabolism and their relations to pathology. Functional and cytoplastic stimuli. Relations of the cytoplastic effects of stimuli to the functional. Hypertrophy of activity and atrophy of inactivity. Metabolic alterations during growth of the cell. Primary and secondary effects of stimulation. Scheme of effects of stimulation. In the foregoing lectures we have had occasion to touch more or less often on the subject of the effects of the stimuli. This was the case, however, only when it appeared necessary to obtain a systematic knowledge of the stimuli and the differentiation of the individual factors. We will now proceed to consider the effect of stimulation in a more systematic manner. The conditional method of observation, however, will remain our guide. We have already pointed out the relations between the conception of stimulation and that of vital conditions, now we will consider that of the effect of stimulation with that of vital processes. Nevertheless, the _effect_ of stimulation being a manifestation of the vital process is not, therefore, in opposition to the latter as such. Hence the question presents itself as to the connections between vital process and the effect of stimulation. When we study the motile flagellate infusorium _Peranema_ swimming undisturbed in water, we observe that the swimming movements are absolutely regular in character. The elongated cell body remains unaltered in shape. The long flagellum is extended in a perfectly straight line in the axis of the body and only the extreme end lashes with regularity through the water (Figure 8, A). There is majestic grace in this perfect uniformity of motion. The picture suddenly alters the moment the _Peranema_ is influenced by the slightest jar. The whole flagellum at once executes a few violent movements (Figure 8, B), the body draws together, soon stretches itself again and swims immediately after, in another direction, with the same majestic calm as before.