medium temperatures exciting with increasing intensity, and higher temperatures from a certain height upwards again depressing. The effects of stimulation are not, however, always manifested in merely quantitative changes of the normal vital phenomena. Sometimes, especially in the case of long uninterrupted and chronic stimuli, stimulation is found gradually to produce phenomena which are apparently quite foreign to the normal vital phenomena of the cell in question. Such qualitative alterations of normal vital phenomena are perceptible chiefly in chronic maladies in the cells of different organs (the heart, liver, kidneys, spleen, &c.), in which the vital conditions become gradually more and more modified by the cause of the malady. To this category pertain all the so-called chronic processes of degeneration which in pathology are known as fatty degeneration, mucous degeneration, amyloid degeneration, and so forth. The characteristic element in all these processes is that the normal metabolism is diverted into a wrong channel by the altered vital conditions of the cells of the organ affected, so that substances are formed and accumulated in the cell which are entirely foreign to its normal life. But this class of stimulation is still very obscure as regards causes and inner processes, and it is within the range of possibility that the ultimate cause of the qualitative changes in the normal metabolism is to be found simply in the processes of excitation and depression which chronic stimulation produces in separate parts of the metabolism. Thus, at least with regard to fat-metamorphosis (fatty degeneration), it is highly probable that fat is deposited in the protoplasm simply because, owing to an inadequate supply of oxygen, it cannot, when it originates, be oxidized in the same proportion as it is formed, whereas in the normal cell all fat which originates in metabolism is consumed as soon as it is produced. According to this conception, therefore, fatty degeneration is attributable primarily to a depression of the processes of oxidation in the cell. If we may accept this view as correct with regard to the other metamorphic processes also, the qualitative changes in vital phenomena under the influence of stimuli would after all depend simply upon the excitation or depression of the constituent parts of the vital process, and, according to such a view, all stimuli would act primarily only as exciting or as depressing agents upon the normal process of life.
In accordance with the three groups into which general vital phenomena are divided, it follows as a matter of course that the excitation or depression produced by a stimulus can manifest itself in the cell's metabolism, assumption of form, and manifestation of energy. The effects of excitation upon the production of energy are the most striking, and were therefore in former times frequently thought to have a claim par excellence to rank as stimulating effects. These reactions attract most attention in cases where the production of energy is proportionately very great—as with muscle, for instance, which is made to twitch and perform work by a feeble stimulus. Processes of discharge (Auslösungsvorgänge), however, lie at the bottom of cases like these. Potential chemical energy, which is stored up in a considerable quantity in living substance, is converted by the impulse of the stimulus into kinetic energy. Therefore the amount of the effect of stimulation—that is to say, the quantity of work performed—bears no proportion whatever to the amount of energy acting as a stimulus upon the muscle. The amount of energy thus acting may be very small as contrasted with an enormous production of energy on the part of the living substance. It will not do to make generalizations, however, with regard to this proportion, as was frequently done in former times. All processes of stimulation are not processes of discharge. The influence of many stimuli, as has been observed, consists far more in depression than in excitation, so that in certain circumstances a stimulus actually diminishes the normal liberation of energy. There is therefore no general law as to the proportion which the amount of energy acting as a stimulus upon living substance bears to the amount of energy liberated.
Among special varieties of stimulation there is one class of stimuli which has attracted particular attention—namely, those which act unilaterally upon free-moving organisms. It is principally with the lowest forms of life that we have here to do—unicellular protista and free-living cells in the bodies of higher Directive Stimulation. organisms (sperm-cells, leukocytes, &c.). When from one direction a stimulus—be it chemical, thermal, electrical, or of any other kind—acts upon these organisms in their medium, they are impelled to move in a course bearing a definite relation to the source of the stimulus—either directly towards that source or directly away from it, more rarely in a course transverse to it. This directive action of stimulation is under such a fixed conformity to law, that it vividly recalls such purely physical processes as, for instance, the attraction and repulsion of iron particles by the poles of a magnet. For example, if light falls from one side upon a vessel full of water containing unicellular green algae, according to the intensity of the light these organisms swim either towards the illuminated side, where they form a compact mass on the edge of the vessel, or away from it, to cluster on the opposite edge. In the same way infusoria in water are observed to hasten towards or to flee from certain chemical substances, and leukocytes in our bodies act in the same manner towards the metabolic products of pus-forming bacteria which have penetrated into an open wound. The suppuration of wounds is always accompanied by an amazing conglomeration of leukocytes at the seat of the lesion. Perhaps the most striking effects are those of the constant electric current upon unicellular organisms, since in this case the motion follows the cause with absolutely automatic regularity, certainty and rapidity. Thus, for example, after the establishment of the current many Infusoria (Paramaecium) accumulate at the negative pole with great celerity and without deviation, and turn round again with equal celerity as soon as the direction of the current is altered. As such cases of directive stimulation may occur among all varieties of stimuli whenever stimuli act unilaterally, they have been designated, according to the direction in which they occur in relation to the source of the stimulus, as positive or negative chemotaxis, phototaxis, thermotaxis, galvanotaxis, and so forth. The strange and perplexing element in these phenomena becomes clear to us as soon as we know the characteristic method of locomotion for each form of organism, and whether the stimulus in question in the given intensity exercises an effect of excitation or of depression upon the special form. The direction of motion is the essential mechanical result of unilateral stimulation of the organs of locomotion. Seeing that these reactions are exceedingly widely distributed throughout the whole organic world, and possess a deep biological significance for the existence and continuance of life, the interest they have awakened is thoroughly justified.
One of the most important physiological discoveries of the 19th century was that of the “Specific Energy of Sense-substances.” “Specific Energy.” Johannes Müller was the first to establish the fact that very different varieties of stimuli applied to one and the same organ of sense always produce one and the same variety of sensation, and that, conversely, the same stimulus applied to the different organs of sense produces a different sensation in each organ—the one, in fact, which is its specific attribute. Thus, for example, mechanical, electrical and photic stimuli applied to the optic nerve produce no other sensation than that of light; and, conversely, any one variety of stimulus—take the electrical, for example—produces sensations of light, hearing, taste or smell, according as it affects the optic, auditory, gustatory or olfactory nerves. This law of the “Specific Energy of Sense-substances,” as Johannes Müller (1809-1875) called it, has come to have a highly important bearing upon scientific criticism, since it proves experimentally that the things of the outer world are in themselves in no way discernible by us, but that from one and the same outward object—the electric current, or a mechanical pressure, for instance—we receive altogether different sensations and form altogether different conceptions according to the sense-organ affected. But this law does not possess significance for psychology alone; as regards physiology also it has a much more general and more comprehensive force than Müller ever anticipated. It holds good, as demonstrated by Ewald Hering (b. 1834) and others,
