This definition gives to the conception of stimulation its most complete, that is to say, its generally applicable and simplest form. The great importance from a methodical standpoint of this definition of stimulation for the research of life is evident. Our whole experimental natural science always employs for investigation of any state or process the same method: the state or process to be observed is studied under systematically altered conditions. By stimulating the living substance it is brought under changed external conditions. A systematic employment of stimulus is, therefore, the experimental means for the research of life.
CHAPTER III
THE CHARACTERISTICS OF STIMULI
Contents: The quality of the stimulus. Positive and negative alterations of the factors which act as vital conditions. Extent of the alteration in vital conditions or intensity of the stimulus. Threshold stimuli, sub-threshold, submaximal, maximal and supermaximal intensities of stimulus. Relations between the intensity of stimulus and the amount of response. The Weber and Fechner law. All or none law. Time relations of the course of the stimulus. Form of individual stimulus. Absolute and relative rapidity in the course of the stimulus. Duration of the stimulus after reaching its highest point. Adaptation to persistent stimuli. Series of individual stimuli. Rhythmical stimuli. The Nernst law.
We have found that stimuli are alterations in the external vital conditions and that the irritability of living substance consists in the capability to respond to stimuli by changes of the vital processes. It now behooves us in the interest of experimental research to investigate the relations between the nature of the alterations in the external vital conditions on the one hand, and that of the alterations of the vital process on the other; that is to say, to systematically study the effects of stimulation on the living organism. For this purpose it is above all necessary to become acquainted with the almost countless numbers of alterations which take place in the external vital conditions of an organism, and to create a systematic scheme of stimulation which differentiates and presents in comprehensive order those various elementary factors which, among the innumerable varieties of stimuli, would prove effectual. For this purpose it is necessary to select the various factors which are involved in an alteration of the external vital conditions.
The first of these factors is the quality of the stimulus. The external vital conditions are, in short, a series of chemical factors, such as foodstuffs, water and oxygen; the presence of a certain temperature; the existence of a certain light intensity; the existence of a definite static pressure; and finally the presence of an equal osmotic pressure. The stimulus according to its quality can be differentiated into chemical, thermal, photic, mechanical and osmotic varieties. To these must be added other forms of stimuli not ordinarily operative, for instance, many uncommon chemicals, and certain kinds of rays. The form of stimulation, par excellence, which has acquired the greatest importance for the experimental investigation of life, is electricity. In its manifold forms it permits, as no other, of such fine gradations of intensity and duration that it has become in the hand of the physiologist an invaluable means of research.
Alterations in those factors which act as vital conditions compose the great mass of physiological stimuli which act continuously on every living organism. The first point to be considered in every alteration is its direction. The alterations produced by stimuli may be of two different kinds, either positive or negative. The quantity of foodstuffs, water or oxygen, in the surrounding medium, can undergo an increase or diminution; as may the temperature, intensity of light, the atmospheric and osmotic pressure. The strength of the electric current, which may be applied, can also be regulated. In accordance with the definition of stimulation already referred to, we must consider these alterations, whether negative or positive, as forms of stimulation. Now the question arises: Is this point of view justifiable? Should one also consider, for example, the lessening or total removal of a vital condition as a stimulus? Should one consider the removal of water or oxygen, cooling or darkening, as a stimulus? It has, in point of fact, been occasionally attempted not to regard these negative deviations as forms of stimuli. These observers permitted themselves to be led by the dogma, that only that which produces an excitation, that is, an increase of the processes in the living substance, should be regarded as a stimulus. Such a limitation of the conception of stimuli would only result from the one-sided consideration of an all too limited circle of facts. Considered from the point of view which results from a broader range of experience, this narrow view becomes untenable.
In the first place it does not follow that only positive fluctuations of a factor, acting as a vital condition, result in excitation in the existing vital processes. The withdrawal of water produces a diametrically opposite effect. A muscle, from which water has been removed, if exposed to dry air or placed in a hypertonic salt solution, shows violent excitation, which manifests itself in great increase of irritability and development of fibrillary contractions. The breaking of a constant current which has for a long time flowed through a nerve or muscle also elicits a momentary excitation. Further, the abrupt removal of light may also bring about stimulation. To cite an example from the physiology of the single cell, I should like to call to your attention the interesting observations of Engelmann[17] on the Bacterium photometricum, of which he was the discoverer. When the field containing these organisms is suddenly darkened, all the individuals contained in the drop immediately dart forward for some distance, at the same time, as is usually the case, quickly rotating around their own axis, and then after a moment of immobility, swim on quickly in another direction. An analogous responsivity has also been shown by other single cell organisms, as has been pointed out by several observers and especially by Jennings.[18] In all these cases the excitation was produced by a lessening or total withdrawal of the factors which act as vital conditions; and even those who take the standpoint that only such factors are to be considered as stimuli which produce an exciting effect, are compelled to regard these alterations as stimuli, in spite of the fact that they are negative variations of external vital conditions.
But further, the restriction of the term stimulation to those alterations which increase the course of the changes in the living substance involves the observer in still greater contradictions. It can easily be shown that one and the same factor in one and the same form of living substance has now an exciting, now a depressing effect on the vital processes. This fact can be readily demonstrated[19] by means of the infusoria Colpidium colpoda, which can be grown without difficulty in a hay infusion. A number of individuals in a drop of fluid may be placed in a warm stage and observed under the microscope; one then sees that at room temperature they swim about by moving their ciliary processes at a definite rate. Now if the temperature is raised to about 35° C., the ciliary movement becomes enormously increased. The infusoria swim madly through the field of vision. They are in a state of violent excitement. The increase has, therefore, acted as a strong, exciting stimulus. But if one allows the temperature to further increase only a few degrees the ciliary movements are suddenly greatly retarded. The infusoria now swim sluggishly through the field of vision and finally remain stationary. In this case the increase in the temperature has had a depressing effect. If the infusoria are not quickly removed, the depression is followed by death. Should the increase in temperature be regarded in the first instance as a stimulus, and not as such in the second, in which the temperature rises only a few degrees higher? Here the change in the vital conditions concerned is in both instances positive. In all cases of overstimulation we are confronted by the same question. Nevertheless it is not at all necessary to refer to such strong or even life-endangering stimuli for the observation of these conditions. In this connection I would like to cite an even more striking instance and which is of special interest for the understanding of the phenomena in nerve centers. If the posterior spinal roots of a Rana temporara are severed, and the eighth root stimulated with a faradic current, whilst the musculus Gastrocnemius of the same side is connected with a writing lever, one obtains, as Vészi[20] has found, at the moment of the beginning of stimulation a contraction of the muscle. The faradic stimulus has, therefore, produced an excitation reflexly. If instead of the eighth the ninth posterior root is stimulated, the result obtained is also an excitation of the muscle. In this case, however, the excitation in the form of a tetanic contraction lasts for some time, provided that the stimulation is not at once stopped. If now during tetanic stimulation of the ninth root the eighth is at the same time stimulated, with a strength of current equal to that which previously brought about contraction of the muscle, instead of an increase and a strengthening of contraction there is, on the contrary, an inhibition which continues throughout the time during the stimulation of the eighth root. If the stimulation of the eighth root is discontinued, the tetanic response of the ninth root reappears. If, on the other hand, the faradic stimulation of the ninth root is interrupted and the eighth root now again stimulated, one obtains once more, as in the beginning, with each stimulation a contraction of the muscle. This fact is illustrated by the accompanying tracings. (Figure [2].) In this investigation undertaken in the Göttingen laboratory it was further shown that a faradic current of the same strength and the same frequency had at one time an augmenting, at another an inhibitory effect, and these effects could be produced alternately at will. Should the faradic current at one time be called a stimulus, at another not? It is here clearly shown to what absurd consequences it leads if the conception of stimulation is limited solely to the cases in which an external factor has an exciting effect; and yet an immense number of instances of a like nature could be cited to show the untenability of this view.
Fig. 2.