Irritability: A Physiological Analysis of the General Effect of Stimuli in Living Substance

by Max Verworn

Fig. 1.

These facts show clearly that some stimuli can also be considered as vital conditions. In the absence of certain stimuli, life could not exist for any length of time. In the highly differentiated cell community of the animal organism, for instance, as a result of the coexistence of the cells and the tissues, many parts have forfeited in a measure their independence. An example of this is the skeletal muscle, which, in the absence of impulses from the nervous system, reaches a low level of chemical change and energy transformation. Here the nervous impulses which act as momentary stimuli, are also in the course of time indispensable vital conditions. Without them the muscle would gradually become atrophied from inactivity. The same applies to all other tissues of our bodies. The functional stimuli are for them at the same time vital conditions. These vital conditions undergo fluctuations and interruptions but at each alteration from a given state they act as stimuli.

Stimulus is every change in the vital conditions. But is this definition complete? Are we really justified in regarding every alteration in the vital conditions as a stimulus?

In considering this question, one point must not be omitted. This is the fact that one of the chief characteristics of the vital process is, that it undergoes continuous change. A vital process involves not simply an alteration in metabolism or transformation of energy in the sense that the same chemical processes continuously reoccur in the same manner. Such a view could only be admissible for the observation of living substance during a limited period. An investigation over a long period of time shows rather that every living system alters as long as it exists, although this alteration is very gradual. The constituent processes, in short, continuously undergo metabolic change both quantitative and qualitative in nature.

If we observe the occurrences in a living system at various moments of the cycle of life, we will find that the condition differs qualitatively at each period. The progressive alteration of the system is such that every state of living substance conditions another, by which it is followed. No state can permanently exist as such. Every state is the product of the preceding, as it in turn conditions its successor. Consequently the relations of the system to the surrounding medium also undergo alteration, even when the external factors themselves in no way alter. That which today is still a vital condition, is not in consequence necessarily one tomorrow. These progressive changes exist continuously until the death of the system takes place. They characterize life. It is development, and life cannot exist without development. Death is only the last phase of development. The individual constituent processes of metabolism gradually change to such a degree that they can no longer work harmoniously together. Then the chain of processes is interrupted at one point or another. The system develops into death or, on the other hand—and this, as Weissman especially emphasizes, is realized in the case of unicellular organisms—a corrective process takes place, a process of cell division by which the original state of the cell is restored and development begins anew and in a similar manner.

Ought we to designate these constant alterations in the inner vital conditions as “stimuli”? Usage in this connection has already answered in the negative, by applying to them the word “development.” And this use is in a certain sense justified. Let us imagine an organism or any other object for the purpose of investigation as isolated from its surroundings. This conception, which we have already stated, proves untenable on closer analysis, but it, however, is based on the nature of the methods of human observation and is indispensable for practical use within certain limits. Then the inner vital conditions belong to the organism, the external to the medium. They differ in so far that the external vital conditions can exist permanently without alteration, that is, independently of the development of living systems, whilst the inner vital conditions of every living organism continuously and progressively undergo alteration. In this sense, but only in this, there is evidently a difference between the inner and outer vital conditions, which permits a separation of the two groups. But we should always bear in mind that this separation cannot be sharply defined. On the same basis we assume that the organism for purposes of study is separated from its surroundings as an independent system, which leads us in consequence to contrast the alterations in the internal with those in the external vital conditions, in which we designate the first as processes of development, the latter as stimuli. This distinction, as all differentiations and separations in nature, gives us only a practical working basis.

In this way we confine the conception of the stimulus to all alterations in the external vital conditions of a living system, considered as isolated. This view does not exclude the fact that stimuli can also occur and act within an organism. If a nervous impulse is conducted from the cerebral cortex through the pyramidal tract to a skeletal muscle, this impulse acts upon the muscle cells as a stimulus. Although the explosion of the impulse is an alteration within the body, nevertheless, as far as the muscle is concerned, it may be looked upon as an external vital condition, therefore as a stimulus. As the conception of stimulus involves the relation to a given state, it likewise involves at the same time the relation to a given living system, upon which it acts from the exterior.

What is the value then of all this theoretical discussion?

In presenting the conception of stimulation from a conditional standpoint, I desired to show what difficulties stand in the way of a theoretical isolation of a fundamental conception in the field of physiology, which indeed is used in our practical research work at every step. “Natura non facit saltus.” I wished to demonstrate that the sharp separation of the conception of stimulation, like all artificial divisions which we make in nature, must always contain an arbitrary note, as in reality isolated systems do not exist in the world. I wished to show that, for this reason, the conception of vital system, the conception of life, the conception of vital conditions are not sharply defined. I wished likewise to show that as a necessary consequence of this fact a sharp separation of the conception of stimulation, which can only be made in relation to that of vital conditions, cannot be maintained theoretically. I wished to show further that there is no sharp line of division between inner and outer vital conditions, and that we cannot, therefore, make a strictly theoretical distinction between the conception of stimulation and that of the processes of development. I wished to show that, for these reasons, we must not expect from the conception of stimulation, as we understand it, anything beyond its possibilities. But finally I wished also to show that, whilst fully conscious of and with due consideration of all these difficulties, it is possible to work out a definition of stimulation which is of great practical working value. The definition in short is: “Stimulus is every alteration in the external vital conditions.”