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

by Max Verworn

A condition is in itself a thing of quite distinct properties. The properties of a thing are, however, determined by the specific combination of conditions which characterize the thing. The conditions by which a thing, that is to say, a state or process, is determined, are identical with its being and nature; in other words, they are the thing itself. Purely formal relations without essence would be altogether an absurd fiction not in accord with reality, and which even the science of mathematics does not acknowledge, for we cannot have a conception without concrete content, just as in nature we do not find a form existing independently of a thing. Every thing is equal to the sum of all its conditions and depending upon the uniform constancy in accordance with natural laws is solely determined by its conditions. The problem of all scientific research consists wholly in the ascertaining of the conditional interdependency.

A state or process is solely determined by the sum total of its conditions. A state or process is identical with all of its conditions in totality. From this it follows that equal states or processes are always the expression of equal conditions and wherever unequal conditions exist, unequal states or processes will result; and further, a state or process is completely investigated when the entire number of its conditions is ascertained.

This fundamental statement of conditionism should be engraved over the portals to the entrance of every scientific investigation.

That there is not the least difficulty in presenting scientific observations strictly according to these principles of conditionism, and that one can perfectly well do without the causal conception in a scientific description, I have shown by a concrete example, namely, in the fifth edition of my “General Physiology.” In the whole volume the conception of cause is only mentioned in one place, where its theoretical value is criticised, elsewhere not at all, and yet I do not think that any one will miss this conception, and indeed, if their attention is not especially called to the fact, even notice the omission.

These principles of an exact conditional investigation must also guide us in the analysis of the processes of stimulation. The process of stimulation is especially apt to tempt one to employ the old conception of cause, for it belongs to that group of processes which originate from an already existing system by the addition of a new factor. An electric stimulus acts on the muscle. The muscle contracts. The stimulus is considered the cause of the contraction. But what would I explain if I were to prove that the stimulation is the cause of the contraction?

The history of physiology shows us that this subject has advanced long since far beyond the stage of being satisfied with such an explanation. Today the process would only then be fully investigated if we knew the entire number of its conditions and had traced the dependency of the individual partial constituents of the whole complex process upon one another. For this, however, it is essential that we study the conditions already existent in the entire system previous to the action of the stimulus.

That which we describe with the word life is an exceedingly complex process. If we analyze life, it is found to be composed of an immense number of separate constituent processes, each one being conditioned by the others. These constituent processes are the vital conditions. A vital process occurs, and must occur, where and when the whole sum of vital conditions is realized. It is identical with the sum total of the vital conditions. If only one condition is absent, then life does not exist. It is then expedient to reserve the expression “life” for the entire sum of the vital conditions. When we speak of the individual constituent processes as “vital processes” in the plural, we must bear in mind that in reality each is not in itself life. Only the whole complex “lives,” not an individual constituent of the same. Living substance is rather the whole system, and not a constituent part of the same, not a piece of protoplasm, not a nucleus and not a specific protein combination in the cell.

A property of this system should receive our consideration at this point. It is a characteristic of every system in the world, namely, the fact that a system is not isolated from its surroundings. It is a deception resulting from the selective action of our sensory organs, if we consider the bodies as separated and isolated from their environment. This deception disappears upon further analysis and when we assist our organs of sense, which only respond to certain parts of the whole process, by experimental methods of investigation. Our experience then shows us that an isolated system does not exist, but that there are instead everywhere connections which extend further and further into the infinity of the world. An organism is consequently no delimitated system and the vital process cannot, therefore, be sharply separated from the processes in the medium. We cannot draw a sharp line between vital processes and say: on the right we have factors which are necessary for the maintenance of life, and on the left factors which are not necessary. The conditional connection between individual processes extends to the entire world, and likewise a great series of constituents, each influencing the others, extend from the medium into the organism. The nature of our sense perception, and consequently the knowledge derived therefrom, is such that we are obliged to arbitrarily take into consideration merely fragments from the endless interdependence of all things in the world, and so we separate the vital conditions of the organisms from their surrounding factors, as though they were independent. A conscientious theoretical analysis requires that we should never forget that in reality such an isolation does not exist. Only with the recognition of this can we distinguish for practical purposes between internal and external vital conditions. In such a differentiation the internal vital conditions which compose the living system conceived to be isolated, are the organs, the tissues, the cells, the protoplasm and the cell nucleus, and within the protoplasm and the nucleus the arrangement and quantitative relations of certain substances, such as proteins, salts, water and the thousands of special components with their interactions and continued alterations. On the other hand, the external vital conditions, which act on the periphery, are the conditions of the surrounding medium, as foodstuffs, water, oxygen, static and osmotic pressure, temperature, light, etc. But this distinction has only a practical value for the study of the organism as an independent system. Theoretically it is as impossible to make a sharp distinction between internal and external vital conditions, as to distinguish between the vital conditions generally and the more remote conditions of the environment. All these conditions form a widely branching system of factors of which one is conditioned by the other reaching continually from the interior of the vital system into the surrounding medium, so that on the periphery of the system it cannot always be said whether or not a component still belongs to life. Considering these circumstances we can roughly for the present define the conception of stimulus as follows:

A stimulus is every change in the vital conditions.

The most essential point in this definition is the relation of the conception of stimulus to that of vital conditions. These relations, however, call for a brief explanation. Here again the conditional method of observation saves us from error, for it would be wrong to place the conception of stimulus and vital conditions in contrast to one another, one excluding the other. On the other hand, this method of observation shows that the stimuli are likewise only conditions, but conditions producing certain changes in the vital system. If a stimulus acts, that is, if there is any change whatever in the vital conditions, the whole complex of life in consequence of the dependency of the constituent parts upon each other is also changed, and a new state of living substance occurs. Stimuli are, therefore, also only vital conditions, but vital conditions for new vital manifestations. The relation of one given state to another, forms an indispensable point in the understanding of vital conditions as well as that of the stimulus. The stimulus becomes a vital condition for the new state which it produces. It is only a stimulus relatively to the original state, which previously existed. The essential point, therefore, in the conception of the stimulus is that of alteration. An example will serve to make this clearer. If Amœba limax are bred in a hay infusion they appear in countless masses. Observed in water in a watch glass they show at first the well-known form of Amœba proteus with short, broad, lobate pseudopods. (Figure [1], A.) After a period of rest, however, they gradually assume the characteristic elongated limax form. (Figure [1], B.) In this shape they constantly move about. But if I add to the water only a faint trace of diluted solution of caustic potash, the amœbæ first assume the shape of a ball (Figure [1], C), and then after a time, stretch out long, pointed pseudopods, which give them the characteristic form of Amœba radiosa. I have observed them for several hours at a time. (Figure [1], D and E.) They remain permanently[16] in this form. They move in the same manner as Amœba radiosa. They draw in one pseudopod, stretch out another and float freely in the water in contrast to their limax state, in which they are always attached to some support. The long, pointed, often threadlike pseudopods, yield to every movement of the water, bending in consequence like whipcords. In this example the amœbæ under the vital conditions existing in tap water have limax form. The vital conditions undergo a change by the addition of a solution of caustic potash, which acts as a stimulus. The consequence is a reaction, in which the animal assumes radiosa form. By the action of the stimulus a new state of the living substance is produced, and remains as long as the solution of caustic potash is contained in the medium. The solution of caustic potash is, therefore, a stimulus for the state of the vital system, which is manifested in the limax form, whilst for the state of the system which shows itself in the radiosa form, it is a vital condition. If I place the amœbæ of the radiosa form once again in tap water, they assume the proteus and then the limax form. The withdrawal of the solution of caustic potash, the presence of which is a vital condition for the radiosa state, acts as a stimulus, which results in a transition of the vital system to another state. By altering the medium I can at will bring about this change of form in the same individuals. In this way one and the same factor can figure as stimulus and vital condition, according to the state of the vital system on which it acts. Whilst its addition acts as stimulus in the one state, its withdrawal acts as a stimulus in the other state, which it has produced. The same fact is shown by the well-known example of Artemia salina, which on being placed in fresh water changes into Branchipus stagnalis and, when again introduced into sea water, becomes once more Artemia salina.