THE NATURE OF STIMULATION
_Contents_: Principles of scientific knowledge and research. Origin
and meaning of the conception of cause. Cause and condition. Criticism
of the conception of cause. The conditional point of view. Conception
of cause. The conditional point of view applied to the investigation
of life. Conception of vital conditions. Definition of the conception
of stimulation.
The common problem of all scientific research is the investigation
and formulation of natural laws. The assumption of a unity in the
happenings and of existence in the world, in accordance with definite
laws, forms the indispensable foundation of all scientific study and
is fully justified by experience. Experience has taught us, as a
result of innumerable individual observations, the existence of such
an accordance, whereas in not a single instance has it been shown
that this is not the case. We are thus justified in assuming without
further discussion that every scientific research, every new problem
which we approach, is likewise founded on this unity of occurrences in
accordance with natural laws. Only on the firm basis of this assumption
has scientific investigation a purpose, and every success is a new
proof of this. There is an unanimity of opinion concerning this among
scientific investigators in all fields.
Not such complete agreement, however, exists in regard to the question
by what symbols of human thought and speech these laws can be described
in part as well as _in toto_, so that existing laws can not only be
_fully_ and conclusively defined, but at the same time without the
use of _superfluous_ terms. According to _Ernst Mach_, thought is an
adaptation to facts. Our speech is simply a method of expression of
our thoughts and indeed the most satisfactory form we have. We must,
therefore, use those symbols which are most closely adapted to facts
as the most precise expression of these existing laws. What forms of
expression have we?
It might appear that a discussion of this fundamental question
has not a close connection with our special subject of physiology
of stimulation. This, however, is not the case. Indeed, it is an
irremissibly previous requirement not only for the elucidation, but
also for the understanding itself in this particular field. We _could
not_ come to a clear understanding in this field without such analysis.
The interpretation of the unity of being and happenings in accordance
with natural laws, which today is widely accepted in the scientific
world as the only exact one, implies the assumption of a “_causation_”
according to which things are explained by the law of “_cause_”
and “_effect_.” I[14] have already on various occasions taken the
opportunity to criticise this view and to show the error and confusion
to which it leads. I should like here to enter somewhat more in detail
into the reason for this criticism. It is particularly directed against
the scientific use of the term “_cause_” on the basis of our best-known
theoretical principles. It is clear that all scientific observations
and explanations are founded on experience. Can it be said that the
conception of “cause” originates from experience?
[14] Compare with this _Max Verworn_: “Die Entwickelung des
menschlichen Geistes.” Jena, Gustav Fischer, 1910.
_Max Verworn_: “Die Erforschung des Lebens.” II Auflage. Jena,
_Gustav Fischer_, 1911.
The same: “Die Fragen nach den Grenzen der Erkenntniss.” Jena,
_Gustav Fischer_, 1908.
The same: “Allgemeine Physiologie.” V Auflage. _Gustav Fischer_, 1909.
We can say with absolute certainty that the conception of cause dates
from prehistoric times. Its beginning reaches back to the stone age,
at least to neolithic, possibly to palæolithic culture. This is
demonstrated by the careful reconstruction of these prehistoric races
based on a critical comparison of the remains of their culture with
that of primitive races living today. The ideas of these primitive
races show an inclination to an extraordinary degree to explain
all happenings in the world anthropomorphously. All happenings in
surrounding nature are given the same origin as the activities of man
himself. To man, on this plane of phantastic religious speculation, all
events in nature appear as acts of the will of invisible powers, which,
having originally proceeded from the souls of dead human beings, think,
feel and act exactly as _he does_. This anthropomorphic conception of
the occurrences in the surrounding world is one of the many conclusions
which ensue from the supposition of an invisible soul, which can be
separated from the body. It was this conception which gave the impetus
for the transition of human thought from the era of the naïvely
practical to the era of the theoretical spirit in that far removed
age. In this anthropomorphic transference of personal subjective
impulses of will to the objectively observed events of the surrounding
world, lies the origin of causal conception, which since then has been
generally used as the explanation of the happenings in the world. One
cannot assert that the formation of the conception of cause is purely
a product of _experience_, but rather a result of _naïve speculation_.
Even if a later evolution of human thought shows a continued endeavor
to dismantle the conception of cause of its primitive trappings and
to modernize, as it were, its outer appearance, we still find today
many inner components clinging to it, which do not agree with the
strict demands of critical scientific exactness, demands which must
particularly be made concerning a conception which has been given such
fundamental importance in theoretical knowledge.
I wish to observe here, however, that the conception of cause, even
though more or less unconsciously so, is still the remains of a part of
the old anthropomorphic mysticism carried over into our own times. This
shows itself especially in the conception of _force_, which is nothing
more than a form of the conception of cause. Force is the cause of
movement. One has here in anthropomorphic manner transferred the action
of the _will_ of man, which produces movement of the muscles, into
lifeless nature. The force of the sun attracts the earth, that of the
magnet attracts iron, etc. In short, one has introduced a mysterious
unknown factor instead of being content with the simple description of
facts, such as _Kirchhoff_[15] has advanced in the field of mechanics.
Although of late natural science has also dispensed more and more with
conception of force as a means of explanation, it is still today not
wholly done away with. That which applies to the conception of force is
likewise true of the conception of cause.
[15] _Gustav Kirchhoff_: “Vorlesungen über mathematische Physik.
Mechanik.” Leipzig 1876.
Another point concerning the application of the conception of cause
seems to me, however, to be of much more importance, namely that a
single cause is held responsible for the taking place of a process. One
endeavors to explain a process in general by seeking for its “cause.”
The cause being found, the process is considered as fully accounted
for. This idea is not only widely spread in everyday life, but is even
found frequently in natural science, especially in biology, although
here, it should be known, the processes are decidedly more complicated.
The search for the “cause” of development, for the “cause” of heredity,
for the “cause” of death, for the “cause” of the respiration, for the
“cause” of the heart beat, for the “cause” of sleep, for the “cause”
of disease, etc., was for a long time and frequently even today a
characteristic of biological investigation. As if such a complicated
process as development, death or disease could be explained by a single
factor! In reality, one has obtained very little as a result of the
analysis of a process by discovering its cause; and in addition the
false impression arises that through the finding of this one factor the
process has been definitely explained. It has been generally recognized
in the natural sciences in recent times that no process in the world is
dependent upon one single factor and attempts have been made to give
this fact more consideration.
It is the custom at the present time to hold the view that every
process or state is brought about by its _cause_, but that a series
of _conditions_ are also necessary to the production of the process.
Such a view, however, which considers that two different factors
existing at the same time are necessary to the accomplishment of every
happening or state, namely, the cause and the conditions, leads to new
difficulties, for then, upon a more exact analysis arises the question:
Which is the cause and what are the conditions? It is very soon found,
however, that this does not permit of any strict differentiation, as
the two conceptions can not be sharply separated. This difficulty
was brought to my notice with particular force during an animated
discussion with a friend and colleague about twenty years ago, which I
have always remembered. I had observed at that time the dependence of
pseudopod formation of amœboid cells on the oxygen of the medium, and
had found that the expansion phase of protoplasmic movement, that is,
the extension of pseudopods, the centrifugal flowing of the protoplasm
into the surrounding medium and with this the enlargement of the
surface of the cell body, only takes place when oxygen is contained
in the surrounding medium and never occurs in its absence. Being at
that time wholly under the influence of the conception of cause, I
believed that oxygen was the cause of the formation of the pseudopods.
To this my friend made the objection: “Yes, I quite acknowledge the
fact of the dependence of the formation of pseudopods on oxygen, but
what informs me that the oxygen is really the _cause_? It might be
simply a necessary _condition_.” This objection led to a long debate,
which ended, however, without our being able to agree. We were not in
a position to distinguish between the conception of cause and that
of condition, and at that time the idea _did not occur_ to us to
emancipate ourselves from the conception of cause deeply implanted in
us as a result of our training. In fact, one is greatly embarrassed if
one attempts to sharply distinguish by a definition the conception of
cause and that of condition. A condition is a factor on which a state
or a process is dependent for its existence or its taking place. To the
conception of condition belongs, besides the factor of _relation_, that
of _necessity_. Every condition is necessary to the existence or taking
place of this state or process. Without the condition in question the
state or process does not occur. The same must be demanded for the
conception of cause. No state exists, no process takes place, without
its cause. The cause then has itself the specific character of a
condition, it is itself a condition. Has it perhaps then some specific
peculiarity in contrast to the other conditions, which would give it a
prominent place? Experience teaches us that nothing, that is to say,
no state or process in the world, is dependent upon a single factor
alone. There are always numerous factors which bring about the state or
process. Would it be possible to distinguish which of these particular
conditions is of the greatest importance?
First of all, it must here be taken into consideration that the
importance of a condition is not one which is capable of increase
or decrease, for the simple reason that necessity, which forms an
essential component of the conception of cause cannot be varied. A
factor cannot be _more_ than necessary for the existence of a state or
the taking place of a process. If, however, it is less than necessary,
then it is not necessary at all, and the state or process exists also
without it, that is to say, the factor is not a condition. In other
words: _all conditions for a state or process are of equal value for
its existence, as they are all necessary_.
If one attempts to prove by means of concrete examples this statement
obtained by purely logical deduction--a control which, considering the
experimental nature of modern thought, never should be neglected even
in the simplest of reasoning--it might appear that an objection could
still be made against its general validity. From various instances it
might be concluded that there are conditions, which as such are not
absolutely necessary for a state or process, but can be replaced by
other factors. An example may serve to make this clear. I pour diluted
hydrochloric acid on powdered carbonate of sodium, and carbon dioxide
is set free. The addition of hydrochloric acid is here a condition
for the liberation of the carbon dioxide. Without the presence of
the hydrochloric acid the process does not occur. Nevertheless I can
substitute diluted sulphuric acid for the hydrochloric acid. Here it
would appear that one condition can be replaced by another. But one
must not be deceived. A closer observation soon shows that the process
has not been sufficiently analyzed if we look upon the addition of
hydrochloric acid as a condition for the liberation of carbon dioxide.
It is not the presence of hydrochloric acid or sulphuric acid, as such,
which is a condition for the process, but rather the separation of the
sodium atoms from their combinations with the oxygen in the molecule of
the carbonate. This reaction can occur as a partial component in very
different complexes of processes. Or to quote another example, taken
from the subject with which we are especially here concerned. I allow
an induction shock to act on the nerve of a nerve muscle preparation of
the frog. The muscle contracts. The electric stimulus is the condition
for the muscle contraction. But I can substitute for the induction
shock a mechanical stimulus by sudden pressure of the nerve. The
muscle again contracts. The analysis again shows that the induction
shock as such was not the condition for the muscle contraction, but
the excitation of the nerve which it produced and which is conducted
as a specific impulse to the muscle. This excitation of the nerve can,
however, be induced by very different kinds of processes, namely,
by all processes which possess in common the condition that they
suddenly increase certain disintegration processes in the living
nerve substance. Indeed, the further analysis of the whole process
shows in addition that the nerve impulse as such likewise does not
form a condition for the contraction of the muscle, but it first of
all produces the necessary condition for the muscle contraction by
suddenly greatly increasing certain chemical processes, which take
place in the living substance of the resting muscle. The nerve impulse
can, therefore, also be replaced by other processes, if only these
contain the condition for an increase of disintegration of the muscle
substance, as in the case of the direct stimulation of the curarized
muscle, where the influence of nervous impulses is totally eliminated.
In a further analysis of this process we should penetrate even more
deeply into the differentiation of the individual constituent processes
and the isolating of the special conditions on which each link in the
chain is dependent.
Such an analysis then shows us the following: Every thing, every state
or process, is a complex of numerous components, of which _one_ always
conditions the other in the manner that the individual conditioning
components are themselves in their turn contained as constituents of
other complexes and are conditioned here again by other factors. These
factors in themselves as such are not directly necessary to the taking
place or existing of the special component and can, therefore, be
replaced by others. Closer observation shows that there is a constant
interdependence between all things in the world. _Every_ thing in the
world is _indirectly_ dependent upon _every other_, although often so
remotely that we are not able to trace the connection. Absolute things,
completely isolated and independent of others, _do not_ exist in the
world. In observing and studying complexes individually, we must not
forget that we only _think_ of them as isolated from the great eternal
coherence, from which they are in reality not separated. The conception
of condition, however, only then has meaning, if we refer to it in
connection with the direct dependence of one factor upon another.
Nevertheless if we understand by conditions those which are connected
by multitudinous intermediate components, then we would render the
conception of conditions useless. For if every thing in the world were
the condition for every other, the conception of relation would lose
its value in special states or processes. Should the conception of
condition have a meaning in regard to a _certain_ state or process,
then we should only look upon _that_ part of a complex upon which the
other is _directly_ dependent as a condition. When, however, we meet
with a factor for a process or state, which can apparently be replaced
by another factor, we have not carried the analysis far enough. Upon
deeper penetration into the subject, it is found that the essential
condition for the process, which exists, is a component common to both
factors, one of which in consequence can replace the other.
It is the task of all scientific research to penetrate deeper and
deeper into these relations, these connections and the order of
succession of states and processes and to separate them into their
individual components, and in this way gain a more thorough knowledge
of the constancy of existence and happenings in the world.
This analytical process, it is true, only advances very gradually, and
we must accept for the present, especially in the complex biological
processes, that a whole complexity of members appear conditioned,
and that a complex aggregate is a condition of the whole process.
We are not yet in the position to define the special components of
the constituent processes. It is only step by step that we are able
to differentiate the necessary from the accessory parts in these
complexes. However, we are here only concerned for the present with
a purely theoretical question and we may be permitted to say: If we
maintain that the conception of condition has as an integral part the
element of necessity and of relation to a special thing, then there
are no substituting conditions. For then every condition for a state
or process is of equal value. There is no justification to give more
prominence to one condition and place it in the position of being the
“_cause_.”
If the cause is elevated, then it is done from some superficial motive.
This is confirmed by a glance at the practical use of the term cause.
The cases in which the cause is always at once clearly recognized and
named without doubt or hesitation are those where a new factor is
added to an already existing system of conditions, which bring about
a process. When such a process is produced, the last added condition
is considered as “cause.” A shock acts on an explosive body, the body
explodes: the shock is considered the cause. An induction shock acts
on a muscle, the muscle contracts; the induction shock is looked upon
as the cause of the muscle contraction. To regard only the last added
condition as being of especial importance to the taking place and the
explanation for a process is, however, a standpoint which could satisfy
only the most superficial of observers.
In a scientific investigation such methods should play no rôle. For to
every careful observer it must appear quite clear from the beginning,
that the previously existing conditions have as great a value for the
taking place of the process and its explanation as that last added.
The induction shock would not have produced the characteristic effect
had not the other conditions been already previously combined, had not
certain special atoms in the molecule of the explosive combination
in consequence of former processes assumed quite a peculiar labile
position, had not in the evolution of the muscle in the growth and
metabolism certain combinations been formed, and certain chemical
processes taken place.
Therefore if I do not analyze these previously existing processes and
the conditions brought about by them in the system of the explosive
substances or the muscle, and simply know the condition added last,
then I have learned nothing of the process itself, have _explained_
nothing. The time of application of a new condition does not justify
in any degree the assignment of a dominant position to a factor. But
more: in many cases there is not a question at all of the _addition_
of a process to an existing state, but rather of the _simultaneous_
interference of two or more processes. Several conditions can appear
at the _same_ time. In other cases the sequence of the combination can
be reversed. Which then is the cause? Has the process several causes,
or has it no cause? Here one sees plainly to what absurd results it
leads if time alone is used as a basis of the conception of cause.
To illustrate this I return to the case of the liberation of carbon
dioxide from carbonate of sodium. I place anhydrous carbonate of sodium
in a beaker and add hydrochloric acid. The carbon dioxide escapes. Here
the addition of hydrochloric acid would be assumed to be the cause of
the freeing of the gas. Then I put hydrochloric acid in a beaker and
add carbonate of sodium. The same process takes place, but now the
addition of _carbonate of sodium_ would be considered the cause for the
formation of gas. Now I put both simultaneously into a beaker. Again
the same process. Which was now the cause? Has the process now _two_ or
has it _no_ cause at all? Finally I put anhydrous carbonate of sodium
and hydrochloric acid in ether solution into the beaker. The formation
of gas does not take place, and _yet_ both causes for this formation
of gas are present, the carbonate of sodium and the hydrochloric acid.
Only when I add water to the mixture does the formation of carbon
dioxide take place. Here water would be considered the cause. Hence
every condition would be in succession the cause for one and the same
process. Under some circumstances the same process would have _several_
and in others _no_ cause at all. It is scarcely necessary for further
comments upon the value of the conception of cause for the scientific
explanation of a state or process. If we do not seek to introduce
into exact science the antiquated symbols which have become useless
and belong to a primitive phase of development of human thought,
there cannot be a moment’s doubt that a strict scientific analysis in
whatever field of investigation it may be carried on can consist only
in the study of all the conditions concerned in a state or process. If
this is done, then the work of exact research is accomplished. Further
problems do not exist. The use of superfluous terms or symbols for
the definition of things would be in opposition to the fundamental
principle, already brought forward by _Kirchhoff_, especially for
mechanics, namely, that of formulating comprehensively and in the
simplest manner the processes which take place in nature.
At first glance one might be tempted to find an incompleteness in the
observation and description, when a conditional standpoint is adopted.
It might be thought that conditionalism were a purely _formal_ method
of observation, and only considered the _interdependence_ of things,
but not the _properties_, the _nature_ of the objects themselves.
Regarded more closely, however, it is seen that this objection does not
hold good. For what is a condition?
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_. (Figure 1,
D and E.) They remain permanently[16] in this form. I have observed
them for several hours at a time. 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_.
[16] _Max Verworn_: “Die polare Erregung der lebendigen Substanz
durch den galvanischen Strom.” In Pflügers Archiv. f. d. ges.
Physiologie Bd. 65, 1896.
[Illustration: _A_
_B_
_C_
_D_
_E_
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._”
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.