THE ACTIVITIES OF THE ORGANISM
The rather lengthy discussion of the last chapter was necessary in
order to show just how far the principles of energetics established by
the physicists applied to the organism. We have seen that the first law
of thermodynamics does so apply with all its exclusiveness. The more
carefully a physiological experiment is made; the more closely do its
results correspond with those which theory demands. It is true that
relatively few experimental investigations can be controlled in this
way, but in those that can be checked by calculation (as, for instance,
in the well-known calorimetric experiments) everything tends to show
that precisely the same quantities of matter and energy enter the body
of an organism in the form of food-stuff, that leave it as radiated and
conducted heat, as work done, and as the potential chemical energy of
the excretions. Even when we are unable (as in most investigations) to
apply the test of correspondence with theory, we have the conviction
that the law of conservation holds with all its strictness.
Then, whenever it was possible to apply the methods of chemistry and
physics to the study of the organism, it was seen that the processes
at work were chemical and physical. The substance of the living body
was seen to consist of a large (though limited) number of chemical
compounds, differing mainly from those which exist in inorganic nature
in their greater complexity. It was also seen that physico-chemical
reactions occurred in living substance analogous with, or quite
similar to, those which could be studied in non-living substance. The
conclusion, then, was irresistible that the life of the organism was
merely a phase in the evolution of matter and energy, and differed in
no essential respect from the physico-chemical activities that could be
observed in the non-living world.
These conclusions were stated so well by Huxley in his famous lecture
on “The physical basis of life,” over forty years ago, that all
subsequent utterances have been merely reiterations of this thesis in
a less perfect form. The existence of the matter of life, Huxley said,
depended on the pre-existence of certain chemical compounds--carbonic
acid, water, and ammonia. Withdraw any one of them from the world
and vital phenomena come to an end. They are the antecedents of
vegetable protoplasm, just as the latter is the antecedent of animal
protoplasm. They are all lifeless substances, but when brought together
under certain conditions they give rise to the complex body called
protoplasm; and this protoplasm exhibits the phenomena of life. There
is no apparent break in the series of increasingly complex compounds
between water, carbon dioxide, and ammonia, on the one hand, and
protoplasm on the other. We decide to call different kinds of matter
carbon, oxygen, hydrogen, and nitrogen and to speak of their activities
as their physico-chemical properties. Why, then, should we speak
otherwise of the activities of the substance protoplasm?
“When hydrogen and oxygen are mixed in certain proportions and an
electric spark is passed through them they disappear, and a quantity
of water, equal in weight to the sum of their weights, appears in
their place. There is not the slightest parity between the passive and
active powers of the water and those of the oxygen and hydrogen that
have given rise to it.... We call these and many other phenomena, the
properties of water, and we do not hesitate to believe that in some
way they result from the properties of the component elements of the
water. We do not assume that a something called “aquosity” entered into
and took possession of the oxide of hydrogen as soon as it was formed
and guided the aqueous particles to their places in the facets of the
crystal, or among the leaflets of the hoar frost.”
“Is the case in any way changed when carbonic acid, water, and ammonia
disappear, and in their place, under the influence of pre-existing
protoplasm, an equivalent weight of the matter of life makes its
appearance?”
“It is true that there is no sort of parity between the properties
of the components and the properties of the resultant. But neither
was there in the case of water. It is also true that the influence of
pre-existing protoplasm is something quite unintelligible. But does
anyone quite understand the _modus operandi_ of an electric spark
which traverses a mixture of oxygen and hydrogen? What justification
is there, then, for the assumption of the existence in the living
matter of a something which has no representative or correlative in the
non-living matter which gave rise to it?”
All the investigations of over forty years leave nothing to be added
to this statement of what, in Huxley’s days, was called materialistic
biology. It was a very unpopular statement to make then, but it has
become rather fashionable now. Let the reader compare it with all
that has been spoken and written since 1869, even with the utterances
of the British Association of the year 1912, and he will find that it
expresses the point of view of mechanistic biology far better than
all the subsequent restatements. The only difference he will find is
that the latter have become (as William James has said about academic
philosophies), rather shop-soiled. They have been reached down and
shown so often to the enquiring public, that each display has taken
away something of their freshness.
[Illustration: FIG. 8.]
Now Huxley’s example leads up so well to the consideration of the
differences between the chemical activities of the organism and those
of inorganic matter that we may consider it in some detail. What, then,
_is_ the difference between the explosion of a mixture of oxygen and
hydrogen, and the photo-synthesis of starch by the green plant?
In the case of the synthesis of water we have an example of an
exothermic chemical reaction. We are to think of the mixture of oxygen
and hydrogen as existing in a condition of “false equilibrium.” It may
be compared with a weight resting on an inclined plane.
Suppose that the plane is a sheet of smoothly polished glass, and that
the weight is a smooth block of glass. By canting the plane more and
more an angle will be found at which the slightest push starts the
weight sliding down. Now in the case of the explosive mixture of oxygen
and hydrogen we have a chemical analogue. Either the gases do not
combine at all at the ordinary temperature or they combine “infinitely
slowly.” But the slightest impulse, an electric spark requiring an
almost infinitesimally small quantity of energy, starts the combination
of the gases, and this continues until all is changed into water
vapour. In this reaction a large quantity of energy is liberated in
the form of heat. This heat becomes transformed into the kinetic
energy of the water particles which condense from the steam formed in
the explosion, and these particles assume the temperature of their
surroundings. The energy which was potential in the explosive mixture,
and which was capable of doing work, still exists as the kinetic energy
of the water formed, but it has become unavailable for any natural
process of work.
We have seen what is the general character of the reaction series
in the course of which carbon dioxide and water become starch; and
then this, becoming first soluble, and becoming associated with the
ammonia or nitrate taken into the plant, becomes protoplasm. It is a
reaction which differs from that just described, in that available
energy becomes absorbed and accumulated, and retains the power of doing
work. It is not a reaction which can be initiated by an infinitesimal
stimulus, but one in which just as much energy is required in order
that it may happen as is represented in the energy which becomes
potential in the living substance generated. The first reaction is one
which may take place _by itself_;[17] the other is one which requires
a compensatory energy-transformation in order that it may happen.
In the first reaction energy is dissipated; in the second one it is
accumulated.
[17] It is no use saying that apart from the electric spark the
combination would not take place, for we do not know that the _O_ and
_H_ of the mixture do not combine _very_ slowly, molecule by molecule,
so to speak. At all events there is no functionality between the
infinitesimal quantity of energy supplied by the spark, and the energy
which becomes kinetic in the explosion.
We are thus led to the consideration of the second principle of
energetics and its limitations, but before entering upon this
discussion we must consider the nature of the activities of the
organism.
By the term “metabolism” we understand the totality of the
physico-chemical changes which occur in the living substance of the
organism. In physiological writings we usually find that two categories
of metabolic changes are described: (1) anabolic processes, in the
course of which simple chemical compounds possessing relatively little
energy are built up into much more complex substances, containing a
relatively large quantity of available energy, and therefore capable of
doing work. The transformations constituting an anabolic change must
be accompanied by corresponding compensatory energy-transformations,
to account for the energy which becomes potential in the substances
formed. The formation of starch from carbon dioxide and water, by
the green plant, is such an anabolic change, and the compensatory
energy-transformation is the absorption of radiation from the ether by
the cells of the plant. A further anabolic change in the plant organism
is the formation of amido-substances from the ammonia or nitrate
absorbed from the soil, and from the soluble carbohydrates formed from
the starch manufactured in the green cells.
The typical activities of the chlorophyll-containing organism are of
this nature; they are anabolic. The organism may be a green land-plant;
a marine green, red, or brown alga; a yellow-green diatom, a yellow,
green, red, or brown peridinian or other holophytic protozoan; an
ascidian, mollusc, echinoderm, polyzoan, worm, or coral containing
“symbiotic algæ” (that is the chlorophyll-containing cells of some
plant organism which have become associated with the animal and
incorporated in its tissues). In all these cases the presence of
this chlorophyllian substance confers on the organism the power of
effecting the compensatory energy-transformation, by the aid of
which carbon dioxide and water are built up into starch. What this
transformation is, and what are the steps by which the carbon dioxide
and water become carbohydrate we do not exactly know. Solar radiation
impinging upon an inorganic substance is partly reflected and partly
absorbed. The absorbed fraction may become transformed in such a way
as to render the substance phosphorescent, or it may transform into
chemical energy, as when light impinges on a photographic plate, but
as a general rule it is transformed into heat. In the green plant,
however, the transformation of radiation into heat does not occur--at
least the heating is very small--and it passes directly or indirectly
into the potential chemical energy of the starch which is synthesised.
We must regard this power of absorbing radiation and utilising it in
compensatory transformations as a general character of protoplasm.
It is true that it is now specialised in the cells containing the
chlorophyll bodies, but there are indications that it may be present in
the tissues of the animal devoid of chlorophyll.
Other anabolic transformations occur in the animal. The food-stuffs
which are absorbed from the intestine are substances which have
undergone dissociations, the nature of which is such as to render them
capable of absorption and of reconstruction. These anabolic changes in
the higher animal are exceptional, and their usefulness lies in the
fact that by their means substances become capable of being transported
by the tissue fluids of the body.
(2) Katabolic changes in the animal body correspond in their frequency
of occurrence to the anabolic changes of the plant organism. In them
complex chemical substances undergo transformation into relatively
simple substances, and the contained energy at the same time undergoes
a parallel transformation, passing into the form of heat and mechanical
energy, while a fraction becomes dissipated. Food-stuffs taken into the
alimentary canal break down in this way, but to a very limited extent.
Proteids undergo dissociation or decomposition into amido-substances,
while fats are dissociated into fatty acids and glycerine.
Doubtless energy is dissipated in these processes, serving no other
purpose but to heat the contents of the alimentary canal, but this
energy-transformation has not been worked out very completely and it
is a question whether, given a healthy animal and perfect food-stuffs,
any energy would necessarily be lost during the digestive processes.
The reactions involved in the latter do not belong to the category
of chemical changes proceeding from the complex to the simple, with
a liberation of energy; but appear to involve rather a rearrangement
of the constituents of a complex molecule, a process in which the
contained energy need not undergo change in quantity. These processes
involve the action of _enzymes_.
Enzymes play a great part in modern physiological theory and we must
consider them in detail. Let us attach a concrete meaning to the
general notion of enzyme-activity by considering the phenomena known
as _catalysis_. The metal platinum can be brought into a very fine
stage of division when it is known as platinum black. In this condition
it brings about reactions in chemical mixtures or substances which
would not otherwise occur: a mixture of oxygen and hydrogen explodes
when brought in contact with platinum black, and a mixture of coal
gas and air inflames, a reaction which is made use of in the little
gas-lighting apparatus which most people have seen. If, again, a
powerful electric current be passed between platinum wires which are
a little distance apart, and are immersed in water, the metal becomes
torn away from the points of the wire in the form of an impalpable
powder, colloidal platinum. The liquid containing this colloid then has
the power of setting up chemical changes in other substances, changes
which would not otherwise occur, or, at least, would occur very slowly.
In general such catalysts, platinum black or colloidal platinum for
instance, have the following characters: (1) a small quantity is
sufficient to cause change in a large (theoretically an infinite)
quantity of the substance acted upon; (2) the nature and quantity
of the catalyst remain at the end the same, as at the beginning of
the reaction; (3) a catalyst does not start a reaction in any other
substance or substances, it can only influence the rate at which
this reaction may occur: apparently it does, in some cases, start a
reaction, but in such cases we suppose that the latter proceeds so
slowly as to be imperceptible; (4) the final state of the reaction
is not affected by the catalyst; it depends only on the nature of
the interacting substance or substances; (5) the final state is not
affected either by the nature or quantity of the catalyst: it is the
same if we employ different catalysts, or a large or small quantity of
the same catalyst. Finally, it appears that the phenomena of catalysis
are universal: “There is probably no kind of chemical reaction,” says
Ostwald, “which cannot be influenced catalytically, and there is no
substance, element, or compound which cannot act as a catalyser.”[18]
[18] A statement of interest in view of the enormous number of
“ferments” or enzymes discovered by physiologists. It would appear that
any tissue in any organism is capable of yielding an enzyme to modern
investigation.
Enzymes, then, are agents which are produced by the organism, and which
act by influencing (accelerating or retarding) chemical reactions.
An enzyme, as such, need not exist in a tissue; it is there as a
_zymogen_, a substance which may become an enzyme when required. An
enzyme need not be active: it may be necessary that it should be
“activated” by a _kinase_, another substance produced at the same time.
Associated with many enzymes are _anti-enzymes_, substances which undo
what their corresponding enzymes have done. Finally some, perhaps
most, enzymes are reversible, that is, if they produce a change in a
certain substance they can also produce the opposite kind of change:
the meaning of this will become clearer a little later on. We have
spoken of enzymes as “agents” or “substances,” but it is not at all
certain that they are definite chemical compounds. In the preparation
of an enzyme what the bio-chemist obtains is a liquid, a glycerine
or other extract _which possesses catalytic properties_. An actual
catalytic substance, like platinum black, cannot be obtained from
this liquid. A white powder may be obtained, but this usually proves
to be proteid in composition; it is not the actual enzyme itself but
is the impurity associated with the latter. Now the very great number
of enzymes “isolated” by the physiologists has rather destroyed the
original simplicity of the idea of enzyme activity and suggests a
parallel statement to that made by Ostwald about catalysts: any tissue
substance may influence the reactions that may possibly occur in other
tissue substances. But while pure chemistry has to deal with definitely
known chemical compounds in the phenomena of catalysis, this cannot be
said to be the case with physiology in dealing with enzymes. Reasoning
by analogy, we may say that it is probable that enzymes are definite
proteids, or chemical substances allied to these, but this has not been
clearly demonstrated, and it is possible that the phenomena of enzyme
activity may belong to some other category of energy-transformations.
However this may be, the conception is a useful one in describing the
reactions of the organism, and it may be illustrated by considering the
digestion and absorption of fat in the mammalian intestine, a process
which appears to be better known than that of proteid digestion. A
neutral fat consists of an acid radicle, oleic, palmitic or stearic
acids, for instance, united with glycerine. The action of the
pancreatic or intestinal enzymes is to dissociate this fatty salt. Let
us write the formula of the latter as _G F_, _G_ being the glycerine
base, and _F_ the fatty acid; then
_G F_ ⇄ _G_ + _F_
which means that the enzyme can cause the neutral fat to dissociate
into glycerine and fatty acid. This action will go on until a state of
equilibrium is attained, in which there is a certain quantity of each
of the radicles, and a certain quantity of unchanged neutral fat, the
ratio of all these to each other depending on various things. When
this state of equilibrium is attained the enzyme does indeed go on
splitting up more neutral fat, but it is a _reversible_ enzyme, and it
also causes the glycerine and fatty acid already split up to recombine,
forming neutral fat. A condition is, therefore, reached in which the
composition of the mixture remains constant.
Now there is dissociated fat in the intestine after a meal, but there
is only neutral fat in the wall of the intestine. The fat itself cannot
pass through the cells forming the intestinal wall, but the glycerine
and fatty acid into which it is dissociated can so pass, since they are
soluble in the liquids of the intestine. We suppose that the cells of
the wall of the intestine also contain the fat-splitting ferment; this
ferment in the cells acts on the glycerine and fatty acid immediately
they enter and recombines these radicles again into neutral fat, the
above equation now reading from right to left. But after a time this
reaction in the cells will also begin to reverse, for the enzyme
will begin to split up the synthesised neutral fat when the state of
chemical equilibrium in the new conditions is attained. Fatty acid
and glycerine will then diffuse out from the cells into the adjacent
lymph stream or blood stream--perhaps neutral fat will also pass from
the cells into these liquids, we are not sure. At all events the lymph
and blood after a meal containing much fat are crowded with minute fat
globules. But why are there no fatty acids or glycerine in the blood,
for the latter also contains lipase (the fat-splitting enzyme)? The
explanation is, apparently, that either an anti-enzyme is produced, or
that the enzyme passes into a _zymoid_ condition. Why also does fat
accumulate in the tissues? Here, again, the activity of the enzyme,
which from other considerations we may regard as being universally
present almost everywhere in the body, must be supposed to be arrested
by some means.
The conception of a catalytic agent, such as we can study in pure
chemistry, thus carries us a long way in our description of the
processes of digestion, absorption, and assimilation. We have applied
it to the case of fat-digestion, but very much the same general scheme
might also apply to many other processes in the body. Obviously it
enables us to describe these processes in terms of physico-chemical
reactions, but we cannot fail to see that ultimately we are compelled
to assume the existence of reactions which were not included in the
original conception--the activation of the enzyme at the proper moment
by the kinase, the operation of the anti-enzyme, and the passage of the
enzyme into the zymoid. Just why these things happen as they do we do
not know, yet the whole problem becomes shifted on to these reactions.
In the same way we apply the purely physical processes of the osmosis
and diffusion of liquids to the circulation of substances in the animal
body. The nature of these processes will probably be familiar to the
reader, nevertheless it may be useful to remind him that by diffusion
we understand the passage of a liquid, containing some substance in
solution, through a membrane; and by osmosis the passage of a solvent
(but not of the substance dissolved in it) through a “semi-permeable
membrane.” The molecules of the solvent (water, for instance) pass
through the membrane (the wall of a capillary, or lymphatic vessel),
but the molecules of the substance (salt, for instance) dissolved in
the solvent do not pass. Let us suppose that a strong solution of
common salt in water is injected into the blood stream: what happens
is that osmosis takes place, the water in the surrounding lymph spaces
passing into the blood stream because the concentration of salt there
is greater than it is in the lymph. While this is happening, the
capillary walls are acting as semi-permeable membranes, allowing the
molecules of water to pass through but not the molecules of salt.
Very soon, however, the process of osmosis becomes succeeded by one of
diffusion, and the salt molecules pass through the capillary wall into
the lymph and are excreted.
[Illustration: FIG. 9.]
Undoubtedly the purely physical processes of diffusion and
osmosis occur all over the animal body and are the means whereby
food-materials, secretory, and excretory substances are transported
from blood to lymph, or _vice versa_, from lymph to cell substance or
to glandular cavities, and so on. But it is also the case that in very
many processes the activity of the cells themselves plays an important
part. It may even be the case that a particular process, after all
physical agencies are taken into account, reduces down to this action
of the cells. To understand this we must consider the mode of working
of some well-known organ, and the best possible example of such an
organ, considered as a mechanism, is that of the sub-maxillary salivary
gland of the mammal.
What, then, is this mechanism and how does it act? The gland is a
compound tubular one, its internal cavity being prolonged into the duct
which opens into the mouth. The saliva prepared in the gland issues
from this duct. Blood is carried to the gland by twigs of the facial
artery, and, after circulating through it, is carried away by factors
of the jugular vein. Two nerves supply the gland: one is the chorda
tympani, a branch of a cranial nerve, and the other is a sympathetic
nerve. Lymph also leaves the gland by a little vessel.
Now suppose we have laid bare all this mechanism in a living animal and
make experiments upon it. If we stimulate the chorda tympani there is a
copious flow of thin watery saliva, but if we stimulate the sympathetic
there is a less copious flow of thick viscid saliva. Why is this? We
find on closer analysis that the chorda contains fibres which dilate
the small arteries so that there is an increased flow of blood through
the gland; but that, on the other hand, the sympathetic contains fibres
which constrict the arteries, thus leading to a reduced flow of blood.
This accounts for the fact that “chorda-saliva” is abundant and thin,
while “sympathetic-saliva” is scarce and thick. It was thought at one
time that the chorda contained fibres which stimulated the gland to
produce watery saliva, while the sympathetic contained fibres which
stimulated it to produce mucid saliva. This, however, is not the case.
Both nerves contain the same kind of secretory fibres: their other
fibres differ mainly in that they act differently on the arteries.
It might be the case--indeed it was at one time thought that it was the
case--that secretion of saliva was simply a matter of blood-flow: an
abundant arterial circulation gave rise to abundant saliva, a sparse
flow to a sparse saliva. Undoubtedly the secretion depends on blood
supply, but not _solely_. If it did, then the whole process might be
conceived to be a very simple mechanical one--filtration or diffusion
of the saliva from the blood stream through the thin walls of the blood
vessels, and the walls of the tubules into the cavity of the gland.
If this were the case, then the liquid in the gland would be the same
in composition and concentration as the liquid part of the blood--the
plasma. But it is really different in composition and it is not so
concentrated. Now osmotic pressure--on the action of which so much is
based--cannot help us, for the liquid in the gland is less concentrated
than that in the blood vessels, so that water ought to pass from gland
to blood instead of from blood into gland. Again, if we tie the duct,
so that the saliva cannot escape, secretion still goes on, though
the hydrostatic pressure of saliva in the cavity of the gland may be
considerably greater than that of the liquid in the blood vessels. Yet
again, if we stop the blood flow by tying the artery, secretion of
saliva may still go on for a time.
Therefore the only physical agencies we can think of do not explain the
secretion. The latter is actually the work of the individual cells,
stimulated by the nerves. If the volume of the gland be measured
just while it is being stimulated to secrete, it will be found that
the organ becomes _smaller_, yet while it is being stimulated the
blood-vessels are being dilated so that the volume of the whole
structure ought to become greater. Obviously part of the substance of
the gland is being emptied out through its duct as the secretion.
If we examine the cells of the gland in various states we see clearly
that granules of some material, different in nature from the substance
of the protoplasm itself, are being formed within them. Evidently these
granules swell up during secretion and discharge their contents into
the ducts. Further changes in the characters of the cell-substance,
and in the nucleus, can be observed, and all these indicate that the
protoplasm of the cells, as the result of stimulation, elaborates
certain substances; that these substances are then washed out, so
to speak, into the duct by the withdrawal of water from the cell;
and that thereafter the cell absorbs fresh nutritive material from
the lymph which exudes from the blood vessels, along with water.
The distinctive part of the whole train of processes is, then, this
elaboration of material by the cells themselves; while the concomitant
changes in the calibre of the blood vessels and in the flow of blood
and lymph are subsidiary ones. In the process of secretion of saliva
energy is absorbed from the chemical substances of the blood to bring
about the passage of water from a region of high to a region of low
osmotic pressure; oxygen and nitrogen, with other elements of course,
are withdrawn from the arterial blood stream for the purpose of the
secretion, and carbon dioxide and other substances are given off to the
venous blood and lymph.
The problem thus is pushed back from the mechanical events occurring
in the nervous and circulatory processes, to the physico-chemical ones
occurring in the cells of the gland tubules; and it thus becomes much
more obscure. It is true that we can formulate a hypothesis which
describes, in a kind of way, these intra-cellular metabolic changes,
in terms of physico-chemical reactions, and, without doubt, reactions
of this kind must occur within the cell. But if we could test any such
hypothesis as easily as the mechanical ones suggested, should we find
it any more self-sufficient?[19]
[19] We have not referred to “psychical secretion.” If we smell some
very savoury substance our “mouth waters,” that is, secretion of
saliva occurs. If we even see some such substance the same secretion
occurs. All this is clear and can be “explained” mechanistically: the
stimulation of the olfactory or visual organs begins a kind of reflex
process. But if we even _think_ about some very savoury morsel saliva
may be secreted. We must suppose now that our consciousness, something
which has nothing to do, it must be noted, with energy-changes in the
body, can react on the body. If we show a dog an attractive bone it
will secrete saliva; if we show it again and again, the same thing
occurs. But after certain such trials the dog will realise that he is
being played with, and the exhibition of the bone no longer evokes a
flow of secretion. Why is this? The whole process has now become more
mysterious than ever.
Irritability and contractility are general properties of the organism.
These properties are illustrated by the irritability of an _Amœba_
or _Paramœcium_ to stimuli of many kinds; by the movements of the
pseudopodia of the former animal, or of the cilia of the latter; by
the nervous irritability of the higher animal, and the contraction of
its muscles when they are stimulated. They are among the fundamental
properties or functions of living protoplasm, and their study is of
paramount interest, and carries us to the very centre of the problem
of the activities of the organism. Naturally physiologists have never
ceased to attempt to describe irritability and contractility in terms
of physics, but though we may be quite certain that the things that do
occur in these phenomena are controlled physico-chemical reactions, it
must be remembered that what we positively know about their precise
nature is exceedingly little.
What is the nature of a nervous impulse? When a receptor organ is
stimulated, as, for instance, when light impinges on the cone cells
of the retina, or when the nerve-endings in a “heat-spot” in the skin
are warmed, or when the wires conveying an electric current are laid
on a naked nerve, an impulse is set up in the nerve proceeding from
the place stimulated, and we must suppose that approximately the same
amount of energy moves along the nerve as was communicated to the
receptor or the nerve itself by a stimulus of minimal strength. How
does it so move? Several facts of capital importance result from the
experimental work. (1) The impulse travels with a velocity variable
within certain limits, say from 8 to 30 metres per second; (2) it
travels faster if the temperature is raised (up to a certain limit);
(3) it is difficult to demonstrate that the passage of this impulse is
accompanied by definite chemical changes in the nerve substance: it
is stated that carbon dioxide is produced, but this is not certainly
proved; (4) an electric current is produced in the nerve as the result
of stimulation; (5) no heat is produced, or at least the rise of
temperature, if it occurs, is less than 0.0002° C.
Thus it is quite certain that physical changes accompany the
propagation of the nerve-impulse, for the latter has a certain
velocity, which depends on the temperature, and an electric change
also occurs in the substance of the nerve. Is this electric change
the actual nerve impulse? It is hardly likely, since the velocity
of the impulse is very much less than that of the propagation of an
electric change through a conductor; besides, the passage of the
impulse is not accompanied by a measurable heat evolution, although
the flow of electricity along a poor conductor must generate heat
and dissipate energy. Is it a chemical change? Then we should be
able to observe metabolism in the nerve substance--that is if the
energy-change is a thermodynamic one--while it is not at all certain
that metabolic changes do occur. Nevertheless it seems probable that
a physico-chemical change is actually propagated when we consider
the chemical specialisation of the substance of the axis-cylinder of
the nerve. Now the velocity of propagation of the nervous impulse
is of the same order of magnitude as that of an explosive change in
chemical substances (using the term “explosion” to connote chemical
disintegrations rather than combustions). If we imagine a long rod of
dynamite, or picric acid, or a long strand of loosely-packed gun-cotton
to be exploded by percussion at one end, then a transmission of the
chemical disintegration of any of these substances will pass along
the rod, etc., with a velocity which will certainly vary with the
physical condition of the material. It would be a high velocity in
a rod of dynamite, or fused picric acid, but a lower velocity in a
loosely aggregated strand of gun-cotton, or a trail of picric acid
powder. Is this what happens in the nerve when an impulse travels
along it? Obviously not, since the substance of the nerve is not
altered appreciably, while that of the explosive substance passes into
other chemical phases. We might imagine, then, such a change in the
nerve fibrils as that of a reversible transformation of some chemical
constituent:--
(2) (1)
-------------------------------------------------------------
: _a_ + _b_ : _a_ + _b_ : _a_ + _b_ : _a_ + _b_ : _a_ + _b_ :
: ⇣⇡ : ⇡⇣ : ⇡⇣ : ⇡⇣ : ⇡⇣ :
: _c_ + _d_ : _c_ + _d_ : _c_ + _d_ : _c_ + _d_ : _c_ + _d_ :
-------------------------------------------------------------
Let us imagine the substance of the fibril to be composed of, or at
least to contain, the substances _a_ + _b_ which dissociate reversibly
into the substances _c_ + _d_. At any moment, and in any particular
physical state, as much of _a_ and _b_ pass into _c_ and _d_ as _c_
and _d_ pass into _a_ and _b_. There will be equilibrium. But now let
a stimulus alter the physical conditions: prior to the stimulus the
phase was _a_↓{m} + _b_↓{n} = _c_↓{p} + _d_↓{r}--the suffixes _m_, _n_,
_p_, _r_, denoting the concentrations of _a_, _b_, _c_, and _d_--but
after the stimulus the phase may be _a_↓{m1} + _b_↓{n1} = _c_↓{p1} +
_d_↓{r1}. Now the element of the nerve substance (1) forms a system
with the element (2). The condition in (2) is _a_↓{m} + _b_↓{n} =
_c_↓{p} + _d_↓{r}, and that of (1) _a_↓{m1} + _b_↓{n1} = _c_↓{p1}
+ _d_↓{r1}, but these two together now fall into a new state of
equilibrium and this is transmitted along the whole nerve-fibril with
a velocity which belongs to the order of magnitude of that of chemical
changes. If the stimulus remains constant (a constant electric current
for instance), the new condition of equilibrium will be established
throughout the whole length of the fibril and the nervous impulse will
be a momentary one (as it is in this case). But if the stimulus is an
intermittent one (an interrupted electric current, light-vibration,
sound-vibrations), then in the intervals the former condition of
equilibrium will become re-established and the nervous impulse will
be intermittent (as it is). There would be no work done on the whole
in the changes, except that done by the transmission of the changed
state of equilibrium to the substance of the effector organ in which
the nerve-fibril terminates--the substance of a muscle fibre, or the
cell of a secretory gland, for instances. There would, probably, be a
certain dissipation of energy as in the case of the propagation of an
electric impulse through a poor conductor, but all our knowledge of the
chemistry of the nerve fibre points to this amount of dissipation as
tending to vanish.
Something analogous to this may be expected to take place in a muscle
fibre when it contracts; except that, of course, energy is transformed
in this case. What precisely does happen we do not know and at the
present time no physico-chemical hypothesis of the nature of muscular
contraction exactly describes all that can be observed to take place.
Certain positive results have, of course, been obtained by chemical
and physical investigation of the contracting muscle: carbon dioxide
is given off to the lymph and blood stream, and the amount of this is
increased when an increased amount of work is done by the muscle; heat
is produced and this too increases with the work performed; glycogen
is used up, and lactic acid is produced; finally oxygen is required,
and more oxygen is required by an actively contracting muscle than
by a quiescent one. Now the obvious hypothesis correlating all these
facts is that the muscle substance is oxidised, and that the heat so
produced is transformed into mechanical energy. “We must assume,” says
a recent book on physiology, “that there is some mechanism in the
muscle by means of which the energy liberated during the mechanical
change is utilised in causing movement, somewhat in the same way as the
heat energy developed in a gas-engine is converted by a mechanism into
mechanical movement.”
Now, must we assume anything of the kind? To begin with, life goes
on, and mechanical energy is produced in many organisms living in a
medium which contains no oxygen. Anaerobic organisms are fairly well
known, and we cannot suppose that in them energy is generated by
the combustion of tissue substance in the inspired oxygen. A muscle
removed from a cold-blooded animal will continue to contract in an
atmosphere containing no oxygen, and it will continue to produce
carbon dioxide. It is true that the contractions soon cease, even
after continued stimulation under conditions excluding the fatigue
of the muscle, but do the contractions cease _because_ the oxygen
supply is cut off, or because the muscle dies in these conditions?
We know that some complex chemical substance is disintegrated during
contraction and that mechanical energy and heat are produced and that
carbon dioxide is also produced. We know that the carbon contained in
the latter gas corresponds roughly with the carbon contained in the
muscle substance which undergoes disintegration, but does all this
justify us in saying that this substance is oxidised in order that its
potential chemical energy may be transformed into mechanical energy?
Obviously not, since we might equally well suppose that the complex
metabolic substance of the muscle splits down into simpler substances
and that in this transformation energy is generated. Suppose that these
simpler substances are poisonous and that they must be removed as
rapidly as formed. The rôle of the oxygen may be to oxidise them, thus
transforming them into carbon dioxide, an innocuous substance which
can be carried away quickly in the blood stream. This line of thought,
according to which the rôle of oxygen is an anti-poisonous one, is
held at the present day by some physiologists, and many considerations
appear to support it; the existence of “oxidases,” for instance,
enzymes which produce oxidations which would not otherwise occur in
their absence. Such enzymes exist in very many tissues, and they may,
apparently, be present in an inactive form, requiring the agency of a
“kinase” before they are able to act.
The usual view among physiologists is that the muscle fibre is a
thermodynamic apparatus transforming the heat generated during
metabolism into mechanical energy. How is this transformation effected?
It cannot be said that we have any one hypothesis more convincing than
another. It has been suggested that alterations of surface tension
play a part, or that the heat produced by oxidation causes the fibre
to imbibe water and shorten. Engelmann has devised an artificial
muscle consisting of a catgut string and an electrical current passing
through a coil of wire, and by means of this he has reproduced the
phenomena of simple contraction and tetanus. But it remains for future
investigation to verify any one of these hypotheses.
When Huxley published his _Physical Basis of Life_, probably few
physiologists had any doubt that protoplasm was a definite chemical
substance, differing from other organic substances only by its much
greater complexity. But in 1880 Reinke and Rodewald published the
results of an analysis of the substance of a plant protoplasm and these
appear to have demonstrated that the substance was really a mixture
of a number of true chemical compounds and was not a single definite
one. Now all of these substances might exist apart from protoplasm, and
in the lifeless form, and a simple mixture of them could hardly bring
forth vital reactions. These results were followed by the morphological
study of the cell--the discovery of the architecture of the nucleus,
and so on, and so opinion began to turn to the hypothesis that the
vital manifestations of protoplasm were the result of its _structure_.
Microscopical examination of the cell appeared to disclose a definite
arrangement, the “foam” or “froth” of Butschli, for instance. But,
again, it was easily shown that the foam, or alveolar structure of
protoplasm was merely the expression of physical differences in the
substances composing the cell-stuff--they reduced to phenomena of
surface tension and the like. Artificial protoplasm and artificial
_Amœbæ_ were made--at least mixtures of olive oil and various other
substances were made which simulated many of the phenomena of
protoplasm in much the same way as crystalline products may be made
which simulate the growth of a plant stem with its branches. For
instance, one has only to shake up a little soapy water in a flask
to see what resembles surprisingly the arrangement of certain kinds
of connective tissues in the organism. Obviously these artificial
phenomena have nothing to do with living substance.
Yet if we grind up a living muscle with some sand in a mortar we do
destroy something. The muscle could be made to contract, but after
disintegration this power is lost. We have certainly destroyed a
structure, or mechanism, of some kind. But, again, the paste of muscle
substance and sand still possesses some kind of vital activity,
for with certain precautions it can be made to exhibit many of the
phenomena of enzyme activity displayed by the intact muscle fibres,
or even the entire organism. Mechanical disintegration, therefore,
abolishes some of the activities of the organism, but not all of them.
If, however, we heat the muscle paste above a certain temperature, the
residue of vital phenomena exhibited by it are irreversibly removed,
so that heating destroys the mechanism. This we can hardly imagine
to be the case (within ordinary limits of temperature at least) with
a physical mechanism, but again a mechanism which is partly chemical
might be so destroyed. We see, then, that protoplasm possesses a
mechanical structure, but that all of its vital activities do not
necessarily depend on this structure. The full manifestation of these
activities depends on the protoplasmic substance possessing a certain
volume or mass, and also on a certain chemical structure.
If living protoplasm has a structure, and is not simply a mixture of
chemical compounds, what is it then? Two or three physico-chemical
concepts are at the present time very much in evidence in this
connection. When the substances known as _colloids_ were fully
investigated by the chemists, much attention was paid to them by the
physiologists, so that life was called “the chemistry of the colloids,”
just as after the investigation of the enzymes it was called the
“chemistry of the enzymes,” and when the discovery of the relative
abundance of phosphorus in cell-nuclei and in the brain was discovered,
it was called the “chemistry of phosphorus.” Colloids (_e.g._ glue)
are substances that do not readily diffuse through certain membranes,
in opposition to crystalloids (_e.g._ solution of common salt) which
do readily so diffuse. They form solutions which easily gelatinise
reversibly, that is, can become liquid again (glue); or coagulate
irreversibly, that is, cannot become liquid again (albumen); which have
no definite saturation point; which have a low osmotic pressure (and
derived properties), etc.; and the molecules of which are compound
ones consisting of combinations of the molecules of the substance with
the molecules of the solvent, or with each other, that is, they are
molecular aggregates.
Colloids pass insensibly into crystalloids on the one hand and into
coarse suspensions (water shaken up with fine mud, for instance)
on the other. We may replace the concept of a colloid by those of
“suspensoids” and “emulsoids.” A suspensoid is a liquid containing
particles in a fine state of division--if the division is that into
the separate molecules we have a solution, if into large aggregates
of molecules we have a suspension. If the substance in the liquid is
itself liquid, the whole is called an emulsoid. On the one hand this
approaches to a mixture of oil in soap and water--an emulsion--and
on the other hand to such a mixture as chloroform shaken up with
water, when the drops of chloroform readily join together so that two
layers of liquid (chloroform and water) form. What we see, then, in
protoplasm is a viscid substance possessing a structure of some kind,
and containing specialised protoplasmic bodies in its mass (nuclei,
nucleoli, granules of various kinds, chlorophyll, and other plastids,
etc.). It may contain or exhibit suspensoid or emulsoid parts or
substances, or it may contain truly crystalloid solutions. These phases
of its constituents are not fixed, but pass into each other during
its activity. Nothing that we know about it justifies us in speaking
about a “living chemical substance.” On analysis we find that it is a
mixture of true chemical substances rather than a substance. It is no
use saying that in order to analyse it we must kill it, for what we can
observe in it without destroying its structure or activities indicates
that it is chemically heterogeneous.
This is not a textbook of general physiology, and the examples of
physico-chemical reactions in the organism which we have selected have
been quoted in order to show to what extent the chemical and physical
methods applied by the physiologists have succeeded in resolving the
activities of the organism. The question for our consideration is this:
do these results of physico-chemical analysis fully describe organic
functioning? Dogmatic mechanism says “yes” without equivocation.
Now it is clear, from even the few typical examples that we have
quoted, that physiological analysis shows, indeed, a resolution of
the activities of the organism into chemical and physical reactions.
How could it do otherwise? How could chemical and physical methods of
investigation yield anything else than chemical and physical results?
The fact that these methods _can_ be applied to the study of the
organism with consistent results shows that their application is
valid; that we are justified in seeing physico-chemical activities in
life. But are these results _all_ that we have reason to expect?
We turn now to Bergson’s fertile comparison of the physiological
analysis of the organism with the action of a cinematograph. If we
take a series of photographic snapshots of, _e.g._, a trotting horse
and then superpose these pictures upon each other, we produce all the
semblance of the co-ordinated motions of the limbs of the animal. Yet
all that is contained in the simulated motion is immobility. From a
succession of static conditions we appear to produce a flux. Yet if we
could contract our duration of, _e.g._, a week, into that corresponding
to five minutes--if we could speed up our perceptual activity--should
we not see the cinematographic pictures as they really are--a series
of immovable postures and nothing more: truly an illusion? If, again,
we reverse the direction of motion of the film, we integrate our
snapshots into something which is absolutely different from the reality
which they at first represented; and by such devices the illusions and
paradoxical effects of the picture-house farces are made possible.
Well, then, in the physiological analysis of the activity of the
organism do we not do something very analogous to this? The complexity
of even the simplest function of the animal is such that we can only
attend to one or two aspects of it at once, arbitrarily neglecting all
the rest. We find that the hydrostatic pressure of blood, and lymph,
and secretion, the osmotic pressure, the diffusibility, vaso-motor
actions, and other things must be investigated when considering the
question of how the submaxillary gland secretes saliva. One, or as many
as possible, of these reactions are investigated at one time, and then
the results are pieced together--integrated--in order to reproduce the
full activity of the whole indivisible process. But in doing this do we
not introduce something new--a _direction_ or order of happening--into
the elements of the dissociated activity of the organism? Each
elemental process must occur at just the right time.
What right have we to say that the activity of the organism is _made
up_ of physico-chemical elements? Just as much as we have in saying
that a curve is made up of infinitesimal straight lines. Let us adopt
Bergson’s illustration, with a non-essential modification.
[Illustration: FIG. 10.]
The curve 1–8 is a line which we draw freehand with a single
indivisible motion of the hand and arm and eye. It is something unique
and individualised, in that no other curve ever drawn, in a similar
manner, exactly resembles it. Let us investigate it mathematically. We
can select very small portions of it--elements we may call them--and
each of these elements, if it is small enough does not differ
_sensibly_ from a straight line. Let us produce each of these straight
lines in both directions, it is then a tangent to the curve, and it
does actually coincide with the curve at one mathematical point--the
points 1–8 in the figure. The tangent then has _something in common
with the curve_, but would a series of infinitesimally small tangents
reproduce the curve? Obviously not, for the equations of the tangents
would have the form _ax_ + _b_, while that of the curve itself would be
quite different, containing _x_ as powers of _x_, or as transcendental
functions of _x_. In this investigation what we succeed in obtaining
are the derivatives of the curve, and to reproduce the latter from
its elements we have to integrate the derivatives; that is, another
operation differing in kind from our analytical one must be performed.
Now in this illustration we have doubtless something more than an
analogy with our physico-chemical analysis of life. The activities of
the organism do reduce to bio-chemical ones (the elemental straight
lines on the curve), and each of these reactions has something in
common with life (it is tangent to life, touching it at one point).
But if we attempt to reconstitute life from its physico-chemical
derivatives we must integrate the latter, and in doing so we over-pass
the bounds of physics, just as integrating a mathematical function we
necessarily introduce the concept of the “infinitely small.”
The physico-chemical reactions into which we dissociate any vital
function of the organism have, then, each of them, something in common
with the vital function. But their mere sum is not the function.
To reproduce the latter we have to effect a co-ordination and give
directions to these reactions. In all physiological investigations
we proceed a certain length with perfect success; thus the elements,
so to speak, of the function of the secretion of saliva are (1)
the blood-pressure, (2) the hydrostatic pressure of the secretion
in the lumina of the gland tubules, (3) the diffusibility of the
substances dissolved in the blood and lymph through the walls of these
vessels, (4) the osmotic pressure of the same substances, and (5)
the stimulation of the gland cells by “secretory nerve fires.” Now
the investigations carried out--and no part of the physiology of the
mammal has been so patiently studied as the salivary gland--fail, so
far, completely to describe the function in terms of these elements.
In the end we have to refer the secretion to intra-cellular processes,
and then we begin to invoke again processes of osmotic pressure,
diffusibility, and so on with reference to the formation of the drops
of secretion which we can see formed in the gland cells. We are forced
to the formulation of a logical hypothesis as to the nature of these
intra-cellular processes, and since much that goes on in the cell
substance is, so far, beyond physico-chemical investigation, our
hypothesis will be as difficult to disprove as to verify.
* * * * *
Let us return now to Huxley’s comparison of the activity of the green
plant with the chemical reaction which occurs when an electric spark
is passed through a mixture of oxygen and hydrogen. The lecture on the
“Physical Basis of Life” was published in 1869; in 1852 William Thomson
published his paper “On a Universal Tendency of Nature to Dissipation
of Energy,” and a year or two before that Clausius had applied Carnot’s
law to the kinetic theory of heat: the second principle of energetics
had therefore even then been exactly formulated, but its significance
for biological speculation had not been recognised by Huxley, any
more than it has generally been recognised by most biologists since
1869. What, then, does the comparison of Huxley show? Clearly that the
physical changes which occur in the explosion of a mixture of oxygen
and hydrogen _trend_ in a different direction from those which occur in
the photo-synthesis of starch by a green plant. Generally speaking,
chemical activity, that is, the possibility of occurrence of chemical
reactions, is a case of the second law of energetics. Energy passes
from a state of high to a state of low potential. A chemical reaction
will occur if this change of potential is possible.
In all such changes energy is dissipated. What exactly does this mean?
It means that, generally speaking, the potential energy of chemical
compounds tends to transform into kinetic energy; while differences
in the intensity factor of the kinetic energy of the bodies forming
a system tend to become minimal. In a mixture of oxygen and hydrogen
there is energy of two kinds, (1) potential energy due to the position
of the molecules (O and H molecules are separated); and (2) kinetic
energy of the molecules (which are moving about in the masses of gas).
After the explosion the potential energy acquired in the separation of
the molecules of O and H has disappeared (the molecules having combined
to form water), but the kinetic energy has greatly increased, since the
explosion results in the formation of steam at high temperature. But
now this steam radiates off heat to adjacent bodies, or becomes cooled
by direct contact with the envelope which contains it. The energy of
the explosion is therefore distributed to the adjoining bodies, and
the temperature of the latter becomes raised. But these again radiate
and conduct heat to other bodies, and in this way the heat generated
becomes indefinitely diffused.
The general effect of all physico-chemical changes is therefore the
generation of heat, and then this heat tends to distribute itself
throughout the whole system of bodies in which the physico-chemical
changes occur. The energy passes into the state of kinetic energy,
that is, the motion of the molecules of the bodies to which the heat
is communicated. This molecular motion is least in solids, greater in
liquids, and greatest in gases. If solids, liquids, and gases are in
contact, forming complex systems, the kinetic energy of their molecules
becomes distributed in definite ways, depending on the constants of
the systems. After this redistribution the kinetic energy of these
molecules is unavailable for further energy transformations, so that
phenomena or change in the system ceases. There is no longer effective
physical diversity among the parts of the system.
We find that this conception of dissipation of energy cannot be applied
to the organism, at least not with the generality in which it applies
to physical systems. Why? Not because the conception is unsound, or
because the physico-chemical reactions that occur in material of the
organism are of a different order from those that occur in inorganic
systems--they are of the same order. The second law of energetics is
subject to limitations, and it is because it is applied to organic
happenings without regard to these limitations that it does not
describe the activities of the organism as well as it describes those
of inorganic nature.
What, then, are these limitations? We note in the first place that
the laws of thermodynamics apply to bodies of a certain range of
size; or at least the possibility of mathematical investigation (on
which, of course, all depends) is limited to “differential elements”
of mass, energy, and time. We cannot apply mathematical analysis to
bodies, or time-intervals of “finite size,” since the methods of the
differential and integral calculus would not strictly be applicable.
But molecules are so small (1 cubic centimetre of a gas may contain
about 5.4 × 10^{19} of them) that even such a minute part of a body, or
liquid, or gas as approximates to the infinitesimally small dimensions
required by the calculus, contains an enormous number of molecules.
Obviously we cannot investigate the individual molecules. Even if
experimental methods could be so applied, such concepts as density,
pressure, volume, or temperature would have no meaning. Physics, then,
is based on collections of molecules, and the properties of a body are
not those of a molecule of the same body. Such concepts as temperature
and pressure are _statistical_ ones, and are applied to the mean
properties of a large number of molecules.
[Illustration: FIG. 11.]
We can best illustrate this by considering Maxwell’s famous fiction
of the “sorting demons.” Let us imagine a mass of gas contained in a
vessel the walls of which do not conduct heat. Let there be a partition
in this vessel also of non-conducting material, and let there be an
aperture in this partition greater in area than a molecule, but smaller
than the mean free path of a molecule. Now this mass of gas has a
certain temperature which is proportional to the mean velocity of
movement of the molecules. The second law says that heat cannot pass
from a cold region in a system to a hot region without work being done
on the system from outside, nor can an inequality of temperature be
produced in a mass of gas or liquid except under a similar condition.
But “conceive a being,” says Maxwell, “whose faculties are so
sharpened that he can follow every molecule in its course; such a
being, whose attributes are still as essentially finite as our own,
would be able to do what is at present impossible to us.”[20] For the
temperature of the gas depends on the velocities of the molecules, and
in any part of the gas these velocities are very different. Suppose
that the demon saw a molecule approach which was moving at a much
greater velocity than the mean: he would then open the door in the
aperture and let it pass through from - to +. On the other hand, should
a molecule moving at a velocity much less than the mean approach he
would let it pass from + to -. In this way he would sort out molecules
of high from those of low velocity. But the collisions between the
molecules in either division of the vessel would continually produce
diversity of individual velocity, and in this way the difference of
temperature between + and - would continually be increased. Heat would
thus flow from a region of low to a region of high temperature without
an equivalent amount of work being expended.
[20] Impossible, in the sense that while we are unable to “abrogate” a
physical law, Maxwell’s finite demon could, although his faculties were
similar in nature to ours.
Now we must not introduce demonology into science, so, lest this
fiction of Maxwell’s should savour of mysticism, or something
equally repugnant, we shall state the idea involved in it in quite
unexceptionable terms. The conclusions of physics are founded on
the assumption that we cannot control the motions of individual
molecules. In a mass of gas, or liquid, or in a solid, the molecules
are free to move and do move. Their individual velocities and free
paths vary considerably from each other. These motions and paths
are un-co-ordinated--“helter-skelter”--if we like so to term them.
Physics considers only the statistical _mean_ velocities and free
paths. The irreversibility of physical phenomena, the fact that energy
tends to dissipate itself, the second law of thermodynamics, depend
on the assumption that Maxwell’s demons exist only in imagination. We
must appeal to experience now. There is no _a priori_ reason why the
phenomena of physics should be directed one way and not the other, for
it is possible to conceive a condition of our Universe in which, for
instance, solid iron would fuse when exposed to the atmosphere. In such
conditions organisms would grow backwards from old age to birth, with
conscious knowledge of the future but no recollections of the past.
Experience shows, however, that phenomena do tend in one way--_but this
experience is that of experimental physics_, so that for the latter
science Maxwell’s demons do not exist. Now physiology has borrowed from
physics, not only the experimental methods, but also the fundamental
concepts of thermodynamics. The organism, therefore (so physiology
must conclude), cannot control the motions of individual molecules,
and so vital processes are irreversible. But we have seen that the
processes of terrestrial life as a whole are reversible, or tend to
reversibility. We must therefore seek for evidence that the organism
can control the, otherwise, un-co-ordinated motions of the individual
molecules.
The Brownian movement of very small particles of matter is so familiar
to the biologist that we need not describe it. It is doubtless due
to the impact of the molecules of the liquid in which the particles
are suspended. Groups of molecules travelling at velocities above the
mean hit the particle now on one side, and again on the other, and
so produce the peculiar trembling which Brown thought was life. Now
the particle must be below a certain size in order to be so affected.
Are there organisms of this size? Undoubtedly there are, for many
bacilli show Brownian movements, while we have reasons for believing
that ultra-microscopic organisms exist. Also, on the mechanistic
hypothesis there are “biophors,” the size of which is of the same
order as that of the molecules of the more complex organic compounds.
All these must be affected by the molecular impacts of the liquid in
which they are suspended. Can they distinguish between the impacts of
high-velocity molecules and those of mean-velocity ones, and can they
utilise the surplus energy of the former? This has been suggested by
the physicists. In Brownian movement, says Poincaré, “we can almost see
Maxwell’s demons at work.”
The suggestion is not merely a speculative one, for it is well within
the region of experiment. To prove it experimentally we should only
have to show that the temperature of a heat-insulated culture of
prototrophic bacteria falls while the organisms multiply.
Is it not strange that the biologists, to whom the Brownian movement
is so familiar, should have failed to see its possibly enormous
significance? Is it not strange that the biologists, to whom the
distinction between the statistical and individual methods of
investigation is so familiar, should have failed to appreciate this
distinction when it was made by the physicists? Is it not strange that
while we see that most of our human effort is that of _directing_
natural agencies and energies into paths which they would not otherwise
take, we should yet have failed to think of primitive organisms, or
even of the tissue elements in the bodies of the higher organisms, as
possessing also this power of directing physico-chemical processes?