GENERAL SURVEY AND CONCLUSION
We have seen that stimulus produces a certain excitatory change in
living substances, and that the excitation produced sometimes expresses
itself in a visible change of form, as seen in muscle; that in many
other cases, however--as in nerve or retina--there is no visible
alteration, but the disturbance produced by the stimulus exhibits itself
in certain electrical changes, and that whereas the mechanical mode of
response is limited in its application, this electrical form is
universal.
This irritability of the tissue, as shown in its capacity for response,
electrical or mechanical, was found to depend on its physiological
activity. Under certain conditions it could be converted from the
responsive to an irresponsive state, either temporarily as by
anæsthetics, or permanently as by poisons. When thus made permanently
irresponsive by any means, the tissue was said to have been killed. We
have seen further that from this observed fact--that a tissue when
killed passes out of the state of responsiveness into that of
irresponsiveness; and from a confusion of 'dead' things with inanimate
matter, it has been tacitly assumed that inorganic substances, like
dead animal tissues, must necessarily be irresponsive, or incapable of
being excited by stimulus--an assumption which has been shown to be
gratuitous.
This 'unexplained conception of irritability became the starting-point,'
to quote the words of Verworn,[21] 'of _vitalism_, which in its most
complete form asserted a dualism of living and lifeless Nature.... The
vitalists soon,' as he goes on to say, 'laid aside, more or less
completely, mechanical and chemical explanations of vital phenomena, and
introduced, as an explanatory principle, an all-controlling unknown and
inscrutable "force hypermécanique." While chemical and physical forces
are responsible for all phenomena in lifeless bodies, in living
organisms this special force induces and rules all vital actions.
'Later vitalists, however, attempted no analysis of vital force; they
employed it in a wholly mystical form as a convenient explanation of all
sorts of vital phenomena.... In place of a real explanation a simple
phrase such as "vital force" was satisfactory, and signified a mystical
force belonging to organisms only. Thus it was easy to "explain" the
most complex vital phenomena.'
From this position, with its assumption of the super-physical character
of response, it is clear that on the discovery of similar effects
amongst inorganic substances, the necessity of theoretically maintaining
such dualism in Nature must immediately fall to the ground.
In the previous chapters I have shown that not the fact of response
alone, but all those modifications in response which occur under
various conditions, take place in plants and metals just as in animal
tissues. It may now be well to make a general survey of these phenomena,
as exhibited in the three classes of substances.
We have seen that the wave of molecular disturbance in a living animal
tissue under stimulus is accompanied by a wave of electrical
disturbance; that in certain types of tissue the stimulated is
relatively positive to the less disturbed, while in others it is the
reverse; that it is essential to the obtaining of electric response to
have the contacts leading to the galvanometer unequally affected by
excitation; and finally that this is accomplished either (1) by
'injuring' one contact, so that the excitation produced there would be
relatively feeble, or (2) by introducing a perfect block between the two
contacts, so that the excitation reaches one and not the other.
Further, it has been shown that this characteristic of exhibiting
electrical response under stimulus is not confined to animal, but
extends also to vegetable tissues. In these the same electrical
variations as in nerve and muscle were obtained, by using the method of
injury, or that of the block.
Passing to inorganic substances, and using similar experimental
arrangements, we have found the same electrical responses evoked in
metals under stimulus.
#Negative variation.#--In all cases, animal, vegetable, and metal, we may
obtain response by the method of negative variation, so called, by
reducing the excitability of one contact by physical or chemical means.
Stimulus causes a transient diminution of the existing current, the
variation depending on the intensity of the stimulus (figs. 4, 7, 54).
[Illustration: FIG. 112.--UNIFORM RESPONSES IN (A) NERVE, (P) PLANT, AND
(M) METAL
The normal response in nerve is represented 'down.' In this and
following figures, (A) is the record of responses in animal, (P) in
plant, and (M) in metal.]
#Relation between stimulus and response.#--In all three classes we have
found that the intensity of response increases with increasing stimulus.
At very high intensities of stimulus, however, there is a tendency of
the response to reach a limit (figs. 30, 32, 84). The law that is known
as Weber-Fechner's shows a similar characteristic in the relation
between stimulus and sensation. And if sensation be a measure of
physiological effect we can understand this correspondence of the
physiological and sensation curves. We now see further that the
physiological effects themselves are ultimately reducible to simple
physical phenomena.
#Effects of superposition.#--In all three types, ineffective stimuli
become effective by superposition.
Again, rapidly succeeding stimuli produce a maximum effect, kept
balanced by a force of restitution, and continuation of stimulus
produces no further effect, in the three cases alike (figs. 17, 18, 86).
#Uniform responses.#--In the responses of animal, vegetable, and metal
alike we meet with a type where the responses are uniform (fig. 112).
#Fatigue.#--There is, again, another type where fatigue is exhibited.
[Illustration: FIG. 113.--FATIGUE (A) IN MUSCLE, (P) IN PLANT, (M) IN
METAL]
The explanation hitherto given of fatigue in animal tissues--that it is
due to dissimilation or breakdown of tissue, complicated by the presence
of fatigue-products, while recovery is due to assimilation, for which
material is brought by the blood-supply--has long been seen to be
inadequate, since the restorative effect succeeds a short period of rest
even in excised bloodless muscle. But that the phenomena of fatigue and
recovery were not primarily dependent on dissimilation or assimilation
becomes self-evident when we find exactly similar effects produced not
only in plants, but also in metals (fig. 113). It has been shown, on the
other hand, that these effects are primarily due to cumulative residual
strains, and that a brief period of rest, by removing the overstrain,
removes also the sign of fatigue.
#Staircase effect.#--The theory of dissimilation due to stimulus reducing
the functional activity below par, and thus causing fatigue, is directly
negatived by what is known as the 'staircase' effect, where successive
equal stimuli produce increasing response. We saw an exactly similar
phenomenon in plants and metals, where successive responses to equal
stimuli exhibited an increase, apparently by a gradual removal of
molecular sluggishness (fig. 114).
[Illustration: FIG. 114.--'STAIRCASE' IN MUSCLE, PLANT, AND METAL]
[Illustration: FIG. 115.--INCREASED RESPONSE AFTER CONTINUOUS
STIMULATION IN NERVE AND METAL
The normal response in animal tissue is represented 'down,' in metal
'up.']
#Increased response after continuous stimulation.#--An effect somewhat
similar, that is to say, an increased response, due to increased
molecular mobility, is also shown sometimes after continuous
stimulation, not only in animal tissues, but also in metals (fig. 115).
#Modified response.#--In the case of nerve we saw that the normal
response, which is negative, sometimes becomes reversed in sign, i.e.
positive, when the specimen is stale. In retina again the normal
positive response is converted into negative under the same conditions.
Similarly, we found that a plant when withering often shows a positive
instead of the usual negative response (fig. 28). On nearing the
death-point, also by subjection to extremes of temperature, the same
reversal of response is occasionally observed in plants. This reversal
of response due to peculiar molecular modification was also seen in
metals.
[Illustration: FIG. 116.--MODIFIED ABNORMAL RESPONSE IN (A) NERVE AND
(M) METAL CONVERTED INTO NORMAL, AFTER CONTINUOUS STIMULATION
(A) is the record for nerve (recording galvanometer not being dead-beat
shows after-oscillation); the abnormal 'up' is converted into normal
'down' after continuous stimulation. (M) is the record for metal,
the abnormal 'down' being converted into normal 'up' after like
stimulation.]
But these modified responses usually become normal when the specimen is
subjected to stimulation either strong or long continued (fig. 116).
#Diphasic variation.#--A diphasic variation is observed in nerve, if the
wave of molecular disturbance does not reach the two contacts at the
same moment, or if the rate of excitation is not the same at the two
points. A similar diphasic variation is also observed in the responses
of plants and metals (figs. 26, 68).
#Effect of temperature.#--In animal tissues response becomes feeble at low
temperatures. At an optimum temperature it reaches its greatest
amplitude, and, again, beyond a maximum temperature it is very much
reduced.
We have observed the same phenomena in plants. In metals too, at high
temperatures, the response is very much diminished (figs. 38, 65).
#Effect of chemical reagents.#--Finally, just as the response of animal
tissue is exalted by stimulants, lowered by depressants, and abolished
by poisons, so also we have found the response in plants and metals
undergoing similar exaltation, depression, or abolition.
We have seen that the criterion by which vital response is
differentiated is its abolition by the action of certain reagents--the
so-called poisons. We find, however, that 'poisons' also abolish the
responses in plants and metals (fig. 117). Just as animal tissues pass
from a state of responsiveness while living to a state of
irresponsiveness when killed by poisons, so also we find metals
transformed from a responsive to an irresponsive condition by the action
of similar 'poisonous' reagents.
The parallel is the more striking since it has long been known with
regard to animal tissues that the same drug, administered in large or
small doses, might have opposite effects, and in preceding chapters we
have seen that the same statement holds good of plants and metals also.
#Stimulus of light.#--Even the responses of such a highly specialised
organ as the retina are strictly paralleled by inorganic responses. We
have seen how the stimulus of light evokes in the artificial retina
responses which coincide in all their detail with those produced in the
real retina. This was seen in ineffective stimuli becoming effective
after repetition, in the relation between stimulus and response, and in
the effects produced by temperature; also in the phenomenon of
after-oscillation. These similarities went even further, the very
abnormalities of retinal response finding their reflection in the
inorganic.
[Illustration: FIG. 117.--ABOLITION OF RESPONSE IN NERVE, PLANT, AND
METAL BY THE ACTION OF THE SAME 'POISON'
The first half in each set shows the normal response, the second half
the abolition of response after the application of the reagent.]
Thus living response in all its diverse manifestations is found to be
only a repetition of responses seen in the inorganic. There is in it no
element of mystery or caprice, such as we must admit to be applied in
the assumption of a hypermechanical vital force, acting in contradiction
or defiance of those physical laws that govern the world of matter.
Nowhere in the entire range of these response-phenomena--inclusive
as that is of metals, plants, and animals--do we detect any breach of
continuity. In the study of processes apparently so complex as those of
irritability, we must, of course, expect to be confronted with many
difficulties. But if these are to be overcome, they, like others, must
be faced, and their investigation patiently pursued, without the
postulation of special forces whose convenient property it is to meet
all emergencies in virtue of their vagueness. If, at least, we are ever
to understand the intricate mechanism of the animal machine, it will be
granted that we must cease to evade the problems it presents by the use
of mere phrases which really explain nothing.
We have seen that amongst the phenomena of response, there is no
necessity for the assumption of vital force. They are, on the contrary,
physico-chemical phenomena, susceptible of a physical inquiry as
definite as any other in inorganic regions.
Physiologists have taught us to read in the response-curves a history of
the influence of various external agencies and conditions on the
phenomenon of life. By these means we are able to trace the gradual
diminution of responsiveness by fatigue, by extremes of heat and cold,
its exaltation by stimulants, the arrest of the life-process by poison.
The investigations which have just been described may possibly carry us
one step further, proving to us that these things are determined, not by
the play of an unknowable and arbitrary vital force, but by the working
of laws that know no change, acting equally and uniformly throughout the
organic and the inorganic worlds.
FOOTNOTES:
[21] Verworn, _General Physiology_, p. 18.
INDEX
Action current
in metal, 88
in nerve, 8
in plant, 19
After-images and their revival, 177
After-oscillation in photo-sensitive cell, 159, 163
Anæsthetics, effect on response
in nerve, 72
in plant, 30, 73, 74, 75
Annealing, effect on response in metal, 101, 138
Binocular alternation of vision, 175
Block method, advantages of, 28, 77
for obtaining response
in metal, 82
in plant, 28
Chloral, effect on plant response, 75
Chloroform, effect on
nerve response, 72
plant response, 74
Compensator, 22
Current of injury in nerve, 7
Curves, characteristics of response, 3
Death-point, determination of, in plants, 61, 63
Depressants, effect on inorganic response, 142
Depression, response by relative, 87
Dewar on retinal current, 149
Diphasic variation
in metal, 113, 114, 115, 116, 188
in nerve, 188
in plant, 46, 188
Dose, effect on
inorganic response, 89, 146, 189
plant response, 79, 189
Electrical recorder, 11
Electrical response. _See_ Response, electrical
Electric tapper, 24
Exaltation, response by relative, 89
Fatigue,
absence of, under certain conditions,
in metal, 120
in muscle, 39
in plant, 39
apparent, with increased frequency of stimulation,
in metal, 120
in muscle, 40
in plant, 40
diminution of response under strong stimulus due to, in plant, 57
in metal, 118, 119, 185
in muscle, 118, 185
in plant, 20, 185
due to overstrain, 41
rapid, under continuous stimulation
in metal, 121, 130
in muscle, 42, 130
in plant, 42, 130
removal of, by rest in plant, 43
theory of, in muscle, 38, 185
Holmgren on retinal current, 149
Hysteresis, 137
Injury, current of, in nerve, 7
Inorganic response. _See_ Metal, electrical response in
Kuhne on retinal current, 149
Kunkel on electrical changes by injury or flexion in plant, 14, 70
Light,
after-effect of short exposure to,
on photo-sensitive cell, 171
on retina, 171
decline and reversal of response under continuous,
in photo-sensitive cell, 166
in retina 166
effect of temperature on
response of photo-sensitive cell produced by, 158
retinal response produced by, 158
relation between intensity and response to,
in photo-sensitive cell, 161, 162
in retina, 162
response to,
after-oscillation in photo-sensitive cell, 159, 163
effect of increasing length of exposure
in photo-sensitive cell, 159
in retina, 160
in frog's retina, 150, 151, 156, 164, 166
in photo-sensitive cell, 152, 153, 154, 155, 157, 165, 166
McKendrick on retinal response, 149
Mechanical
recorder, 3
response, 1
stimulus
by electric tapper, 24
by spring-tapper, 23
by vibrator, 24
conditions of maintaining uniformity of, 26
means of graduating intensity of, 22, 24, 96
Metal, electric response in,
abnormal, 125
abolition of, by 'poison,' 143
additive effect of superposition of stimulus on, 135
annealing, effect of, on, 101
by method of
block, 82, 92
negative variation, 87, 183
depressants, effect of, on, 142
diphasic, 113, 114, 115, 116, 188
enhancement of, after continuous stimulation, 127, 128, 186
fatigue, 118, 119, 120, 121, 185. _See also_ Fatigue
maximum effect due to superposition of stimuli, 136
modified, 129
'molecular arrest,' effect of, by 'poison' on, 145
molecular friction, effect of, on, 108, 109
prolongation of recovery
by overstrain, 106
by 'poison,' 145
relation between, and stimulus, 134, 135
staircase effect, 122, 186
stimulant, effect of, on, 141
temperature, effect of, on, 111
uniform, 102, 184
Minchin on photo-electric cell, 165
Molecular
'arrest' in metals by 'poison,' 145
friction, 108, 109
model, 107
voltaic cell, 99
Munck on electric response in sensitive plants, 14
Muscle, fatigue in, 38, 39, 40, 42. _See also_ Fatigue
prolongation of recovery by 'poison' in, 144
relation between stimulus and response in, 52
staircase effect in, 122
stimulus, effect of superposition of, on, 36
Myograph, 2
Negative variation, response by method of,
in metal, 87, 183
in nerve, 9, 183
in plant, 18, 183
Nerve,
current of injury in, 7
injured and uninjured contacts corresponding to Cu and Zn in voltaic
couple, 8
response in,
abnormal, when stale, 124, 187
abolition of, by 'poison,' 139, 189
anæsthetics, effect of, on, 72
by method of negative variation, 9
current of action of, 8
enhancement of, after continuous stimulation, 127
modified, 128
relation between, and stimulus, 52
reversed when stale, 11
uniform, 184
Nomenclature, anomalies of present, 9, 85
Photographic recorder, 11, 22
Plant
chamber, 64
electrical response
in,
abnormal, when stale or dying, 48, 187
abolition of, by high temperature, 32, 64
additive effect of stimulus on, 37
anæsthetics, effect of, on, 30, 73, 74, 75
by method
of block, 28
of negative variation, 18, 183
diphasic, 46
fatigue, 20, 39, 40, 41, 42, 43, 57, 185. _See also_ Fatigue
physiological character, 30
'poison,' effect of, on, 30, 32, 78, 79
relation between, and stimulus, 52, 53, 54
staircase effect, 37, 185
stimulus,
effect of single, on, 35
effect of superposition of, on, 35
temperature, effect of, on, 32, 59-69
uniform, 36, 184
radial E.M. response in, 49
Poison,
effect of, on response
in metal, 143, 189
in nerve, 139, 189
in plant, 30, 32, 78, 79, 189
'molecular arrest' in metal by, 145
prolongation of recovery by action of,
in metal, 145
in muscle, 144
Record, simultaneous mechanical and electrical, of response, 13
Recorder,
electrical, 11
mechanical, 3
photographic, 11, 22
response, 19
Response-curve,
characteristics of, 3
electrical,
abnormal,
in metal, 123, 125
in stale nerve, 11, 123
in stale or dying plant, 48, 187
in stale retina, 11, 164
converted into normal after strong or continuous stimulation
in metal, 125, 187
in nerve, 124, 187
in plant, 48
abolition of,
by high temperature in plant, 32, 64
by 'poison,'
in metal, 143, 189
in nerve, 139, 189
in plant, 30, 32, 78, 79, 189
additive effect of stimulus on,
in metal, 135
in plant, 37
anæsthetics, effect of, on,
in nerve, 72
in plant, 30, 73, 74, 75
annealing, effect of, on, in metal, 101, 138
by method of block, 28, 82, 92
by negative variation, 9, 18, 87, 183
by relative depression, 87
by relative exaltation, 89
conditions for obtaining, 6, 86, 87
continuous transformation from positive to negative in metal, 115
decline and reversal of, under continuous light in photo-sensitive
cell, 166
decline and reversal of, under continuous light in retina, 166
depressants, effect of, on inorganic, 142
diminution of. _See_ Fatigue
diphasic
in metal, 113, 114, 115, 116, 188
in nerve, 188
in plant, 46, 188
dose, effect of,
on inorganic, 89, 146, 189
on, in plant, 79, 189
enhancement of, after continuous stimulation in metal, 127, 128, 186
enhancement of, after continuous stimulation in nerve, 127, 186
maximum effect due to superposition of stimulus, 35, 136
measure of physiological activity, 13
molecular
friction, effect of, on, 108, 109
modification, effect of, on, 11, 48, 123, 125, 129, 164, 187
physiological character of, in plant, 30
positive and negative, 11
prolongation of recovery in, by 'poison' in metal, 145
prolongation of recovery in, by 'poison' in muscle, 144
prolongation of recovery in, from overstrain, 106
relation between, and stimulus
in metal, 134, 135
in muscle, 52
in nerve, 52
in plant, 52, 53, 54
in real and artificial retinæ, 162
staircase effect,
in metal, 122, 186
in plant, 37, 186
stimulant, effect of, on, in metal, 141
temperature, effect of, on. _See_ Temperature
threshold of, 135
to light. _See_ Light
uniform
in metal, 102, 184
in nerve, 184
in plant, 36, 184
universal applicability of, 12
mechanical, 1
retinal. _See_ Light
simultaneous mechanical and electrical record of, 13
Retina. _See_ Light
Sanderson, Burdon-, on electrical response in sensitive plants, 14
Spring-tapper, mechanical stimulus by, 23
Staircase effect
in metal, 122, 186
in muscle, 122, 186
in plant, 37, 186
Steiner on retinal response, 149
Stimuli, maximum effect due to superposition of,
in metal, 136
in muscle, 36
in plant, 36
Stimulus,
advantages of vibrational, 25
and response, relation between,
in metal, 134, 135
in muscle, 52
in nerve, 52
in plant, 52, 53, 54
in real and artificial retinæ, 162
effect of different kinds of, 2
mechanical,
by spring-tapper, 24
conditions for maintaining uniformity of, 26
means of graduating intensity of, 22, 96
vibrational, 24, 25, 26
Temperature,
death-points in plants, 61, 63
effect of, on response
in metal, 111
in photo-sensitive cell, 158
in plants, 32, 60-69
in retina, 158
increased sensitiveness in plant due to variation of, 66, 67
Vibrational stimulus, 24, 25, 26
Vision,
binocular alternation of, 175
effect of various conditions on the period of binocular alternation
of, 177
Visual
images, revival of, 177
impression, unconscious, 178
impulse,
chemical theory of, 148
electrical theory of, 149
phantoms, 179
recurrence, 174
Vital force, 13
Vitalism, 182
Waller
on enhancement of nerve-response after continuous stimulation, 127
on relation between stimulus and response in muscle, nerve, and retina,
52, 162
on retinal response, 150, 156, 165
on reversal of response in stale nerve and retina, 11, 124, 164
on transformation from abnormal to normal response in nerve after
continuous stimulation, 124
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| Transcriber's note: The following printer errors have been |
| corrected: |
| Diaphasic changed to Diphasic (fig. 26 caption) |
| Dash added (section on Continuous Transformation) |
| Blurr changed to blur (after fig. 111) |
| In creased changed to increased (after fig. 114) |
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| The inconsistent hyphenation of "break-down", "electro-motive" |
| and "vibration-head" is as in the original. |
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