Chapter XX: 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).

#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.

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).

#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.

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.

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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