180. The physiological independence of organs, together with their intimate dependence in the organism, and the fact that this organism is incessantly stimulated from many sides at once, assure us a priori that the “waves” of molecular movement due to each stimulus must sometimes interfere and sometimes blend with others, thus diverting or neutralizing the final discharge in the one case, and in the other case swelling the current and increasing the energy of the discharge. We are accustomed to speak of one part “playing on another,” sympathizing with another, and so on; but what is the process expressed in these metaphors? When an idea, or a painful sensation, quickens the pulse, or increases the flow of a secretion, we are not to imagine that from a spot in the cerebrum, or the surface, there is a nerve-fibre going directly to the heart, or the gland, transmitting an impulse; in each case the central tissue has been agitated by a sudden change at the stimulated point, and the discharge on heart and gland is the resultant of this agitation along the lines of least resistance. The nerves of the great toe, for example, pass into the spinal cord at a considerable distance from the spot where the nerves of the arm enter it; when, therefore, the great toe is pinched, the arm does not move by direct stimulation of its nerves, but by the indirect stimulation which has traversed the whole central substance.
181. This is intelligible when we know that the whole central substance is continuous throughout; but the difficulty arises when we have to explain why, if this central substance is stimulated throughout, only arms and legs respond; in other words, why the toe-centre “plays upon” the arm-centre, and not on the others? When a frog is decapitated, if we gently touch one leg with the point of the scalpel, the leg will move, but only this leg. Prick more forcibly, and both legs will move. Keep on pricking, and all four legs are drawn up, and the frog hops away. Each excitation was propagated along the cord; but the discharge was restricted in the first case to one limb, in the second to two, in the third it involved all the muscles of the trunk. At the sight of a friend a dog wags his tail gently: as there is no direct connection between the optic nerves and the tail, this playing of one centre on another must be by the agency of intermediate centres; and we know that if the dog’s spinal cord be divided, this excitation from the optic centre is no longer possible, yet the tail will wag if the abdomen be tickled, or the leg pinched. Now compare the effect on the dog produced by the sight of his master, or of a friend accustomed to take him out. There is no longer a gentle wagging of the tail, but an agitation of the whole body: he barks, leaps, and runs about; the central stimulation is discharged through many outlets; and could we test the effect, we should find an appreciable alteration in the thermal and electrical condition of the whole organism, with corresponding changes in circulation, secretion, etc. So different are the consequences of two slightly different retinal impressions mingling their stimulations with the same mass of central substance!
182. The discharge is determined by two conditions: the state of tension, and the energy of the stimulation. The state of tension is increased by every stimulation which falls short of a discharge; that is to say, faint and frequent stimulation augments the excitability, whereas powerful stimulation exhausts it. When, therefore, one wave succeeds another in the same direction, it reaches a centre more disposed to discharge; or, as Cayrade expresses it, “a certain agitation of the cells is necessary for the manifestation of their property of reaction, in the same way that the concentric circles produced on the surface of water by a falling stone are more rapid and more numerous if a stone has already agitated the surface.”
183. So much for the tension. What has been called the energy of the stimulation is more complicated. It is not measurable as a simple physical process; we cannot say that a given quantity of any external force will determine a given discharge. It is mostly complicated by psychical processes, and these so modify the result that instead of the predicted discharge there is arrest, or discharge from another centre. Press a dog’s skin with increasing violence, and the effect increases from pleasurable to painful irritation; but whether the dog will cry out and bite, or cry out and struggle to escape, depends upon whether the pincher is a stranger or a friend. If you hurt a dog while removing a thorn from its foot it will cry out, but although the pain causes it to initiate a biting movement, by the time your hand is reached that movement will have been changed, and the dog will lick the hand which he knows is hurting him in the endeavor to relieve him of the thorn. The co-operation of the mind is here evident enough. A purely psychical process has interfered with the purely physiological process. And I shall hereafter endeavor to show that psychical processes analogous in kind though simpler in degree are really co-operant in actions of the spinal cord. The dog would be said to discriminate between the pain inflicted by a friend, and the same pain inflicted by a stranger. In other words, the sensitive mechanism would be differently determined in the direction of discharge, although the initial stimulation was the same in each case. If we admit that the resulting action is in each case the consequence of the particular group of elements co-operating, there will be no ground for denying that analogous discrimination is manifested by the brainless animal, who also responds differently to different external stimuli, and differently to the same stimulus under different central conditions. The brainless frog croaks if its back be gently stroked with the handle of a scalpel; but if the point be used, or if the handle be roughly pressed, instead of croaking, the frog raises his leg in defence. Here the difference in the peripheral irritation has excited a different reaction in the centre; and this might be interpreted as purely physical; if now the leg be fastened, and the movement of defence be thus prevented, the frog will employ the other leg; or adopt some other means of relieving itself from the irritation. It was a mass of registered experiences which determined the dog not to bite his master. An analogous registration of experiences determines the changed reactions of the brainless frog. But this is a point which can only be touched on in passing here, and it is touched on merely to facilitate our exposition of the complicated conditions of neural discharge. These may be formulated in
184. Law IV. The simultaneous influence of several stimuli, each of which separately excites the same centre, is cumulative: stimuli then assist each other, and their resultant is their arithmetical sum.
Simultaneous stimuli, each of which excites a different centre, interfere with each other’s energy, and their resultant is their algebraical sum.
In this law there is a condensed expression of that composition of forces which may either result in Discharge or Arrest. By simultaneity is not to be understood merely the coincidence of impressions, but also the reverberations of impressions not yet neutralized by others. Thus when Sensibility is tested by the now common method,[195] it is found that if one leg is withdrawn after a lapse of, say, ten pendulum beats, the other leg, which has not been irritated, will nevertheless, on irritation, be withdrawn in less than ten beats, provided the central agitation caused by the first stimulation has not yet subsided. But, on the contrary, the withdrawal will be considerably deferred, or even prevented altogether, if at the same time that the leg is acted on by the acid, a more powerful excitation takes place in some other part of the body. In the one experiment we see simultaneous excitation in the same centre and the same direction. In the other simultaneous excitation in different centres. The more powerful excitation suppresses the discharge from the less powerful; but although it prevails, it loses just as much force as it arrests.[196]
185. There is another very interesting experiment by Freusberg, which must be cited here.[197] When the sciatic nerve is divided, the frog’s leg is of course not withdrawn from the acidulated water, because in that case no sensory excitation is propagated from the skin to the centre; but although there is no stimulation from the skin, there is one from the muscles, as appears in the fact that if a small weight be suspended on this leg, the other leg is more rapidly withdrawn from the acidulated water—the action of the muscles having affected the centre and increased its excitability.
186. When the motor group of one leg is moderately stimulated, the discharge is confined to the muscles of that one leg; and according to Herzen the excitability of the motor group of the other leg is thereby somewhat diminished. But if the stimulation be increased, there is an irradiation to the other group, which irradiation, although not sufficient to excite a discharge, renders it much more ready to discharge, so that a feeble stimulus suffices. This accords with Setschenow’s observations, and is confirmed by Freusberg’s experiment, in which, when one leg was stimulated by acid, if the acid were not wiped off but allowed to keep up the irritation, the other leg moved without being irritated; and this other leg having come to rest, when in its turn dipped in the acid, was more rapidly withdrawn than the first leg had been on first being stimulated; showing that the central groups had become more excitable by the stimulation of either leg.
187. While it is intelligible that an excitation of one group should increase the activity of neighboring groups, by an increase of the vascular activity of the region, it is not so readily intelligible why the feebler excitation of one group should diminish the excitability of its neighbor; yet the facts seem to warrant both statements.
188. The conditions which determine Discharge are obscure. We may, however, say that anatomical and physiological data force the conclusion that whenever the central tissue is powerfully stimulated in any one part, there is either a discharge, or a greater tension (tendency to discharge) in every other part; in consequence of which, every fresh stimulus in the same direction finds the parts more prepared to react; while every fresh stimulus in a contrary direction meets with a proportional resistance. Stated thus generally, the principle is clear enough; but the immense complication of stimulations, and the statical variableness of the organs, renders its application to particular cases extremely obscure. Why does the ticking of a clock arrest the attention, even with unpleasant obtrusiveness, at one time, and presently afterwards cease to be heard at all? Why does the cut of a knife cause intense pain, and a far greater cut received during the heat and agitation of a quarrel pass unfelt? Why will the same external force excite convulsions in all the muscles, and at another time scarcely be distinguishable? These are consequences of the temporary condition of the centres; but there are permanent conditions which in some organisms determine equally variable results. Thus the shock of terror which will simply agitate one person, will develop an epileptic attack in another, and insanity in a third; just as exposure to cold will in one person congest the liver, in another the lungs. A loud and sudden sound causes winking in most persons, and in many a sort of convulsive shock. The harsh noise of a file causes a shiver in some persons, and in others “sets the teeth on edge,” while in others it causes an increased flow of saliva.