Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗

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 Thus when Sensibility is tested by the now common method,195 it is found that if one leg is withdrawn after a lapse of, say, ten pendulum beats, the other leg, which has not been irritated, will nevertheless, on irritation, be withdrawn in less than ten beats, provided the central agitation caused by the first stimulation has not yet subsided.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.

189. Nerves and centres have different degrees of excitability. The nerve-terminals in the skin are more sensitive to impressions than those in the mucous membrane; those in the alimentary canal are more sensitive than those in the peritoneum; and all nerve-terminals are more sensitive than nerve-trunks. A touch on the surface of the larynx will produce a cough, but the nerve-trunk itself may be pinched or galvanized without producing any such reflex. Moreover, there is the difference of grouping. If the skin of the abdomen be tickled, there is a reflex on the adductor and extensor muscles of the leg; but these movements are reversed if the skin of the back be tickled. Nor indeed are these movements invariable in either case; the one series will sometimes quite suddenly change to the other, if the irritation is kept up. That one and the same stimulus applied to the same spot should now excite this group and now the other, shows that both motor groups are affected, and that the discharge takes place from the one which at the time being is in the highest tension. The alternation of tension explains rhythmical discharge.

THE LAW OF ARREST.

190. The Law of Arrest is only another aspect of the Law of Discharge, and may be regarded as the conflict of excitations. If a stranger enters the room where a woman lies in labor, there will often be caused a sudden cessation of the uterine contractions.198 Again, every one knows how the breathing and the beating of the heart are arrested by the idea of danger. The arrest is in each of the three cases only temporary, because when the shock of the new stimulus has caused its discharge (arrest), the peripheral irritation which caused the former discharges resumes its influence, and uterus, heart, and diaphragm begin to move again, even more energetically. Note, moreover, that not only will the cerebral excitation arrest the spinal discharge—an idea check the contractions of the uterus or the heart—but the reverse also takes place. The brain of the woman may be intently occupied with some scheme for the education or welfare of her expected child, but no sooner do the labor pains set in, than all these cerebral combinations are arrested.

191. One sensation arrests another; one idea displaces another. If the foreleg of a headless frog be irritated, the hind-leg will also be moved by the stimulation; or vice versa. Here there has been a propagation of the excitation in either direction. But if while the legs are thus irritated, and the centres are ready to discharge, another and more powerful irritation reach the centre—say by pinching the skin of the back—there will be no discharge on the legs. If the vagus be irritated, the heart is arrested; but this does not take place if at the same time, or immediately before, the foot has been sharply pinched. A few gentle taps on the abdomen suffice to stop the heart; but if a drop of acid be previously placed on the skin, we tap in vain, the heart continues to beat. Brown Séquard cites several cases in which convulsions were arrested by irritation of sensitive surfaces;199 and Dr. Crichton Browne records a case of a patient in whom there was abolition of spinal reflex, due to cerebral irritation: tickling the soles of the feet, or pricking the toes, which normally excites reflex movements, in this case excited none whatever. “This seems to prove that nerve currents, set in motion by irritation of the brain, or some of its convolutions, transmitted down the cord, may inhibit reflex action.”200 Examples might indefinitely be multiplied. Pinch the skin of a rabbit between the eyes, and you will observe that pulse and respiration are slackened; but if the tail, which is very sensitive, be pinched, this slackening is only momentary, and is succeeded by a quickening—unless the pain be great. Even the effect of intense pain may be neutralized by stimulating the vagus—just as the effect of stimulating the vagus may be neutralized by pain. Claude Bernard found that having dropped ammonia on the eyelid of a dog, the pain caused a convulsive closure of the lid; but on galvanizing the vagus, the lid opened again, to be closed when the galvanism ceased.201 When the heart is beating faintly (as in syncope), any irritating vapor applied to the nostrils will cause a more energetic pulsation; yet a very irritating vapor lowers the action of the heart beating normally, and will even arrest that of a rabbit. Over-stimulation has almost always the opposite effect of moderate stimulation.

192. While there seems every reason to believe that an excitation necessarily affects the whole cerebro-spinal axis, there is no doubt that there is a certain restriction of this irradiation to definite paths, i. e. the responsive discharge is confined to definite groups. Some of these restrictions are connate pathways: we bring them with us at birth; but most of them are pathways acquired after birth. The boy who sheds tears at parting from his mother when he goes to school, will shed no tears when he parts from her to go to college, nay, perhaps will shed none when he parts from her forever: not that his love has lessened, but that the idea of such expression of it as “unmanly” has become an organized tendency and arrests the tears. A youth of southern race, who has not learned to be ashamed of