Licence: Public Domain Mark
Credit: Physiology of the nervous system / by Robert B. Todd. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![and of longer duration than the direct current: hence the tetanic contractions which remain after the interruption of the current. It is sufficiently obvious why a contraction should occur at the moment of completing the circuit in a nerve. But why the same phenomenon should occur on breaking the cir- cuit is not easily explained. Marianini sup- posed that during the passage of a direct cur- rent through a nen-e a part of the electricity accumulated in it, and on the interception of the current discharged itself, traversing the nerve in an opposite direction, and thus giving rise to contractions. It is not, however, likely that such an accumulation would take place, when the conducting power of muscle is so much better than that of nerve. And further, it is evident that this will not explain the ab- sence of contractions in the direct limb after a time on breaking the circuit. The truth is, that when a continuous current has been passed through the limb of a frog for some time a different state of excitability is established in the nen'e of each limb ac- cording to the direction which the current had taken. That in which the direct current passes becomes exhausted in its powers, while that in which the inverse current passes has its excitability augmented. In the quiescent state a nerve maintains a certain state of tension: the application of a stimulus modifies this tension and causes the nerve to assume a new polar state, which displays itself in the contraction of muscles or the excitation of a sensation or of pain. The electric current is a powerful sti- mulus of the nervous force, and the greatest disturbance of the quiescent state of tension is produced by making the direct current. Upon this current beginning to pass, a new state of tension is established, which is disturbed by breaking the circuit; but if the current have continued to pass too long, the maintenance of the state of unnatural tension exhausts the ner- vous power, and the nerve ceases to respond to any stimulus. Whilst, however, the nerve of the direct limb has assumed one condition, that of the inverse limb has taken on a different one, in which the molecules of the nerve may be conceived to have a disposition the opposite to that which the direct current would produce. Hence only two electric stimuli would restore the particles of the inverse nerve, and so disturb the state of tension into which it had been thrown, namely, making a direct current through the nerv e, or simply breaking the inverse. The tetanoid state which results from the continued passage of the inverse current through a nerve is a phenomenon resulting from the ex- treme augmentation of its polarity. This state is never produced by the direct current; and the instantaneousness with which it is removed by resuming the current, thereby restoring the state of tension which had been disturbed by breaking the circuit, is highly favourable to this supposition. Anything which weakens the force of the current, or diverts a portion of it from the nerve, as the contact of muscles with the nerve, or of much moisture, or the occa- sional reversal of the current making it direct where it had been inverse, will materially re- tard and diminish, or altogether prevent the developement of this phenomenon. The rapidity with which the changes in the nerves, however they may have been excited, are propagated, and the precision with which they are perceived by the mind in the case of sentient nerves, or produced by it in the case of motor nerves, are well calculated to excite our admiration. If the communication between the nerve and the centre be cut off, the will can exert no influence upon the muscles supplied by the nerve below the section ; nor will the mind perceive any stimulus applied to parts which derive their nerves from below the sepa- ration . And this for an obvious reason; be- cause the solution of continuity of the nerve interrupts the propagation of the change which the mental or physical stimulus excites in it. In the case of the voluntary nerves, the effects of the mental stimulus are propagated no further peripherad than the point of section; and in that of the sensitive nerve, the change travels no further centrad than the same point. That this interruption is caused solely by the solution of continuity, and not by any alteration in the pro- perties of the nerve, is proved by the fact that the lower segment of the motor nerve will still continue to respond to a physical stimulus. Mechanical or chemical irritation, or the pas- sage of an electric current along it, will cause its muscles to contract. Such a degree of in- jury to a nerve as will break the continuity of the nervous matter within the tubular fibres is likewi.se sufficient to destroy its power as a propagator of nervous change. This effect may be produced by tying a ligature very tightly round a nerve, or by pressing it with great force between the blades of a forceps. The paralysis, which results from the compression of a nerve by a tumour or in any other way, is, no doubt, due to a similar solution of continuity in the nervous matter. These facts strongly denote the important principle in nervous physiology, that, in pro- pagating the influence of a stimulus, either from periphery to centre, or vice versfi, the whole extent of the nerve-fibre between the point stimulated and its peripheral or central coii- nection is the seat of change; and that the power of develojnng the nervous force is inherent in the nerve-fibre itself is shown by the fact that the stimulation of a muscular nerve, which has been separated from the centre, below the point of section is capable of exciting muscular action. The conducting power of a neive, then, results from its proneness to undergo certain changes, physical or chemical, under the influence of stimuli. We may perceive, then, how important it must be to the healthy action of nerves to pre- serve them in a sound physical condition. A morbid fluid impregnating a nerve at any point may irritate it, or may suspend or destroy its inherent property by modifying its nutrition or impairing its ])hysical condition. Thus we may paralyse nerves by soaking them in a so- lution of opium, or of belladonna, aconite, or to- bacco, in sulphuric ether, or other sedative or](https://iiif.wellcomecollection.org/image/b22332807_0011.jp2/full/800%2C/0/default.jpg)
