PHYSIOLOGY OF THE NERVOUS SYSTEM. 



720v 



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 nerve 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 nerve 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 maintain? 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 nerve, 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 inslantaneousness 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 centrnd than the same point. That this 

 interruption is caused solely by the solution of 

 continuity, and not by anyalteration in the pro- 

 perties of the nerve, is proved by the fact I hat 

 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 

 likewise 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 versa, the 

 whole extent of the nerve-fibre between the point 

 stimulated and its peripheral or central con- 

 nection is the seat of change; and that the power 

 of developing 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 physical condition. Thus we 

 may paralyse nerves by soaking them in a so- 

 lution of opium, or of belladonna, aconite, of to- 

 bacco, in sulphuric ether, or other sedative or 



