720M 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



and no contractions are produced. They may, 

 .however, be reproduced by inverting the 

 direction of the current by transposing the 

 conducting wires of the battery. The cur- 

 rent will now be inverse in B, and direct in 

 A, fig. 398. Or the fact may be illustra- 

 ted by another disposition of the legs of the 

 frog. Let both feet be immersed in one vessel 

 and the pelvis in the other. The direct cunent 

 may now be passed along the nerves in both 

 limbs at the same time, until the phenomena 

 of contraction on making or breaking cease. 

 Inverse the current, and the contraction will 

 again become manifest. This fact was first 

 discovered by Volta, and this mode of exhibiting 

 it has been described under the title Alternatives 

 Voltianes. If the inverted current continue 

 some time, exhaustion will be produced ; but on 

 inverting it again or restoring it to its former 

 course, the actions will recommence. 



4. These effects cannot be produced unless 

 the nerves be in a state of integrity. If a liga- 

 ture be tightly applied to the nerve of either 

 limb close to the muscles, the contractions in 

 that limb will no longer take place. Or to give 

 a more striking illustration of this important 

 fact, if a drop or two of pure sulphuric ether be 

 applied to H point of either nerve, the contrac- 

 tions in the limb of that side will be suspended 

 until the effects of the ether pass off. These ex- 

 periments unequivocally shew that the nerves 

 are not merely conductors of the electrical cur- 

 rent, but that the passage of the current through 

 them developes in them a change which influ- 

 ences the contractile force of the muscles. 



5. The influence of the galvanic current af- 

 fords the most striking results when motor 

 nerves are made the subject of the experiments, 

 but Matteucci has shown that sensitive nerves 

 are affected in an analogous way by the inverse 

 and direct current. In a living rabbit the sciatic 

 nerves were exposed, and one nerve was devoted 

 to the direct current, the other to the inverse. 

 Opening and closing both currents were accom- 

 panied with marked signs of pain, which, 

 however, were greatest at the closure of the 

 inverse current. After a short time, the signs of 

 pain are manifested only on opening the direct 

 current and closing the inverse. 



The reader will scarcely fail to observe that 

 both as regards the sensitive and motor nerves, 

 the effect of the electric current, whether in 

 causing pain or in producing contractions, is 

 greatest when the current passes through the 

 nerve in the course in which the nervous force 

 would naturally proceed in the ordinary nervous 

 actions. It is further worthy of notice that the 

 continuance of the direct current exhausts the 

 power of the nerve, while the reversal of the 

 direction of the current, if not too long delayed, 

 restores it. The continuous passage of the 

 current, however, is not marked either by con- 

 tractions or by pain. The interruption of 

 the current by any means at once developes 

 these phenomena; or even the diversion of a 

 portion of it produces the same effect, as Mari- 

 anini showed long ago. If, for instance, the two 

 vessels in which the frog's paws are immersed 

 be connected by a conductor, as an arc of copper 



or silver wire, contractions will take place on 

 making or breaking the connection ; or if the 

 wires of the battery be connected by a third wire 

 of the same material before they dip into the 

 cups, the same effects will be produced. 



The continued transmission of an inverse 

 current through a nerve increases to a remark- 

 able extent its excitability. This is shewn by 

 the following experiments: let the limbs of a 

 frog be placed in two vessels of water and the 

 current be passed through them in the manner 

 above described, and let this be continued for 

 a few minutes. After the lapse of this period, 

 if the circuit be broken by taking one of the 

 wires out of the water, the limb in which the 

 current was inverse will be thrown into a state 

 of tonic or tetanoid spasm for a few seconds, 

 the tetanus ceasing with a clonic convulsion on 

 the renewed completion of the circuit.* 



That these phenomena are due to a change 

 developed in the nerve (not to any affection of 

 the muscles) by the passage of the galvanic 

 current, is clearly demonstrated by applying the 

 galvanic current to a muscle directly, having 

 first removed as much nerve out of it as pos- 

 sible. The muscle will contract equally on 

 making and breaking the circuit, whatever 

 be the direction of the current; nor is it pos- 

 sible to produce tetanic spasm, however long 

 the current may have been continued through 

 it. The following experiment, suggested by 

 Matteucci, also strongly confirms this view. 

 Let the current be passed through the limbs 

 of a frog in the ordinary way. After the current 

 has passed for 25 or 30 minutes, cut the nerve 

 traversed by the inverse current, at the point 

 where it plunges into the thigh, and there will 

 instantly ensue a violent contraction of that 

 limb, which ceases very quickly. If, however, 

 instead of this the nerve be cut where it issues 

 from the spinal cord, so as to leave a certain 

 length of the nerve attached to the thigh, there 

 will be a violent contraction of the muscles, 

 which will be followed by others, and the limb 

 will remain in a tetanic state for 10 or 15 

 seconds or longer.f 



The tetanoid contractions of the muscles 

 may be produced by a rapid series of currents 

 passed through the nerve alternately in the in- 

 verse and direct course, as by the electro-mag- 

 netic or the magneto-electric instrument. These 

 are always greatest and last longest if a portion 

 of the nervous centre remain connected with 

 the limbs. E.H.Weber has lately made a 

 very interesting series of researches by means 

 of the magneto-electric rotation instrument 

 developing the peculiar mode of action of par- 

 ticular muscles.^ 



We cannot explain these remarkable pheno- 

 mena on any other principle than on that which 

 supposes the developement of the nervous force 

 to be associated with the assumption of a polar 

 condition by the molecules of the nerves under 

 the influence of certain stimuli. The inverse 

 current excites a polar state of greater intensity 



* Matteucci, Phil. Trans. 1846. 



t Comptos Rendus, March 15, 1847. 



t Wagner, Wovterbuch. Art. Muskelbewegung. 



