5. The influence of the galvanic current af fords the most striking results when motor nerves are made the subject of the experiments, but hlatteucci 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 smrcely Lit 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 xvhen the current passes through the nerve in the course in which the nervous force would natnrally 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 Alari 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 dtveloped 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 throtigh it. The following experiment, suggested by AIatteucci, 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 yr here it plunges into the thigh, and there will instantly ensue a violent contraction of that limb, which ceases very quickly. lf, 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.t 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 hy the electro-mag netic or the magneto-elearic instrument. These are always greatest and last longest if a portion of the nervous centre remain connected with the limbs. E. 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 intensil.
and of longer duration than the direct current : hence the tetanic contractions which remain after the interruption of the current.