I shall content myself here with briefly no ticing the points most deserving of attention as bearing upon the laws of action of the nerves.
1. When a galvanic current is passed for how ever short a distance along a nerve which contains motor fibres, muscular contractions will be excited at the moment of completing as well at that of breaking the circuit, but not while the current is passing. These phenomena take place whatever be the direction in which the current be passed, whether from the nervous centre towards the periphery, (when the current is distinguished as the direct current,) or from the periphery towards the centre (when the current is styled the inverse current ).
These effects may be produced in warm as well as in cold-blooded animals. In the former, however, the physical conditions necessary for the display of the vital forces continue for so brief a period that cold-blooded aniinals should be selected for the experiments. On this ac count, as well as because of their peculiar sus ceptibility to the galvanic current, frogs are ge nerally employed for this purpose. The most striking way of exhibiting the influence of the current, direct and inverse, upon the nerves is illustrated by the annexed woodcut. It repre sents a frog prepared in the manner adopted by Galvani. The integuments have been removed from the lower extremities, which have been separated from the trunk by the division of the lumbar region of the spine. The lumbar nerves are carefully raised from the inuscles on which they lie, but are suffered to retain their con nection with the spinal cord and with the thighs. The pelvic bones, however, are re moved so as to admit of the more free separa tion of the extremities, as well as to isolate the nerves more completely. Each leg is immersed in a glass or cup of water, and the current is made to pass through the limbs by immersing each wire of the battery in the water of the cups. It is obvious that in one limb the current is diregt, whilst in the other it is inverse.
The advantage of this arrangement is that it affords great facility in making and breaking the current without bringing the conducting wire of the battery into actual contact with either limb. One wire may be left constantly in the water, while the other can be alternately intro duced or removed from it as we wish to ob serve the effects of completing or of breaking the current.
2. If the current be allowed to pass for a short time through the nerves of a frog, pre.. pared as before-mentioned, contractions will no longer take place in both limbs at the same time, but only in one upon completing the circuit, in the other on breaking. And we shall always find that the contractions occur on rnaking in the limb in which the current is direct, on breaking in the limb in which the current is inverse. I find it useful to adopt the following formula to impress this fact upon the memory ; MD, BI, making direct, breaking inverse.
3. If the current continue to pass for some time longer, these phenomena cease completely 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 novv be inverse in B, and direct in A, fig. 398a. 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 curient may now be passed along the nerves in both limbs at the same time, tintil 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 tinder the title Alternatives l'oWanes. 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 a 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.