The arrangement of the nerves and nervous centres is essentially different from that of muscle or of the electric organ, and so far would suggest a decided difference in the cha racter of the force which they can develope from that produced by the latter textures.
6. A comparison of the muscular with the nervous force throws some light on the nature of the latter, and upon its true relation to elec tricity.
Alattencci has established beyond a shadow of doubt that electricity of teeble tension is gene rated in the ordinary nutrition of the muscles of all animals, arid by a particular arrangement this may be made to assume the current form, passing from the interior to the exterior of the muscle. The source of this electricity is no doubt to be found in the chemical action which accompanies the nutrition of the muscular tissue, " principally. that which takes place in the contact of the arterial blood with' the muscular fibre." The intensity of this cur rent increases in proportion to the activity of muscular nutrition, and in proportion to the rank the animals occupy in the scale of beings. It requires a particular artificial arrangement to accumulate the electricity in such a manner as that it shall affect the galvanometer ; " during life the two electric states evolved in the mus cle neutralize each other at the same points from which they are evolved ;" but in the arrangement of a muscular pile as devised by Matteucci, " a portion of this electricity is put in circulation just as it would be in a pile com posed of acid and alkali, separated from each other by a simply conducting body." During the active contraction of a muscle, however, a force is developed which has re semblance to electricity, and in his early experi ments was regarded in that light by Matteucci. This power is capable of affecting the nerve of the frog in the same manner as electricity. The following experiment displays this :—Take the lower extremity of a frog and skin it; dissect out the sciatic nerve from arnong.the muscles on the posterior part of the thigh, and then sepa rate the thigh by cutting it across just above the knee-joint, leaving the nerve connected . with the knee and leg; this preparation is th galvanoscopiefrog, so called by Matteucci fro the readiness with Nvhich it indicates an elec tric current ; next prepare the loNver extre mities of a frog according to Galvani's method • the nerve of the leg is to be laid upon the mus cles of either thigh, and if these muscles be excited to contraction by mechanically stimula ting the lumbar nerves, or the spinal cord, or by passing a galvanic current through the nerves or the cord, the muscles of the galvanoscopic leg will be simultaneously contracted. If a second and a third galvanoscopic leg be pre pared, and the nerve of the second be laid on the muscles of the first, and that of the third be laid upon the muscles of the second, contrac tions will take place in all three whenever the muscles of the prepared thighs are thrown into contmction. Matteucci, to whom we owe the discovery of this important fact (which he terms induced contractionl has failed to cause a fourth leg to be thus affected.
If the galvanoscopic nerve be laid on the muscles of a frog's thigh in which tetanoid con vulsions have been produced by the cessation of a long continued inverse current, the in duced contractions will be likewise tetanic.t
The annexed woodcuts (figs. 3986 & 398e) will serve to show the manner in which these experiments may be performed.
It is plain, then, that during the contraction of muscles, whatever be the means used to sti mulate them, a force is evolved capable of ex citing a nerve laid upon the exterior of the con tracting muscle to such a degree as to cause contraction of the muscles it supplies. What is thia force ? The readiness with which it excites the nerve of the galvanoscopic leg re sembles the action of electricity, and this view of its nature is favoured by the known fact that during muscular contraction heat is evolved, and in some of the marine animals, light also, ac cording to the observations of Quatrefages. If beat and light be produced during muscular contraction, it is not unreasonable to expect that electricity should be evolved likewise. Matteucci's experiments, however, throw some difficulty in the way of viewing it as such. He finds that this force will freely permeate very imperfect conductors of electricity, whilst it will not traverse substances which are known to con duct electricity. If gold leaf be placed upon the muscle between it and the nerve, the con tractions of the galvanoscopic leg will not take place. If, however, a slight tear be made in the gold leaf, then the nerve may be excited. It is possible that this may arise from the electricity being carried off by the gold leaf, so that it does not affect the nerve at all. Matteucci never succeeded in obtaining the induced con tractions when a solid body WM interposed between the nerve and muscle, however thin it might have been and whatever might be its nature ; for this purpose he used flakes of mica extremely thin, flakes of sulphate of lime, gold leaf, paper smeared with glue, and leaves of vegetables." On the other hand, in interposing sorne sub stances which are known to be bad conductors of electricity, the contractions were obtained. The induced contractions may be excited if the nerve be laid upon the skin over the muscles of the inducing frog. " The experiment," says Alatteucci, " never fails of success, whether the inducing contraction be excited by the electric current or by any stimulus applied to the lum bar plexuses of the inducing froir-." The use of a very bad conducting body, Venice turpen tine, did not prevent the induced contractions. The nearly solid Venice turpentine was ren dered more or less liquid by adding to it a little of the volatile oil of turpentineond with this the Inuscles were smeared over, and the nerve of the galvanoscopic frog was wetted. To prove the bad conducting powers of the mixture em ployed, one pole of the exciting pile was ap plied to the muscle and the other to the galva noscopic frog without exciting the least con traction. Yet the contractions were induced in the galvanoscopic frog by stimulating the muscles of the thigh. This experiment clearly proved, as Matteucci remarks, that the induced contraction may be excited through a stratum of an insulating substance that prevents the propagation not only of the muscular and proper currents, but also of that current which excites the inducing contraction.