It is true that if a motor nerve be separated from the nervous centre, its peripheral segment will evince a susceptibility to stimuli, or, in other words, it will retain the power of gene rating the nervous force for some time after the separation. This is, however, only for a short period, as the experiments of Lonzet distinctly show. Longet cut out a portion of the sciatic nerve in dogs, and irritated the lower segment of the nerve on each succeeding day by means of galvanism from a pile of twenty couples, and by mechanical irritation. The nerve ceased to be excitable on and after the fourth day, (" des le quatrieme jour."), These results, although they appear to differ from those obtained by and Sticker, and Steinruch, are not really inconsistent with them. Theseobservers, instead of examining and irritating the lower segment of the nerve each succeeding day after the sec tion, allowed it to remain for an arbitrary period untouched, and then reopened the wound to try the effect of stimulating the nerve. Thus Miller and Sticker waited eleven weeks in one rabbit, five weeks in a second, and two months and a half in a dog, and in all the cases found the nerve inexcitable ; and Steinruch waited four weeks, at which time he found that the power of the nerve had disappeared. It is obvious that there was nothing in any of these experi ments to cast a doubt on the possibility of the nerve having lost its excitability at a much earlier period after the section, and that the selection of five or eight or eleven weeks, as the period when to inquire whether the nerve re tained its excitability or not, was entirely arbi trary on the part of the experimenters.
The rapidity with which a nerve loses its power after it has been separated from the nervous centres clearly denotes that connection with the centre is a necessary condition for the nutritive activity of nerves, and is, therefore, a necessary condition for their functional activity, or, in other words, for the full developement of the nervous force under its appropriate sti muli. There are, however, other facts which, inasmuch as they enhance the.importance of the vesicular matter in the manifestation oh nervous phenomena, give great weight to thal proposition under consideration. These are—I 1. That there is invariably an accumulation of vesicular matter around the points of im plantation of nerves in the centres, as already referred to. This is true of all nerves in the vertebrate and the higher invertebrate, and we know of no reason to doubt it in tl3e lower invertebrate. 2. The quantity of the vesi cular matter around the point of implantation of a nerve is in the direct ratio of its size and of the activity of its function. Under par ticular circumstances the quantity of vesicular matter becomes so large a.s to cause a special ganglionic enlargement of the portion of the centre in which the nerve or nerves may be implanted. The cervical and lmnbar enlarge ments of the spinal cord are due to this cause : the gangliform swellings on the upper part of the spinal cord in the gurnard (trigla lyre) are connected with the exalted func tions of the nerves of touch distributed to the feelers, and contain a large quantity of vesi cular matter. A remarkable instance of the
developement of vesicular nervous matter under similar circumstances is to he found in the electric lobes of the Torpedo in which are im planted the nerves distributed to the electrical org,an. These lobes are of very considerable size, much exceeding that of any other part of the brain, and they contain vesicular matter in large quantity. The nerves implanted in them are of great size.* Such facts as those cited in the preceding paragraph denote clearly that the developement of the nervous force is to a certain extent con nected with the vesicular nervous matter and to such a degree as to justify the opinion ttiat this element of the nervous centres may be viewed as the dynamic matter, the originator of the force. At the same time it must be borne in mind that this form of nervous matter never occurs alone, and that probably the union of the two is necessary for the developement of nervous power. Just as the union of two metals in the galvanic battery is necessary for the developement of the current, while one of them, that, namely, which possesses the greatest affinity for the fluid interposed between them, seems to ori,ginate the current, and is on that account called the generating plate, whilst the other'is called the conducting plate.
Of the nature of the nervous force.—All that we have said respecting the mode of deve lopernent and the laws of the nervous force denotes its polar character.
We can no more detect by our senses any physical change in the piece of soft iron which , is rendered magnetic by the galvanic current, than we can discover a change in the particles of a nerve stimulated to action by the same I current. That both the iron and the nervous matter are thrown into an analogous state by the same agent seems highly probable. In the case of the iron the indication of the assump tion and of the maintenance of the polar state is afforded by its power of attracting particles of iron; while in a muscular nerve the assump tion arid maintenance of the polar state are shown by the active contraction of certain mus cles, or a more tonic state of passive contrac tion. While the current is passing through a motor nerve there is no active contraction of tile muscles; but that these organs are in a more excited state than the ordinary one of passive contraction seems evident enough, from the readiness with which they assume a tetanic condition upon the cessation of the passage of an inverse current which had been allowed to pass through their nerves for some tinse. And the fact demonstrated by Marianini and Matteucci, that the passage of a continuous current through a nerve will after a time exhaust its excitability, although not so quickly as a current frequently interrupted, denotes that the nerve is in an excited state during the actual passage of the galvanic current.