NERVOUS SYSTEM, PHYSIOLOGY OF THE.—In inquiring into the physiology of the nervous systern, the first step is to determine the vital endowments of nerves and of nervous centres.
When a nerve is laid bare in a living animal, and a mechanical or electrical stimulus is ap plied to it, we do not find as in muscle that a visible change in the nerve takes place ; on the contrary, the nerve seems to be uninfluenced by the applied stimulus, and the evidence we have to the contrary is derived from the con traction of certain muscles, if the nerve be muscular, or from indications of pain, if it be a nerve of common sensation.
We infer, then, from the contraction of the muscle in the one case, or from the affection of the mind in the other, that the application of the stimulus has wrought a change in the nerve, which, however, is of such a nature as not to be discemed by any means of observation within our reach. We get, however, excellent proof of the excitation of the change in the nerve, from the fact that when a ligature is ap plied to a nerve sufficiently tight to produce a solution of continuity in the nerve fibres, the propagation of the influence of the stimulus beyond the ligature is checked. No kind nor degree of stimulation of a muscular nerve above a ligature so applied is capable. of exciting muscular contraction.
The most remarkable feature which we notice in the experiment of stimulating a muscular nerve, is the. instantaneousness with which the muscular contraction takes place. Although the muscles may be at a considerable distance from the point of the nerve to which the stimu lus is applied, there seems no appreciable interval of time between the application of the stimulus and the contraction of the muscle. And the cessation of the muscular contraction, instantly upon the removal of the stimulus, is equally conspicuous.
It would appear, then, that the change in the nerve is produced and is propagated along the nerve to distant parts, as it were at one and the same moment. This rapidity of the pro duction, and the instantaneousness of propaga tion of the change in the nerve, denote that the nerve fibres must be the seat of a molecular change rapidly propagated along the nerve, from molecule to molecule, from the point of application of the stimulus. The change is
obviously analogous to that which takes place in the particles of a piece of soft iron, in virtue of which the iron acquires the properties of a magnet, so long as it is maintained in a certain relation to a galvanic current ; the magnetic power being instantly communicated when the circuit is completed, and as rapidly removed when it is interrupted.
The action of the stimulus, then, excites a state of polarity of th'e particles of the nerve stimulated ; and this polar state may be in duced in other particles, whether inuscular or nervous, with which the nerve stimulated may be in organic connexion. Just as the polar state of the electrical apparatus is capable of being communicated to the piece of soft iron, which thereby acquires the well-known mag netic properties during the continuance of the excited polarity.
Thus, then, we learn that such is the nature of the nerve fibre, that under the application of a stimulus, mechanical, chemical, or galvanic, it is capable of generating a polar force analo gous in many particulam to that of muscle; this force we call the nervous force, vis nervosa, or nervous polarity.* And if we examine the ordinary mode of the development of the nervous force, in the usual actions of the fmme, we find that under the influence of a tnental stimulus, the will, it is propagated from the nervous centre along the nerves to muscles, or under the influence of a physical stimulus it is propagated along the nerves to the centres, where it is capable of exciting either a sensation or muscular motion in a secondary manner, or both.
But the application of a physical stimulus to a nervous centre may cause the development of nervous force, which may be conducted away from it by nerves which are implanted in it. And thus we learn that the same polar condition which may be produced in nerves is equally capable of being excited in nervous centres. The polar condition of the nerve fibre may be propagated to the nervous centre, or that of the nervous centre to the nerve fibre.