These facts strongly denote the important principle in nervous physiology, that, in pro pagating the influence of a stimulus, either from periphery to centre, or vice versa, the whole extent of the nerve-fibre between the point stimulated and its peripheral or central con nection is the seat of change; and that the power of developing the nervous force is inherent in the nerve-tibre itself is shown by the fact that the stimulation of a muscular nerve, which has been separated from the centre, below the point of section is capable of exciting muscular action. The conducting power of a neive, then, results from its proneness to undergo certain changes, physical or chemical, under the influence of sti uli.
We may perceive, then, how important it must be to the healthy action of nerves to pre serve them in a sound physical condition. A morbid fluid impregnating a nerve at any point may irritate it, or may suspend or destroy its inheient property by modifying its nutrition Or impairing its physical condition. Thus we may paralyse nerves by soaking them in a so lution of opium, or otbelladonna, aconite, or to bacco, in sulphuric ether, or other sedative or narcotic substances ; or, on the other hand, we may unduly excite them by applying a strong solution of strychnia. The contact of a solid body with a nerve may irritate and keep up a continual state of excitement, if it do not destroy its properties. A spicultun of bone, in contact with nervous fibres, is often the cause of the severest forms of neuralgia; inflammation may produce like effects. Various physiail agents may produce similar consequences. The ben umb ing influence of cold is explained in this way. Ex posure to a continuous draught of cold air is a frequent cause of facial paralysis. The giving way of a carious tooth will immediately occa sion toothache by exposing the nerves of its pulp to the irritating influence of the air, or of the fluids of the mouth. And undue heat is likewise injurious to the physical constitution, and, therefore, to the action of nerves. These facts are of great interest in reference to the pathology of nervous diseases, and suggest that the attraction of a morbid material in the blood to a nerve or set of nerves, or to that part of the nervous centre in which such nerves may be implanted, may afford satisfactory explanation of many obscure phenomena of nerves of sen sation.
The org,anic change, whatever be its intrinsic nature, which stimuli, whether mental or phy sical, produce in a nerve, developes that won derful power long known to physiologists by the name vis nervosa, the nervous force. This
force is more or less engaged in all the func tions of the body, whether organic or animal. In the former its office is to regulate, control, and harmonize ; in the latter it is the main. spring of action without which none of the phenomena can take place. It is the natural ex citantof muscular motion, and the display of that wondrous poiverdepends upon its energy ; with out vigour in the developement and application of the nervous force, a well-formed muscular system would be of little use, for it would quickly suffer in its nutrition if deprived of that exercise which is essential to it.
In the various combinations of thought which take place in the exercise of the intellect, there can be no doubt that the nervous force is called into play in the hemispheres of the bmin. Here the stimulus is mental ; the inde pendent operations of the mind excite the ac tion of the appropriate fibres of the brain, and the developement of the nervous force in the brain immediately succeeds the intellectual workings. It is thus that we explain the bodily exhaustion which mental labour in duces; and thus, too, we can understand the giving way of the brain—the inducement of cerebral disease—under the incessant wear and tear to which men of great intellectual powers expose it. On the other hand, physical changes in the brain, of a kind different from those which are normal to it, the circulation of too much, or too little, or of a morbid blood, may excite mental phenomena in an irregular way and give rise to delirium or mania.
Of the conditionsnecessary for the maintenance of the power of developing the nervous force.— From what has been already stated, it is rnani fest that a healthy physical state of the nervous matter, whether in the nerves or in the nervous centres, constitutes the main condition neces sary for preserving in them the power of deve loping the nervous force. And as nerves will not maintain their healthy nutrition unless they be in union with the nervous centres, this union becomes an important condition for the main tenance of this power in nerves. In the ner vous centres the nerves form a connexion with the vesicular matter. We therefore infer that this connexion of tite fibrous and vesicular matter is necessary for the exercise of the peculiar power of nerves, because we know of no instance, either in the human econoiny or in that of the inferior creatures, in which the nervous power is developed without this union.