It may be stated in connection with this subject, and in confirmation of the view above taken, that in many cases of coinplete paralysis of a limb from cerebral disease, the patient, although perfectly clear in his general mental perceptions, is not conscious of the presence of the paralysed member, and really feels as if it did not exist. I have known instances in which this unconsciousness has been so great that the patient has actually mistaken the para lysed part for the limb of some other person coming in contact with him, or for some en tirely foreign substance. One man fancied that his paralysed arm was his wife's, and called to her to take it away. In such cases the morbid state of the brain prohibits the developement of that affection of the centre of sensation upon which the feeling dale connection of the limbs depends.t The same law of action applies to nerves of special, as to those of common sensation. Thus, whilst ordinarily they propagate to the centre impressions made at the periphery, we find nevertheless that irritation of the nervous trunk at any part of its course may give rise to its peculiar sensation; and if the brain be stimulated at the part in which the nerve is implanted, similar sensations may be produced. The phenomena of vision and hearing which are excited in these ways are called "subjective:" they are familiarly known to medical men as not unfrequent precursors of more serious symptoms of cerebral disease. Muscm voli tantes, ocular spectra, and tinnitus aurium, are the most common instances of these pheno mena. Pressure on the eyeball, a galvanic current passed through it or very near rota tion of the body, are capable of giving rise to similar phenomena, by exciting the retina or the central connections of the optic nerve, or by disturbing the circulation of the blood in them. A sense of giddiness, similar to that produced by the means last-named, is also a very com mon symptom of cerebral affection arising from a disturbed circulation, or from the blood being deficient in one or more of its staminal princi ples, or vitiated by some morbid element.
The stimai nerves.—Nervous action is ordinarily provoked by stimuli of two kinds, mental and physical. Mental stimuli are those resulting from the exercise of the will, or from thought. Physical are due to some external excitant ; light, heat, sound, mechanical stimu lation, chemical substances, as acids or alkalis, or electricity.
In all voluntary movements an act of the mind is the excitant of the nerve. Sensations are caused generally by the influence of physi cal agents upon the peripheral extremities of nerves, which communicate with the sensorium commune. The change thus produced in the nerve gives rise, through the medium of this communication, to a corresponding affection of the mind. A mental stimulus, hovi.ever, may affect a nerve of sensation. Such stimulus would originate in that part of the brain which is the seat of the changes connected with the intellectual actions, and affecting the centre of sensation, would excite in certain sentient nerves a change similar to that which a physical stimulus applied to their peripheral extremities is capable of producing. In this way the mind
is capable of exciting pain in any part. When the attention has been long directed to any particular situation, whether it has been pre viously the seat of pain or not, painful sensa tions may be excited there. Of this we have many instances in pmctice. In the treatment of cases of hysteria it is of great importance, on this account, to direct the attention of the patient as much as possible away from any local affection.
Motor nerves are never immediately excited , by a physical stimulus in the ordinary actions of the body. A physical stimulus acts upon a motor nerve always through a sensitive neiwe ; the actions thus produced are, commonly, called reflex actions from the apparent reflexion of the change excited by the afferent or sensitive nerve, in the nervous centre into the motor or efferent nerve. This class of actions was first pointed out and described by Prochaska, who viewed them as consisting " in reflexione impressionum: sensoriarum in motorias." The contact of a, foreign substance, pressure, titillation, are the ordinary physical means by svhich such actions may be excited. As a good example of this may be quoted the act of deglutition at the isthmus faucium.
Physical stimuli of other kinds, however, may excite motor nerves. The pressure of n morbid growth of any kind may irritate such nerves and create spasm of the muscles t/ supply. Any virulent fluid applied to a rn nerve will irritate in a similar way—hot Ivo/ —liquor potasste—a mineral acid—a solution of strychnine, tkc. And for the same reaunt certain morbid matters in the blood may irritatt nerves whether sensitive or motor, causing the so-c.alled neuralgic pain in the one case, nod cramp or spasm in the other.
.Uects of the galvanic stimulus.—The most perfect and powerful physical stimulus of motor nerves, and that which most nearly imitates the natural mental stimulus, is the galvanic current. That the nerve should be duly excited by the galvanic current it is necessary that the current should pass along its fibres for however short a distance. If it pass across the fibre, and at right angles to it, it will produce no effect upon the muscles; but if it travel along it, even for the twentieth or a smaller portion of an inch, it will effectually excite the nerve and its muscles, just as when the will stimulates it to action.
The influence of the galvanic current upon nerves is so remarkable that it deserves the careful study of physiologists and of practition ers in medicine who often have recourse to the galvanic stimulus with the hope of rousing the dormant energies of nerves. It is to the Italian school of Physiciens that we owe the highly interesting series of facts which have been col lected upon the influence of the galvanic cur rent upon nerves, to Galvani, Valli, Volta, Marianini, Nobili, and, although last not least, to my distinguished friend, Professor Matteucci, of Pisa, by whose well-devised ex periments and researches a flood of light has been thrown upon this hitherto obscure and difficult subject.