It has also been long known, that many muscles are excited to contraction by such sti muli, when applied to certain nerves, entering their substance, or to certain parts of the spinal cord or brain, even more effectually than by applications to themselves; and likewise, that it is only when those nerves are entire, up to the brain, that those muscles which are natu rally obedient to the mental stimulus of the Will, can be excited by voluntary efforts.
From these different modes of excitation of the contractile power of muscular parts, diffe rent names have been given to the power itself, as by Haller, who applied the term Vis Tonica to the contraction from distension, Vis Insita to the contraction from irritation of the muscular fibres themselves, Vis Nervosa to the contraction from irritation of a nerve, and Vis Animalis to the contraction from volition, acting at the brain and transmitted through a nerve; or again by Bichat, who applied th'e term Contractilite. Organique Sensible to the contractions excited by any kind of irritation, acting on muscular fibres themselves, and the term Contractilia Animale to those excited by stimuli, whether mental or physical, acting on the nerves, spinal cord, or brain. But it is obviously more correct to distinguish the dif ferent varieties of the vital power according to the phenomena, which the contracting part presents, than according to the manner in which the contractions are excited ; and there fore those terms have fallen much into disuse. In most instances, it is the same vital power of Irritability, as above defined, which is called into action in these different ways.
It is only of late years, that it has been fully ascertained, as to the excitement of vital contractions through nerves : 1, that it is, almost exclusively, in the case of muscles which are naturally subject to the Will, that even physical irritation, confined to the nerves, has power to excite contraction; and 2, that these muscles have nerves, or nervous filaments, from tivo distinct sources, viz. from the anterior and posterior columns of the spinal cord, and their prolongations within the cranium ; and that it is by irritation of the first of these only, (or almost exclusively,) that the muscular con tractions are excited:* From these facts, it appears obvious, that the grand and eternal law of separation, as Haller calls it, of the Vo luntary and Involuntary muscles, consists essen tially, not in different powers of the muscular parts, but in different endowments of the ner vous filaments which enter them.
In regard to the excitation of muscular con traction through nerves, it is also to be ob served, that although the action of muscles in obedience to the wi// is the most obvious and striking example, in the living body, where the intervention of a change in a nerve is known to be an essential condition of ,the act, yet there are many examples of movements, per formed by voluntary muscles, in obedience to mental stimuli, blit not to volitions,—to sensa tions, or other involuntary acts of mind, even in opposition to efforts of the will. These con stitute a very important class of vital motions, and are known to be equally excited through the motor nerves of the muscles concerned in them. Of this kind are not only the irregular agitations of the limbs produced by tickling,.or
the convulsive writhing of body from pain, but also, such regular and admirably precise movements as shrinking when pain is excited on the surface, closing the eyelids when the eyes are offended by bright light, swallowing, breathing, coughing, sneezing, vomiting, ex pulsion of fmces and urine, &c. consequent on certain sensations of the fauces, lungs, air passages, nostrils, stomach, rectum, or blad der.. Such muscular actions, excited by irri tation of distant parts, have been generally but vaguely described as the effects of Sympathies of one part of the living body with another. It is well ascertained that they are effected through the motor nerves (or certain of the motor nerves) of the muscles concerned in them; and their dependence on the Sensations, and therefore on the sensitive nerves, of the parts from the irritation of which they orioinate, has been sufficiently illustrated by fialler, Whytt, Monro, and others.* It has also been observed, by Haller and Whytt, but more frequently and carefully by Legalloisif Flourens, and Mayoit. that in many animals, (most remarkably in cold blooded, or young warm-blooded animals,) even after the removal of the brain, as long as the circulation can be maintained, move ments of the kind now in question cro on, or may be excited by irritation of thbe sur faces; and that if the spinal cord be divided into several parts by transverse sections, such movements may still be excited in the muscles supplied from each part, by irritation of the portion of the skin vvhich has its nerves from that part of the cord. These facts have (as is believed) usually been thought to denote, that a certain degree of Sensation remains under these circumstances, in connection with the living state of the spinal cord, or of portions of the spinal cord, and medulla oblongata, indepen dent of the brain ; and that it is still through the intervention of sensation, that irritation of the surface of the body excites any con traction of muscles. Dr. Marshall Hall has lately described phenomena precisely of this description, under the title of Excito-motory phenomena, and as proofs of what he terms the Reflex Function of the Spinal Chord § —a power of exciting- contraction in mus cular fibres connected with it, which he supposes that organ to possess equally inde pendently of sensation as of voiition;i1 and as it seems hardly possible to be quite certain of the existence of Sensation in the case of the mutilated animal, this language is perhaps philosophically correct ; but the probability of the existence of Sensation in such circum stances must be allowed to be very great ; and at all events, that sensation is an essential part of the connection between the irritation of distant parts, and the excitement of involuntary mus cular contractions of voluntary muscles, for useful purposes, in the entire and healthy body,—may be held to be a point well esta blished by the observations of Haller, Whytt, Monro, and others, on such sympathetic actions. Accordingly, those actions, in the entire body, which Dr. M. Hall ascribes to the reflex func tion,* are the same, or similar to those, which have been fully treated by Dr.Whytt and others as sympathetic actions, or actions of voluntary muscles excited by sensations.