This very circumstance affords some presumption in favour of this opinion, as well as their being little affected by those causes which act so powerfully upon the volun tary muscles, and still more from the anatomical facts which w ere adduced by Haller, as well as those more re cently brought Forward by Dr. Philip, many of which, we presume, it would be difficult to reconcile with the hypo thesis of their antagonists. The result of the Galvanic experiments, which have been much insisted upon, when duly considered, must be regarded as less favourable to the nervous theory than has been supposed by its advo cates. It is indeed true, that, in the contraction of the voluntary muscles, the influence may be applied through the medium of the nerves, but the case is different with those parts that arc not under the control of the will ; for we find that we can produce no effect upon these muscles by transmitting the Galvanic influence to them through their nerves, or at least no effect at all proportional to the contractility of the parts. The fair inference from these experiments therefore is, that Galvanism is one of those stimulants that act principally through the medium of the nerves, while they throw no light upon the nature of the connexion between the muscular and the nervous systems. We may farther remark concerning this controversy, that no facts or experiments are strictly applicable to it, ex cept those which refer to, the involuntary muscles ; and that it is riot enough to prove that, in certain cases, the stimulant acts primarily upon the nerve, but to show that It does so in every instance.
The uses of the nervous system are two—to maintain a connexion with the external world, and to unite the dif ferent parts of the body into one whole. No point in physiology is better established, than that the nerves are the media by which the external impressions are convey ed to the brain. The eye, for example, is an optical in strument adapted for receiving the rays of light and bring ing them to a focus, so as to form an image of the object on an expansion of nervous matter, which communicates by the optic nerve with the under part of the brain. Now it is found to be as necessary for vision that the nerve should be in a perfect state as that the eye itself should be sound ; and blindness is as certainly produced by a de fect in the one part as in the other. With respect to the muscles of voluntary motion, if the nerve which passes to them from the brain be divided in its course, or even if it be pressed upon, or have its action in any way im paired, motion is as certainly destroyed as if the muscle itself were injured.' As we always find that the effect of a stimulant upon a muscle is completely obstructed by the division of the nerve, although the muscle itself be in a perfect state, it follows, that the action of the nerve, whatever it may be, is something propagated successively from the extremities to the centre of the system. It is by a reference to this principle, that we account for the different paralytic affections in which a part of the body, without any apparent injury, loses its power of motion. In some cases the disease occupies exactly one half of the body, when it is found that one of the hemispheres of the brain is injured, and that all the parts connected with it have their functions impaired ; often it is only a single limb, or even a single muscle, that loses its power of mo tion, and here we are generally able to detect some mor bid cause situated between the brain and the injured part.
The second use of the nervous system which we point ed out is, to connect the different parts of the animal to gether, and to compose one whole out of its various powers and faculties. The functions, which mare immediately originate in contractility, are all necessarily dependent upon each other; but still it is a kind of mechanical de pendence, where we may conceive the effect to be pro duced by the mere operation of the laws of impulse and chemical affinity, and yet where the animal may have no consciousness of identity, and where there may be no con nexion between the various operations, except what arises from the ordinary properties of matter This is, probably, very much the case with vegetables, which possess some functions that appeal- to be analogous to those of animals, but where we have no evidence of a nervous system. In
animals, on the contrary, the brain and nerves produce a connexion of a different nature, where the effect is trans mitted from one part to another without any change be ing produced which can be referred to mere physical agency, and which is of that kind usually denominated sympathetic. The analogy of the inferior animals leads us to the same conclusion respecting this use of the nervous system ; we find, in general, that they possess many of the individval functions in great perfection, but they have not that intimate connexion with each other, so that their separate parts are more tenacious of life ; they can some times even be completely re-produced when they have been destroyed by external violence ; and, in some cases, the whole animal may be multiplied by being cut into a number of parts, in the mariner of a vegetable In these cases. we always find that the nervous system is either altogether warning, or that it is defective, consisting of a number of nervous cords, without any centre in which they unite ; or, if there be a brain, it is small in propor tion to the nerves that proceed from it.
This view of the use and operation of the nervous sys tem may throw some light upon a question which has been frequently agitated, whether there be a sensorium commune, a part of the nervous system to which all im pressions are referred before they excite perceptions ? Another question has been stated, which is very nearly of the same import, whether, when an impression be made upon an organ of sense, as, for example, upon the eye, the perception exists in the eye or in the brain ? The general result of our experience leads us to believe, that there is a common centre of sensation ; and that, in the human subject, it exists in the brain. \Ve have no con sciousness of any impressions made upon the organs of sense when their nervous communication with the brain is destroyed ; and, farther, it has been found, that, when a change has been produced upon the brain, similar to an impression that had been previously transmitted to it from an organ of sense, it has been mistaken for a recent im pression ; this is a well known occurrence in dreams, and in various morbid states of the brain, where we frequently mistake ideas for perceptions. \Ve likewise find that persons who have had the eyes entirely destroyed after they have arrived at years of maturity, can form as perfect conceptions of visible objects as those whose eyes are in a sound state, when any associations tend to recall such perceptions to the mind, making due allowance for the length of time that may have elapsed since the impressions 'were originally received Many of the facts which were mentioned above, when we were considering the nature of the connexion between the muscular and the nervous systems, lead us to the con clusion, that there is a sensorium commune, and that this has its seat in the brain. Physiologists have not, how ever, been satisfied with assigning the brain in general as the common centre of sensation ; but they have endea voured to find out some particular part of it, which may be regarded as the essential organ to which all the rest are subservient The investigation is certainly a curious one ; and, although it may have been rendered ridiculous by the whimsical opinions to which it has given rise, it is in itself a legitimate object of Inquiry.