The opinion that the nerve controls the nutrition of the parts which it supplies has been advocated by 'Magendie, more particularly with regard to the eye. It has been already stated that we are indebted to this writer for information in regard to results of the division of the entire trunk of the nerve within the cranium. Of these the most prominent is the entire loss of sensibility on the same side of the face, and in regard to the eye especially, loss of sensibility in the conjunc tiva, upon which the most irritating chemical agents then produce no impression. These immediate effects of the section were followed by others not less remarkable : on the next day the sound eye was found inflamed by the ammonia, which had been applied to it, while the other presented no trace of inflatn mation. Other changes, however, supervene. The cornea of the eye of the side on which the section is made, twenty-four hours after wards begins to become opaque; after seventy two it is much more so ; and five or six days after it is as white as alabaster. On the second day the conjunctiva becomes red, inflames, and secretes a puriform matter. About the second day the iris also becomes red and in flames, and false membranes are formed upon its surfime. Finally the cornea ulcerates, the humours of the eye escape, and the globe contracts into a small tubercle. In endeavouring to ascertain the cause of these changes, Ma gendie, on the supposition that they might be owing either to the continued exposure of the eye to the air or to the want of the lachrymal secretion, divided the portio dura in one rabbit, the effect of which is to destroy the power of closing the eyelids; and from others he cut out the lachrymal gland ; but in neither case did opacity of the cornea suc ceed. The sequence of the effects mentioned after the section of the nerve might naturally lead us to infer that the loss of nervous in fluence gives rise to them. But such is not the inference drawn by Magendie, nor indeed can it be admitted : absence or subtraction of an influence cannot be directly the cause of an alteration in the condition of an object otherwise than by allowing it to come or return to a state from which it is preserved by the presence of the influence ; and there is no good reason, either theoretical or experimental, for believing that the state induced in the case under consideration is one in which the eye would necessarily be, which, in fact, would be natural to the organ but for the restraining influence exerted through the fifth nerve.
It is easy to imagine that the absence of such an influence should render a part slow to take on any vital action ; though even this, until proved, is an assumption—an assumption which we are induced to adopt from the fre quency with which sensation and pain are found associated with the establishment of cer tain vital processes, more particularly inflam mation, but which is, on the other hand, con tradicted by the readiness with which inflam mation and its consequences are excited in parts whose nervous faculties are impaired or destroyed by agencies which make little or no impression when those faculties are retained, and which must be demonstrated before admit ted, since it is manifest from the occurrence of that process after the destruction of all trans mitted influence at least, that the principle— the main-spring of it must reside elsewhere; and hence that, if in the natural state the nerve influence the process at all by means of such a property, it can be only in the character of a secondary and controlling power. It does, however, seem proved by the result of Magen die's experiment, that the interruption of the influence did retard the inflammatory process, inasmuch as the eye, on the side of the undi vided nerve, was very actively inflamed the day after the application of ammonia to it, whilst the other eye did not present any trace of in flammation; a circumstance by the way diffi cult, if not impossible, to reconcile with the doctrine that the process of inflammation is directly influenced in either way, whether posi tively or negatively, by the power of the nerve; and further, that the division of the nerve should diminish the vital powers of the eye, and thereby render it less able to resist the effects which inflammatory action tends to pro duce. But indeed there does not appear any
reason for admitting that the alterations which took place in the condition of the eye were produced directly by the loss of nervous influ ence. Having, as he conceived, disproved, by the experiment related, the idea that the alte rations were owing to the continued exposure of the eye to the air, or to the want of the lachrymal secretion,—the only other causes which appear to have occurred to him,—Ma gendie arrived at a conclusion the opposite of that just mentioned, and adopted the opinion that the phenomena " depend upon an influ ence purely nervous"* exerted by the fifth nerve upon the eye,—" an influence independent of the connection of the nerve with the spinal mar row,"t —an influence " proper to the nerve, which has not its source in the cerehro-spinal system, and which is even the more energetic, the farther we remove from that system to a certain distance," of which the following is his proof. "Alterations of nutrition in the eye are the less complete, the less rapid, as we remove farther from the point of branching of the nerves of the fifth pair, and as we cut, within the cra nium, the fasciculus of origin the nearer to its insertion ; finally, the section of the nerve on the margin of the fourth ventricle no longer produces any alteration in the state of the eye." * In this view there are plainly two posi tions advanced, viz. that the nerve does itself exert a proper and independent influence upon the nutrition of the eye, and that it is the sec tion of the nerve which causes the exercise of that influence, or, to use his own words, which is the cause of the inflammation, &c. That the occurrence of the alterations in the eye, in the case in question, is not due to an influence exerted by the brain through the nerve, and that it must proceed from another cause, and that not dependent upon the connection be tween them, is manifest, since it is consequent upon the interruption of that connection ; and therefore, if the nerve do possess the supposed influence, it must be a proper and independent one : but are we, therefore, to infer that the nerve does exert such an influence upon the organ ? It appears to the writer that we cannot : for can we suppose that the nerve is endowed with a property to be displayed expressly tinder cir cumstances, which it is fair to say were not contemplated in the establishment of natural laws, viz. in cases of mutilation ? or is it possi ble that a separate influence can exist in the nerve and increase in energy in proportion as the nerve is curtailed ; for the nearer the section is made to the eye, the more remarkable are the effects; or if any other proof that the nerve does not possess such an influence he wanting, can we suppose that it is possessed for the eye and not for the other parts to which the branches of the nerve are distributed ? Why does not inflammation forthwith assail the nostrils, the mouth, and cheeks upon the mere section of the nerve,t as well as the eye ? Manifestly be cause no such influence exists; and indeed the data upon which it has been assumed, instead of proving the position, leave it precisely as it was; for insomuch as the occurrence of the phenomena upon the section prove the exist ence of the influence of the nerve, in the same degree does the absence of the phenomena upon the section of the nerve disprove it.