A DISSERTATION ON THE FUNCTIONS OF THE NERVOUS SYSTEM. INTRODUCTION. THE nervous system, in which term we comprise the cerebrum, cerebellum, medulla oblongata, and medulla spinalis, and the nerves thence distributed throughout the whole body, is of all organs of the animal economy the most important. It is the seat of the rational soul, and the link by which it is united to the body; it is the instrument by which the soul, so long as it is united to the body, produces its own actions, termed animal, and by which it acts on the rest of the body, and the body in turn acts upon it. But, however great may be the importance of the nervous system in these respects, it is of further importance, because it possesses in addition the singular faculty of exciting in the human body various movements with. out the consciousness or assistance of the soul; nay, plainly against its will it can and does excite them without intermission through the whole of life. The nervous system also influences other functions of the human body, as digestion, nutrition, and secretion, which functions do not remain long undisturbed if the nerves be injured. I say nothing of the share which the nervous system is well known to have in almost every disease. From all this it is manifest how valuable results would follow on sedulous inquiry into the structure and functions of the nervous system, inasmuch as much light might be expected to be thrown on medical art; nor ought it to be lightly esteemed as to its results, with reference to those who desire to know themselves. For he who desires to understand more thoroughly his own mind,-the nobler portion of himself,-can understand it only from its operations. But these are never so pure and so unmixed, that the nervous system,-the immediate instrument of the mind, has no part in them; and consequently it is necessary that the structure and functions of the nervous system should be well understood by those who would determine what should be ascribed in animal actions to the operations and structure of the nervous system, and what should be clearly assigned to the immaterial soul alone. After all the earnest attempts of the greatest philosophers and physicians from the earliest ages, to explain the functions of the nervous system, we can hitherto only say with Haller,' it is but a little that we certainly know, that much remains unknown, and if we may judge of the future by the past, that no little will remain unknown for ever. Nevertheless, I do not think all hope should be abandoned, especially if we should be able to detect and remove the cause of that slow progress hitherto made; and this, in my opinion, partly consists in the difficulties of the subject, which nothing but great labour can overcome; and partly in the love of hypotheses, which have been devised to explain the functions of the nervous system. Many, content with these false resemblances of truth, neglect to inquire into the truth itself, and they who do investigate, unless they discard the prejudices which spring from hypothesis, often fail to perceive the truth, even when it is plain before them. I have therefore entered on this attempt, to explain the natural functions of the nervous system, without any hypothesis, and by simple facts only; and should the attempt be approved, and by additions and emendations be rendered more complete (and these I well know my labours to stand in need of), it may be readily and usefully applied to an explanation of the preternatural functions of the same system. I have taken certain observations and experiments of celebrated men as a foundation; I have spoken doubtfully of what was doubtful, and I have preferred to acknowledge my ignorance of what was inexplicable, rather than with the itch of explaining everything to have recourse to improbable hypotheses. How nearly I have attained to the truth in other respects let the indulgent reader decide. Elem. Physiol., tom. v, p. 529. CHAPTER I. THE PRINCIPAL OPINIONS OF AUTHORS, REGARDING THE USES AND FUNCTIONS OF THE NERVOUS SYSTEM, CONCISELY STATED. SECTION I.-THE OPINIONS OF ARISTOTLE. Ir is remarkable how widely Aristotle with many others of the philosophers and stoics have erred in assigning a use for the brain, having described it as an inert viscus, cold and bloodless, an organ sui generis, not to be enumerated amongst other organs of the body, seeing that it is of no use except to cool the heart. He thus explained how the brain might be the refrigeratory of the heart :-Inasmuch as vapours arise from the waters and earth, and when they reach the cold middle region of the air are condensed into water, which, falling upon the earth, cools it; so also, the hot spirits carried from the heart to the brain with the blood, and there being cooled, are condensed into water, which again descends to the heart for the purpose of cooling it. He placed the seat of the rational soul in the heart, where it can exercise all its functions, and he therefore made the nerves (of the use of which, in sensation and motion, he was not ignorant) to arise from the heart. This opinion of Aristotle as to the heart being the seat of the soul, appears to be preserved, even to our own days, in the popular modes of expression, as when a man of a good disposition is said to have a good heart, and the writers on moral science speak of " the cultivation of the heart." It would appear, that anteriorly to Aristotle, Hippocrates had formed a more correct opinion as to the functions of the brain, for in his book 'de Insania,' he observes, that that man is sane whose brain is undisturbed; although, in another book, 'de Corde,' referred, however, to the spurious works, he places the mind of man in the left ventricle of the heart. Plato, the preceptor of Aristotle, also thought differently, for he recognised three distinct faculties of the mind, having three distinct seats: one was the concupiscent, whose seat was in the liver; the second, the irascible, seated in the heart; the third, the 1 De Animal. partib., lib. ii, cap. vii. |