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element, and principle, — have on the same grounds been applied to denote it; all readily lending themselves to any relation with physical agents which may hereafter be ascertained; but liable in common to the objection of designating, as one principle or element, that which we know not to be really such. For the inquiry brings us directly to the most essential of the questions regarding the nervous power, viz., its unity: whether it be one and the same agent, producing diversity of effect from the manner of its transmission, or from the various fabric and vitality of the parts on which it acts; — or whether there are two or more powers, coming under this common appellation as acting through nervous structure, but really different in nature, and thereby producing different results in the body.
This question, to which I shall afterwards revert, becomes more definite as we proceed to specify the several functions appertaining to the nervous system, and especially those of the nerves of sensation and voluntary power. Here the obvious machinery is nearly or altogether the same; it is everywhere blended for the purposes of mutual action and relation; yet are the functions themselves so utterly unlike to our comprehension, that we can in no way conceive the same physical agent, however modified, to be capable of fulfilling both. The opinion, early admitted by many physiologists, that the difference is simply that of the action being centripetal in the sensitive nerves, centrifugal in the motor, merely translates the difficulty into another form; and is still less tenable since the distinctness of these two classes of nervous fibres has been fully demonstrated.
The inquiry extends, and under equal obscurity, to the nervous influence of organic life; if this term be permitted for the power by which numerous muscles are moved, and the several offices of circulation, absorption, and secretion
performed, without any direct intervention of the two great functions just named; though in such various and close relation to them, that it seems as impossible to dissever the two agencies as to understand their dependence on a single source of power. And yet further, in connexion with these organic functions, we are required to recognise the direct influence upon them of all mental emotions, even of the simple act of attention of mind; — testified in every part of life during sleep and dreams, as well as in waking state -connected with some of the most remarkable sympathies of our nature and depending, we must suppose, upon relations of nervous structure for all that concerns its distribution to the several organs of the body.
Nor can we with certainty stop here, when seeking to analyse those different functions of animal life which are performed through the nervous system, in the general understanding of the term. The question occurs, suggested by unceasing consciousness and observation, whether there is not some proper and independent energy, of which, however it be designated, the brain is the immediate source and seat; by which the higher animal functions are maintained and brought into relation with others, and unity given to every part of the individual being; — a principle various in power in different individuals, and forming what has been termed the temperament of each; -varying also in the same individual at different times, and by its excess or deficiency in the living organisation becoming a source of disorder to the functions both of body and mind.* Description applied
* May we reasonably look to this source for explanation of the great exhaustion which often follows sudden or excessive growth of the body (sometimes even ending fatally), from a seeming disproportion between the size of the frame, and the amount of nervous power ministering to its functions? In one or two cases of this kind, however, which I have seen,
to this sensorial agency might, in truth, extend throughout all the phenomena of health and disease; from the exuberant excitement of high bodily and mental vigour, to the sudden collapse which threatens, or occasionally produces, instant death. And its terms might be borrowed, as well from the language of familiar use, as from that which has been current at all times in the schools of philosophy.
It will be manifest that we are treading in this place on the extreme boundary of human knowledge. The close proximity on one side to the mental functions on the other to the doctrine of a vital principle, in the numerous shapes which this has assumed-places the question under an obscurity we can scarcely hope to remove. It may be that the principle inquired into is not really a separate and single element of power, but merely a quality or degree of the other actions by which mind is connected with material organisation;
that we are not entitled to express more by it than a greater or less sensibility; a higher or lower degree of the voluntary power; or the varying strength of impressions made on the body by mental emotions. This indeed is one part of that question, metaphysical more than physiological in its nature, which has perplexed reasoning men in every age, and been fruitful of dispute in proportion to its obscurity. We have powers before us for contemplation, which we cannot identify as the properties of any physical agents, nor interpret by any analogy beyond their own action; and which, while giving unity to many separate organs and functions, seem to be derived from, and supported by, the combination of these functions themselves. On the other side we find them indis
some part of the effect might be attributed to inability of the heart, not exceeding the average size, to carry on a free and healthy circulation through a vascular system thus unduly extended.
solubly blended with the great separate unity of the mind and will; a relation which, from the very conditions of the inquiry, must ever remain a sealed question to the present comprehension of man.*
On many of these points it is expedient for the highest interests of science, that inquiry should not, by any artifices of language, be pressed beyond this boundary. Though insusceptible of strict definition, it is certain and obvious as a general limit, and becomes more so as our knowledge increases in exactness. The precaution suggested by one of the wisest of the ancients, Το γινωσκειν τινων δεὶ ζητειν αποδειξιν, καὶ τινων ou del, is applicable to every age of philosophy. Within this boundary, indeed, there is space enough for the utmost zeal of research; and it is enlarged, rather than narrowed, by the abandonment of all abstractions which are not absolutely requisite to classify the phenomena observed.
These remarks directly apply to the question of a vital principle; a term which, as expressing an agency independent of organisation, and itself capable of organising and giving life, has found its way into every part of physiology and general philosophy; and which, even where rejected by men of acute understanding, is still often seen to lurk in their writings, under some less palpable form of expression. In reasoning, for example, on the active principle of animal organisation — that which operates in the original evolution and unceasing maintenance of the animal frame modern physiologists have almost given separate existence to this, as the primum movens of the system; therein approaching to the theory of Stahl and early writers; and still more
* These remarks apply to the notion held by certain writers of the existence of another sense, needful to establish a community of feeling and consciousness in all parts of the body; the Gemeingefühl or Selbstgefühl of some German physiologists.
explicitly to the doctrine of a vital principle in one or other of its forms.* Upon the latter subject I do not enter further, having nothing to add to what has been so ably written upon it of late years. To suppose the existence of a principle of life, independent of the organs and functions of living bodies, and superadded to give activity to them, would seem, in the present state of our knowledge, the substitution of a bare phrase for the reality of facts. We gain from it no explanation of the vital actions and affinities, the relations of which are the proper objects of study; nor even of the phenomena of generation, and of propagation by simple division in some of the lower animals, which seem most to authorise the conclusion; — but rather impose injurious limits on all other parts of the inquiry which are fairly open to human research. We interpose a separate agent, (for in no other sense can the doctrine of a vital principle, thus independent, be understood) when all that is warranted to our understanding is the assumption of existing and active laws.
I have dwelt longer on these topics, as illustrating some of the difficulties of the subject, and the manner in which they affect the course of research into the nervous system. Recurring now to the question, what other attributes of life depend directly on this part of the animal economy, the large
* In describing the principle of organisation (the nisus formativus of earlier writers), the German physiologists have availed themselves of the richness and redundance of their language to give a more copious and distinct expression of these powers than any we have ventured to adopt. No other modern language could furnish such phrases as "die bewustloss wirkende zweckmässige Thatigteit,” — "die nach vernunftigen Gesetzen wirkende organizirende Kraft," &c., which we find in the writings of Müller, and which appear to give more of individuality and independence to this power than is usually recognised at the present day. His discussion of the question of identity, or other relation, of the mental principle (das psychische princip) to that of life, is a striking specimen of the acuteness of this eminent physiologist.