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application of some new force to the matter in which they occur, and those which ensue from pre-existing tendencies to change when some force previously operative is neutralised or overcome. The former class of material changes are characterised by an absolute proportion between the force applied and the resulting action; the latter are distinguished by their spontaneity, or the disproportion (often extreme) between the apparent cause and the result.
The endowments of living beings embrace both these forms of action. The first is seen in the processes of nutrition, development, and growth, the forces engaged in which are truly correlated, as Dr. Carpenter has most ably shown, to other forms of force. The changes in which function consists exemplify the second, being effected by the chemical affinities of the elements of the tissues, when the vital resistance is in definite manner and degree diminished.
Treating the question thus on abstract grounds, it can hardly be denied that the view of the vital functions above propounded possesses great simplicity, and by virtue of its wide analogies, a certain amount of à priori probability. It aids in reducing to the smallest number "the assumptions which, being granted, the order of nature as it exists would be the result." But it cannot on such grounds claim acceptance, unless it be capable of an unstrained application to all the phenomena which come within its scope. It would almost appear, indeed, to be so natural an interpretation of the facts of animal existence, that had it been the true one, it could hardly have been overlooked or rejected, and that the class of functional actions must have presented characters which, indicating the direct agency of the vital force, forbade them to be grouped under so simple an expression. I
shall proceed, therefore, to an examination of some of the leading facts connected with the animal functions, and inquire:
I. How far the actions of the nervous system may be interpreted upon the principle suggested. From such an inquiry it is of course necessary to exclude altogether the phenomena of thought and volition, viewed in their psychological relations. Of the mysterious process by which a material change in the brain awakens a perception or kindles a thought, we are entirely ignorant; nor can we form any conception of the mode in which the spiritual will communicates its behests to its obedient instrument. Whatever theory be adopted of nervous action, these relations must remain equally inscrutable. Confining our attention, therefore, to those operations of the nervous system which are strictly physical in their character, it may be remarked that all the stimuli which excite them are adapted to bring into activity the repressed chemical affinities of the organic elements. Thus the nervous force is called into action by mechanical irritation, or motion in whatsoever form applied, by changes of temperature, by chemical reagents, electricity, light, or sound, and by the sapid and odorous properties of matter. It is hardly possible to perceive in these various agents any property in common to which their influence upon the nervous system can with reason be referred, except the power they all (so far as they are known to us) possess of disturbing an unstable chemical equilibrium. They cannot all supply a force which is converted into the nervous force. They have no visible adaptation to cause such a conversion of the vital force. No analogy warrants the assumption that they can immediately produce a state of active polarity. But acting upon a tissue in which the affinities of the component
elements are so delicately balanced, and the inherent tendency to chemical change so strong, it can hardly be otherwise than that they should overthrow that balance, and bring into play the latent and coerced attractions.
"In compounds in which the free manifestation of chemical force has been impeded by other forces," says Liebig, speaking of inorganic substances,1 "a blow, or mechanical friction, or the contact of a substance the particles of which are in a state of transformation, or any external cause whose activity is added to the stronger attraction of the elementary particles in another direction, may suffice to give the preponderance to this stronger attraction, and to alter the form and structure of the compound."
That such an actual change of the composition of the nervous tissue does ensue from the action of the stimulus, is proved by the fact that the same stimulus will not reproduce the effect until after the lapse of a certain interval. This should not be the case if the stimulus merely induced a polar state, or itself assumed the form of the nervous force. The necessity of time for the renewal of the irritability is evidence of an altered composition.
Instances have been adduced from the inorganic world of the production of action in substances prone to change by slight mechanical irritation, which may be referred to as the analogues of the sense of touch. The senses of sight and hearing are susceptible of illustration by similar analogies.
To prepare a plate or paper for photographic purposes, it is only requisite to apply to it a suitable chemical compound, the elements of which tend to assume other relations, and of affinities so weak as to be overcome and
1 Op. cit., p. 207.
neutralised by light. Thus prepared, the paper is called sensitive, and it would appear to furnish a very exact illustration of the process by which vision is affected.
The retina consists of matter prone to change. Its elements break up and enter into new relations immediately the vital force or affinity which holds them in their existing combinations ceases, or becomes impaired. What hypothesis can be more simple than that the luminous rays of the spectrum should have the power, to a certain extent, of neutralising this delicate affinity, and thus causing, or, to speak more correctly, permitting, a definite chemical change to take place in the retina; just as the actinic rays, overcoming the affinities of the photographic salts, cause or permit a new arrangement of their elements?
The sense of hearing also admits of explanation by the application of the same principle. In the texture of the auditory nerve it appears that the chemical and vital forces are so balanced that the sonorous vibrations overthrow the equilibrium, and bring into activity, as in the case of light, the chemical affinity. An illustration of the nature of the action is furnished (if we may compare great things with small) by the fact mentioned by Rogers, that masses of ice and snow of considerable magnitude may be precipitated from the Alpine ridges by the sound of the human voice. The gravitation of the masses, and the resisting forces which maintained them in their places, being in such exact equilibrium, that even so slight a motion of the atmosphere suffices to give the preponderance to the former. This illustration, remote though it may seem, is valuable, as bringing clearly before the mind the essential character of the process which constitutes the animal function. For the stimulus in this case, the aerial vibration, palpably induces the resulting action,
not by any direct agency, nor by a conversion of one form of force into another, but solely by disturbing the equilibrium of the counteracting forces, and neutralising the resistance which opposed the force of gravity.
Such a change of composition in the nervous substance must tend directly, in conformity with all our knowledge of physical laws, to produce a polar state or force, corresponding in every respect with that which we term the "nervous force." The close analogy which exists between the nervous force and electricity, strongly confirms this view of its origin and nature. For we recognise chemical change, and especially the decomposition of compound bodies, by means of stronger affinities acting on their elements, as an invariable source of the electric force; and Mr. Grove has demonstrated its existence as a result of the changes which take place in the photographic process. In the living body, it would appear that the decompositions (if they may be so called) in which the exercise of function consists, give rise to a force-not electric, indeed -but of a peculiar though analogous character, inasmuch as the changes in which it has its origin, though analogous to those which take place in inorganic matter, are yet of a distinct and peculiar order. Thus regarded, the nervous force, in its relation to functional activity, may be defined to be a polar condition, or other molecular change in a nerve, akin to that which exists in a body conveying a current of electricity, and arising from a chemical change either in itself or in any of the tissues with which it is in relation. This change being the result of the chemical affinities of the elements of the tissues, which come into play when the vital resistance is diminished by any force which, so disturbing the equilibrium, is called a stimulus. I have said the nervous force may be thus defined in its relation to functional activity, because there appears to