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The knowledge of the mode in which the various functions of the body are exercised constitutes the science of life. The manifestations of life have consequently been considered already. We have seen, that animal and vegetable substances, possess the ordinary properties of matter, but that these properties are singularly controlled, so that organized bodies, are prevented from undergoing those changes that inevitably occur so soon as they become deprived of vitality. The human body is prone to decomposition. It is composed of substances extremely liable to undergo putrefaction, and is kept at a temperature the most favourable for such change; yet so long as life exists, the play of the ordinary affinities is prevented, and this constant resistance to the general forces of matter prevails throughout the whole of existence, even to an advanced old age, when it might be supposed the vital forces must be enfeebled almost to annihilation. The case of solution of the stomach after death, described in the first volume of this work, is an additional and forcible evidence of such resistance. So long as life continues in the stomach, the gastric secretions exert no action on the organ, but, when life becomes extinct, the same secretions act upon the stomach in the same manner that they do upon ordinary dead animal matter. What, then, is this mysterious power, possessed of such astonishing, such incomprehensible properties?
Our knowledge is limited to the fact above stated, that organized matter, in addition to the general physical and chemical forces, possesses one other,—the vital force or principle, vitality or life. This principle exists, not only in the whole, but in every part, of a living body; and its existence is evidenced by the une quivocal signs afforded by the various functions that have been considered, as well as by others to be presently described. Yet it is not equally evinced in all organs: some appearing to be possessed of more vitality than others,—a result probably produced by peculiar texture, as it would seem irrational for us to admit a different kind of vital principle, wherever its manifestations appear to be modified.
Admitting the existence of this controlling principle, what, it may be asked, are the functions through which it immediately acts in keeping up the play of the living machine? It has been elsewhere seen, that, in animals, the reciprocal action of innervation and circulation are indispensable, and that if one of these functions be arrested, the other quickly ceases. This is only applicable, however, to animals; and it has been doubted whether it applies to all and to
every part of them, whilst to the vegetable it is altogether inapplicable, unless we regard it, with some physiologists, to possess a rudimental nervous system. The function of sensibility exhibits to us the mode in which the nervous system acts in connecting man with the objects around him, through the agency of volition; but a multitude of other acts take place within him, altogether uninfluenced by volition, and yet indispensable for the maintenance of existence. These last acts are equally met with in the animal and vegetable; and hence a division has been made, by Bichat, into animal life, and organic life:—the former evidenced by those functions that are peculiar to animals—sensibility and voluntary motion—which require the presence of a great nervous centre, that may receive from, and transmit to, the different parts of the body, the nervous irradiation,—the necessary excitant of the different functions :the latter evidenced by those functions that are common to animals and vegetables, and are inservient to the nutrition of the frame, as digestion, absorption, respiration, circulation, &c., all of which go on without any direct exercise of volition; and occasionally, it has been believed, independently of all nervous influence.
Physiologists may, indeed, on this point, be divided into two classes:—they who consider that the whole of the organic functions are under the government of the nervous influence; and they who think that the nervous influence does not extend to all the organic functions, but only to the principal of them.
The supporters of the first opinion believe, that the agents, or conductors of the nervous influence, are less and less dependent upon the nervous centres, when such exist, the lower the animal is situated in the animal kingdom, and the lower the function; but they consider the nervous influence to be indispensable to every living being, and to every part of such being. In support of this opinion, they are of course compelled to believe, either that a nervous system exists in the vegetable, or that there is a system which appears to exert over every part of it an influence necessary for its life, and which is, consequently, analogous to the nervous system of animals. The organ of this influence is, by some botanists, considered to be the medulla or pith; whence medullary appendages set out, to be distributed to every part of the vegetable, and which are particularly abundant, in such parts as are charged with very active functions,—as the flower.' Brachet maintains this idea, and compares the knots of the pith to the ganglions of the nervous system,—destruction of the pith, and especially of these knots, occasioning the death of the parts, that receive their filaments from them. Dutrochet, again, considers, that nervous corpuscles exist in the pith of vegetables, which constitute the rudiments of a nervous system; only, in the vegetable, this system is diffused, instead of being collected in a mass.
The believers in the earlier formation of the nervous system in the Vol. II.
fætus will necessarily be in favour of this first opinion, and it would, of course, be strengthened if the results of the experiments of Dumas on generation should be found correct, and if the spermatic animalcules, which, according to him, are the agents of fecundation, should be discovered to be the rudiments of the nervous system of the new individual, a circumstance, which, however, is as doubtful as the confirmation is difficult.
The supporters of the second opinion, that the nervous influence does not extend to all the organic functions, assert, that it is chiefly exerted on those functions which are of the highest moment,—the most elevated in animality; that it is less and less in the inferior functions, and ultimately ceases in the lowest acts,—those that immediately accomplish nutrition and reproduction; and the arguments they adduce in favour of their views are, that these lowest acts exist in every living being—vegetable as well as animal; and that in the superior animal, and in man, there are many parts which do not appear to contain nerves. They moreover consider the nervous system as one superadded to living beings, not only for life, nutrition and reproduction, but also, where necessary, for sensation, motion, &c., and hence the prolongations or extensions of this system ought to be sent to the organs of the internal or nutritive functions, for the purpose of connecting them with the organs of the external or sensorial functions; and that it is in these connexions only that innervation consists. In this view, consequently, the nervous influence arises only from the necessity of connecting the organs; is but an indirect condition of life; exists in the upper animals only, and can in no way be invoked to account for vegetable life.
The last is, in our view, the most accurate opinion. We cannot in the present state of knowledge, admit the existence of nerves in the vegetable: certainly no such thing as a nervous centre is discoverable, and yet we find the most complicated acts of nutrition and reproduction exercised by it, and the principle of instinct, we shall find, as strikingly evidenced as in many animals. We are, therefore, irresistibly led to the conclusion, that the manifestations of vitality are but little, if at all, connected with nervous influence, and that the nerves are added, in the upper animals and functions, for other purposes than that of directly communicating vital properties to the part. This deduction will be found confirmed by the facts to be hereafter mentioned, connected with the independence of the vital property of irritability of the nervous influence.
We have elsewhere alluded to the similarity between the nervous and galvanic fluids, and to the notion which has even prevailed of the similarity if not identity between the vital principle and electricity, as well as to the strange views of endosmose and exosmose, recently promulgated by Dutrochet, and which have been so happily commented on by Dr. J. K. Mitchell. The mode, in which Dutrochet assimilates the phenomena of animal and vegetable life to the actions of endosmose and exosmose, is as follows. It is known
that the sap in vegetables ascends from the roots to the stalk; first, by the action of the
spongioles or terminal buds of the roots, which are evidently organs for the absorption and impulsion of the sap; and secondly, by the action of the leaves, which, by exciting an action of transpiration and evaporation at the top of the plant,--the greater in proportion to the warmth and dryness of the air,—exert a kind of aspiration on the sap received by the spongioles. These spongioles Dutrochet considers to be cellular organs containing organic fluids in their interior; and, consequently, they cannot be plunged into water, without the fluid penetrating by endosmose, not only into their interior, but even as far as the top of the stalk. Hence, according to Dutrochet, endosmose constitutes the action of absorption by the spongioles, and is the cause of the circulation of the sap. It presides, also, over the development and nutrition, the movements of composition and decomposition, of plants; for, as it consists of two opposite electric currents, it not only conveys fresh substances incessantly into the interior of the structures, and removes a part of those existing there, but also induces constant chymical modifications in the organic elements of parts;—every electrical action modifying the chymical nature of matter, as every chymical action induces a development of electricity. It is also the agent of the secretions. The exhalation of vegetables is, according to him, no more a simple physical evaporation than their absorption is the effect of capillarity. It also is a phenomenon of endosmose. He does not doubt, that capillarity, gravity, agitation by the winds, &c. exert an influence on the functions of vegetables, but he considers such influence to be accidental, and the true vital motor to be the electrical agent. He regards the medulla or pith of vegetables to be to their organization, what the nervous system is to the organization of animals, and to be intended to dispense everywhere the vital activity, or electricity.
As the conditions of endosmose,—namely, a vesicular structure and the presence of organic fluids denser than water in the vesicles, -exist in animals as well as in vegetables, Dutrochet invokes a similar influence in the case of the former as in that of the latter. In the same manner, as it occasions the progression of the sap in vegetables, it presides over the capillary circulation in animals, and especially over the progression of the blood in the veins, as well as over absorption, secretion, nutrition, &c. All these actions, however, take place by filtration through permeable, organic membranes,—all that has been said of the agency of the venous radicles in absorption, and of the arterial radicles in exhalation and nutrition, being, according to Dutrochet, physiological mythi. The sanguineous system constitutes a cavity devoid of outlet, and it is by filtration through the parietes of the vessels, which constitute it, that it receives, and parts with, its elements. In short, endosmose is the essence of the life of animals, and as it is an electrical phenomenon, electricity, Dutrochet concludes, is the motor of the
life of animals, as it is of that of vegetables. He, moreover, extends his theory to pathology, asserting, that as endosmose is the vital act par excellence and as it is a phenomenon of electricity, we may conceive that diseases may consist in some defect in endosmose or electricity, and that our therapeutical agents should be directed to the modification of such endosmose. Inflammation, for example, is, according to him, hyperendosmose.
It is obvious, that the foundations of a theory, so extensive in its ramifications, ought to be tested by accurate, and repeated in restigation, and that no deductions can be considered established, until this has been accomplished, and the base found to be impregnable. This has not been done. On the contrary, many of the positions have been seriously assailed by Poisson and Mitchell, and even Dutrochet's own faith seems to have been shaken in his electrical theory.
The system of Bachoué de Vialer on innervation appears to rest on still less foundation. This, according to Adelon, is merely an application of the electro-chymical law of Becquerel, that, when two substances, made to communicate with each other by a conducting wire, simultaneously exert a chymical action with a third, a galvanic current is developed, which is always directed from the substance in which this action is strongest, towards that in which it is least. Now, says M. Bachoué, as the electric fluid is always evidenced during chymical action, and as in every organ, a simultaneous chymical action is constantly exerted by the transformation of arterial into venous blood; whilst by means of conductors,—the nerves,—the nervous centres communicate with every part of the organism,—in each nervous cord, a constant galvanic current must be established, proceeding from its central to its peripheral extremity, or vice versa, according as the chymical action, whence this current emanates, predominates at the one or other extremity. This current, according to M. Bachoué, determines the play of each organ; and he explains, as follows, the mode in which it effects the different functions. First. The circulation being continuous in animals, an agent, which is developed in a continuous manner in their interior, must be looked for, as the cause of this function. This agent is the electric fuid, disengaged by the chymical action exerted simultaneously by the blood on the nervous centres, and on the organic tissues at the periphery; but as this action predominates in the centres, the galvanic current resulting from it is established from these centres towards the circulatory organs, and consequently the action of the latter is excited. To determine the current in this direction, nature occasions the afflux of blood to the ganglions of the great sympathetic to predominate,—these ganglions being, in his view, the nervous centres, that preside over the circulation. A greater chymical action is thus induced in the ganglions, and of course a more marked centrifugal galvanic current. This arrangement has likewise the advantage