Abbildungen der Seite
PDF
EPUB

of diminishing the conducting power of the nerves, in accordance with the principle in physics, that the power of any body as a conductor of electricity is less in proportion as such body exerts a more powerful electro-motive action, whence it results, that the circulation is freed as much as possible from the perturbations that might otherwise be caused in it by the currents incessantly traversing the other parts of the nervous system,—the cerebral and spinal nerves,with which those of the great sympathetic communicate. So that the action of the circulatory organs is constantly provoked by the centrifugal galvanic current, resulting from the chymical action exerted by the blood simultaneously in the nervous centres, and in the organs at the periphery of the body; whilst the uninterrupted arrival of the blood in the organs constantly excites in them also the chymical action necessary for the development of the electricity, on which the continuity of the circulation is dependent. Secondly. M. Bachoué accounts, in the same way, for the mechanism of the sensorial functions. The contact between external agents and the sensitive, nervous extremities, renders the chymical action constantly produced by the contact of arterial blood there predominant; hence the production of a galvanic current passing from the circumference to the centre. This current excites the action of the brain to accomplish sensation; and the brain, excited by the process, becomes the seat of a more marked chymical action, which irradiates another, and a centrifugal, galvanic current to the muscles, that have to execute the movements.

According to Bachoué's theory, therefore, all the phenomena of life are derived from a chymical action which gives rise to the development of electricity. He likewise extends his system to pathology. If the chymical action be comprised within due proportions, all the phenomena of life are performed in health; if, on the contrary, the proportions are inappropriate, disease results, which is always dependent on preternatural chymical actions giving rise to irregular galvanic currents.

The remarks, made regarding the views of Dutrochet, are equally applicable to those of Bachoué. The very foundation, indeed, has been assailed by the experiments of M. Pouillet, at the Hopital Saint Louis, of Paris, which contradict the existence of these centrifugal or centripetal galvanic currents, developed in the organs during the production of the vital phenomena.

In the introductory remarks to the first volume of this work, the characters, which distinguish organized from inorganic bodies, were pointed out. All the characters of the former result from the influence of the vital principle, which produces the body of a definite magnitude, shape, structure, composition and duration. There is, moreover, a power, possessed by bodies endowed with the living principle, of being acted upon by certain stimuli, and thrown into movement without the participation of the will. This has, indeed, by

some physiologists, been considered to be the sole vital property,with what truth we shall see hereafter. An inquiry into its manifestations will aid us materially in determining whether or not the vital principle is effected directly through the medium of the nerves, and will tend to confirm an opinion, which we have already expressed on this subject.

Prior to the time of Haller the nervous system was looked to as the great source of power in the body; and the contractile power of the muscles,—described at length under the head of muscular motion, was considered to be wholly derived from the nerves, which were supposed to transmit the power to the muscular fibre as it was called for,—accurately regulating the quantity supplied.

Haller contended for a vis insita, a power of irritability or contractility, essentially residing in the muscles themselves, independently of any condition of the nervous system, and called into action by stimuli, of which, in the case of the voluntary muscles, the nervous influence is one, contributing, however, like all other stimuli, to exhaust it, instead of furnishing any fresh supply. We have elsewhere shown that a muscle is capable of being thrown into contraction after a limb has been removed from the body, and for a considerable period after the cessation of respiration, circulation, and consequently of innervation, provided the appropriate stimuli be applied, so as to excite the vis insita which remains attached to the muscle for some time after dissolution; and if all the nerves, supplying the limbs of a frog, be divided, and cut out close to the place where they enter the muscles, the muscles will still retain their contractility in as great a degree as when the nerves were entire.

They who believe that the contractility of muscles is wholly derived from the nervous system, maintain, however, that, in such case, the stimulus may still act, through the medium of the portions of nerves that must always remain attached to the muscle, however carefully attempts may have been made to remove them; and some have supposed that these nervous fibres may even constitute an essential part of the muscular fibre. The most satisfactory reply, that has been made to this argument, is the following experiment of Dr. Wilson Philip. All the nerves, supplying one of the hind legs of a frog, were divided, so that it became completely paralytic. The skin was removed from the muscles of the leg, and salt sprinkled upon them, which, being renewed from time to time, excited contractions in them for twelve minutes: at the end of this time they were found no longer capable of being excited. The corresponding muscles of the other limb, in which the nerves were entire, and of which, consequently, the animal had a perfect command, were then laid bare, and the salt applied to them in the same way. In ten minutes they ceased to contract, and the animal had lost the command of them. The nerves of this limb were now divided, as those of the other had been, but the excitability of the muscles to which the salt had been applied was gone.

Its application excited no contraction in them. After the experiment, the muscles of the thighs in both limbs were found to contract forcibly on the application of salt. It excited equally strong contraction on both sides. In this experiment, the excitability of the muscles, whose nerves were entire, was soonest exhausted; and hence Dr. Philip properly concludes, that the nervous influence, far from bestowing excitability on the muscles, exhausts it like other stimuli; and that the excitability or irritability is a property of the muscle itself.

It seems, therefore, that this essential characteristic of living bodies is a distinct vital property, not confined, as Haller supposed, to the muscular structure, but existing over the whole body. In favour of its not being dependent upon the nerves, we have the fact of its presence in the vegetable as well as in the animal. Many plants exhibit the power in a remarkable manner. The barberry bush is one of these. In this flower, the six stamens, spreading moderately, are sheltered under the concave tips of the petals, till some extraneous body, as the feet or trunk of an insect in search of honey, touches the inner part of each filament near the bottom. The irritability of that part is such, that the filament immediately contracts there, and consequently strikes its anther, full of pollen, against the stigma. Any other part of the filament may be touched without this effect, provided no concussion be given to the whole. After a while, the filament retires gradually, and may be again stimulated; and when each petal, with its annexed filament, is fallen to the ground, the latter, on being touched, shows as much irritability as ever.

In another plant,—the Cistus helianthemum, dwarf cistus or lesser sunflower,—the filaments, when touched, execute a motion, the reverse of that of the barberry. They retire from the style and lie down, in a spreading form, upon the petals.

Owing to the possession of this property, the Apocynum androsæmifolium or dogs-bane is extremely destructive to insect life. Attracted by the honey on the nectary of the expanded blossom, the instant the trunk of the fly is protruded to feed on it, the filaments close, and, catching the fly by the extremity of its proboscis, they detain the insect until its struggles end in death, occasioned apparently by exhaustion alone. The filaments then relax, and the body falls to the ground.

These are only evidences, however, of particular parts possessing unusual degrees of irritability. The property exists in every part of the plant, and, as in the animal, is the essential characteristic of the principle of life.

Irritability or contractility forms a medium of communication between the various parts of the living machine, and is excited to action by extraneous influences. All its movements, however, appear to be dependent upon the action of appropriate stimuli, and are, consequently, passively exercised.

There is a power which has been conceived to be nearly allied to irritability, and is highly characteristic of organized bodies,vegetable as well as animal,—whose movements or impulsions are active, and most varied. To this power, the term instinct has been appropriated by Virey, Fleming, Good, and others. It is an extension of the ordinary acceptation of the term, but it enables us to understand the phenomena better than where we restrict it to those manifestations of man, or animals, that bear the semblance of reason. It is this power, which, according to those gentlemen, regulates the movements, that are requisite to obtain a supply of food, to remove or counteract opposing obstacles, and to fly from impending danger, or to repair injuries which may be occa

sioned.

"In every organized system," says Dr. Good, "whether animal or vegetable, and in every part of such system, whether solid or fluid, we trace an evident proof of that controlling, and identifying power, which physiologists have denominated, and with much propriety, the principle of life. Of its cause and nature we know no more than we do of the cause and nature of gravitation, or magnetism. It is neither essential mind nor essential matter; it is neither passion nor sensation; but though unquestionably distinct from all these, is capable of combining with any of them; it is possessed of its own book of laws, to which, under the same circumstances, it adheres without the smallest deviation; and its sole and uniform aim, whether acting generally or locally, is that of health, preservation, or reproduction. The agency, by which it operates, is that which we denominate or should denominate instinct, and the actions, by which its sole and uniform aim is accomplished, are what we mean or should mean by instinctive actions; or, to speak somewhat more precisely, instinct is the operation of the living principle, whenever manifestly directing its operations to the health, preservation, or reproduction of a living frame, or any part of such frame. The law of instinct, then, is the law of the living principle; instinctive actions are the actions of the living principle; and either is that power, which characteristically distinguishes organized from unorganized matter, and pervades and regulates the former, uniformly operating by definite means in definite circumstances, to the general welfare of the individual system or of its separate organs, advancing them to perfection, preserving them in it, or laying a foundation for their reproduction, as the nature of the case may require. It applies equally to plants and to animals, and to every part of the plant, as well as to every part of the animal, so long as such part continues alive. It is this, which maintains, from age to age, with so much nicety and precision, the distinctive characters of different kinds and species, which carries off the waste or worn out matter, supplies it with new, and in a thousand instances, suggests the mode of cure, or even effects the cure itself, in cases of injury or disease. It is the divinity that stirs within us' of Stahl, the vis medica

trix naturæ of Hoffmann and Cullen and the physicians of our own day, &c. &c."

Of the existence of this instinctive principle, we shall adduce a few examples from both the vegetable and the animal kingdom. When the seed of a plant is deposited in the ground, under circumstances favourable for its development, it expands, and the root and stem are evolved. The root descends into the ground, manifestly not from the laws of gravitation, but owing to some inherent force, inasmuch as it penetrates the earth which is of much greater specific gravity than itself. The stem, too, bursts through the earth, and rises into the atmosphere, notwithstanding that the air is of much less specific gravity, until, having attained the height to which the action of the vital principle limits it, its upward development ceases. It rarely happens, however, that the root is capable of procuring nourishment sufficient for its future development in immediate contact with it. It, therefore, sends out numerous filamentous radicles in all directions to search after food, and to convey it to the proper organs. The number and direction of these filaments, and the distance to which they extend, are regulated by the necessities of the plant and the supply of the soil. A strawberry offset, planted in sand, will send out almost all of its runners in the direction in which the proper soil lies nearest, and few, and sometimes none, in the direction in which it lies most remote.

When a tree, which requires much moisture, has sprung up, or been planted in a dry soil, in the vicinity of water, it has been observed, that a much larger portion of its roots has been directed towards the water, and that, when a tree of a different species, and which requires a dry soil, has been placed in a similar situation, it has appeared, in the direction given to its roots, to have avoided the water, and moist soil. When a tree, too, happens to grow from seed on a wall, it has been seen, on arriving at a certain size, to stop for a while, and to send down a root to the ground. As soon as this root has been established in the soil, the tree has continued increasing to a large magnitude. The fact has been often noticed with respect to the ash,—a tree, which, in consequence of the profusion of its seed, is found more often scattered in wild and singular places, than any other not propagated by the agency of birds, or conveyed by the winds.

We find, in all cases, that if the roots of a plant, spreading in search of nourishment, meet with interruption in their course, they do not arrest their progress, but either attempt to penetrate the opposing body, or to avoid it by altering their direction. Dr. Fleming states, that he has repeatedly seen the creeping root of the Triticum repens or couch grass, piercing a potato, which had obstructed its course. It is well known, too, that roots will pass under a stone wall or a ditch, and rise up on the opposite side.

The nearest approximation to these manifestations of instinct, in VOL. II.

62

« ZurückWeiter »