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parts of the system together; but this analogy does not hold good with such a muscle as the heart. The muscles are susceptible of being contracted, and this contractility is the principle by which the various motions are effected. The guiding principle is knowledge: this consists of the various kinds of thought excited in the animal. It is by their knowledge that the various animals are guided in their efforts to procure food and other enjoyments, and in the avoidance of the evils by which they are surrounded.
The impelling principle is feeling. It has its various species, such as hunger, thirst, warmth, chilliness, love, hatred, anger, joy. It is the prime mover of the animal.
The animal is distinguished from the vegetable by the three principles that I have enumerated. Those living beings belonging to species that exhibit the phenomena of thought, feeling, and action, are called by naturalists animate existences or animals; those that do not are termed vegetables.
The organs in which the excitements of knowledge and feeling, or consciousness, are produced, form a system called the nervous system, the central mass of which is the brain; the brain in the more perfect animals is the actual seat of knowledge: in animals, such as the frog and eel, indications of consciousness may be produced in the body after the head has been removed from it, the nerves being susceptible to the excitement of consciousness independently of the brain.
The excitements called thought and feeling exercise a power over the voluntary muscles, causing the various muscular contractions which bring about the animal movements requisite to effect the object to which the thought and feeling relate.
Electrical excitements also have the power of contracting the animal muscle, when forwarded through the nerve communicating with that muscle.
The nervous substances can be demonstrated to be possessed of considerable electrical properties. The mode of producing galvanic circles and shocks with their aid is given in most elementary treatises on gal
vanism. This adequacy of the central masses of the nervous system, to elaborate electricity, together with the fact, that the nerves passing from these central masses to all parts of the system are electric conductors; and that the muscles can be contracted, and the various animal motions produced by the agency of this power has led physiologists to enquire respecting the identity of electricity, and the nervous power and their similarity has now been frequently experimentally proved.
The lungs and stomach depend on the eighth pair of nerves for power to perform their functions adequately. If the nerves be cut, so as to cause a division sufficient to prevent the passage of the nervous agent from the masses of the nervous system through these conductors, a difficulty of breathing immediately takes place, and also a want of the power of digestion, and the animal soon becomes exhausted and dies: the food given to animals in experiments of this nature, has been found, on opening the stomach after death, to be undigested. When the animal is fed, and its eighth pair of nerves divided, those ends communicating with the stomach and lungs may be attached to the positive pole of a galvanic battery, and on the skin over the stomach a piece of flat metal communicating with the negative wire is placed; thus a stream of electricity is brought to act on the lungs and stomach. Under this influence the breathing immediately revives, and on killing the animal, after the lapse of a few hours, the food is found digested, thus shewing the adequacy of electricity to perform the respiratory and digestive processes. Dr. Philip, after much experimental investigation, came to the conclusion, that the nervous power was electricity "collected by the brain and spinal marrow, and sent along the nerves; galvanism being, not only of all artificial means of exciting the muscles, that which seems best adapted to this purpose, but capable of forming the secreted fluids, preserving the healthy structure of the organs, &c."*
Cuvier when referring to this subject says, "All con
* Vide Philip's Experimental Enquiry into the Laws of the Vital Functions.
traction, and, generally speaking, every change of dimension in nature, is produced by a change of chemical composition, though it consists merely in the flowing or ebbing of an imponderable fluid, such as caloric; thus also are produced the most violent movements known upon earth, explosions, &c.
"There is, consequently, good reason to suppose that the nerve acts upon the fibre through the medium of an imponderable fluid, and the more so, as it is proved that this action is not mechanical.
"The medullary matter of the whole nervous system is homogeneous, and must be able to exercise its peculiar functions wherever it is found; all its ramifications are abundantly supplied with blood vessels.
"All the animal fluids being drawn from the blood by secretion, we can have no doubt that such is the case with the nervous fluid, and that the medullary matter secretes it.
"On the other hand, it is certain that the medullary matter is the sole conductor of the nervous fluid; all the other organic elements restrain and arrest it, as glass arrests electricity.
"The external causes which are capable of producing sensations or causing contractions of the fibre are all chemical agents, capable of effecting decompositions, such as light, caloric, the salts, odorous vapours, percussion, compression, &c. &c.
It would appear, then, that these causes act on the nervous fluid chemically, and by changing its composition; this appears the more likely, as their action becomes weakened by continuance, as if the nervous fluid needed the resumption of its primitive composition to fit it for a fresh alteration.
The external organs of the senses may be compared to sieves, which allow nothing to pass through to the nerve, except that species of agent which should affect it in that particular place, but which often accumulates it so as to increase its effect. Thus, the tongue has its spongy papillæ which imbibe saline solutions; the ear, a gelatinous pulp which is violently agitated by sonorous vibrations; the eye, transparent lenses which concentrate the rays of light, &c. &c.
"Brief as our sketch has been, it is sufficient to establish the possibility of accounting for all the phenomena of physical life, from the properties it presents, by the simple admission of a fluid, such as we have defined."
Some animals can discharge electricity from their nervous system at will, thus the Torpedo, when angry, can give an electric shock like that obtained from a Leyden jar.
"The electrical gymnotus, which, from its almost uniform shape and obtuse head and tail, has also been called the Electrical Eel. It is from five to six feet long, and communicates such violent shocks, that men and horses are struck down by them. This power is dependent on the will of the animal, which gives it what direction it pleases, and renders it effective, even at a distance, killing fishes therewith, so situated. It is, however, dissipated by use, and to renew it, the Gymnotus requires rest and nourishing food."*
The animal electricity being elaborated from the blood in its circulation through the nervous system, the nervous power may be said to depend on the circulation and to be, in some respect, proportional to it.
There are two states of animal life, the conscious and the unconscious-the state of being awake and that of sleep; in the former the blood circulates rapidly through the nervous masses; in the latter, the circulation is of a very sluggish or tranquil character. In one case the electric state of the nervous system may be said to be powerful, in the other weak; during the more powerful electro-nervous state, resulting from the circulation, the excitements of thought and feeling are produced in the nervous system, When there is a strong circulation, the consciousness is of a more active character, the flow of animal spirits is great. When the circulation gets languid the consciousness is also so-the spirits are low. When the circulation sinks below a certain degree of force, the consciousness ceases; as in cases of exhaustion, such as sleep, fainting, &c. Intense cold,
Cuvier's Animal Kingdom.
bad atmosphere, bleeding, fear, surprise, often produce exhaustion and weakness in the circulation sufficient to remove all consciousness.
A slight pressure on the human brain is sufficient to check the circulation through it, and hence we find that a blow on the skull adequate to press in a part of it on the brain, immediately removes consciousness, which can be restored only by removing the pressure. From this it appears that the susceptibility to consciousness is constituted by a certain state of the nervous system having a relationship of dependency to the circulation, and that the removal of that state causes an immediate cessation of thought and feeling.
The nervous system ramifies throughout the animal structure, and is of so omnipresent a character in it, that the finest pointed needle pressed on any part of the skin, immediately excites a consciousness, constituted of a knowledge in the brain, of a feeling existing in the part touched. But if the communication of the central masses of the nervous system with any portion of the structure be checked by disease or any other cause, the part so deprived of its proper share of the nervous influence
That the various phenomena of consciousness are so dependent upon certain states of the brain, that on the removal of those states consciousness ceases, is a fact well known, generally acted upon, and one that admits of demonstration. The physiologist, when about to experiment upon a living animal, desirous of giving as little pain as possible, causes pressure upon its brain, by an adequate blow on the skull, being well aware that the immediate consequence of the altered state of brain is a cessation of consciousness. The butcher, in his avocation of preparing animal food for sale, acts on the same well known fact. In our daily experience, we meet with, or hear of, persons who have been rendered temporarily unconscious by a slight concussion of the cranium: indeed, so numerous are such instances that it is almost impossible for any one to arrive at maturity, without being made acquainted with some of them.