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3. It is one of the great discoveries of physiol. ogy that the anterior root of each pair contains only nerves of motion, and the posterior root only nerves of sensation, and that the former, therefore, carry impressions or commands from the spinal marrow, and the latter bring impressions to it.t But what is peculiar to some of these nerves is, that when they run to the heart, lungs, etc., they act independently of the brain. Thus, when the
. right ventricle of the heart is filled with the dark impure venous blood, the nerves of sensation which run to the heart convey a notice of the fact to the gray substance of the spinal marrow; this gray substance, which seems to have a power in itself
independent of the brain, responds to this notice, and sends back a message to the heart by the motor nerves, directing the muscles of the right ventricle to contract, and force the blood into the lungs, that it may be purified there. It is the same with all the other involuntary muscles—with those of the lungs and the stomach; they are put in motion. at the proper time, and in the right manner, through the medium of nerves over which, ordinarily, the will has no control.
4. The exceeding wisdom of an arrangement by which the functions of the heart and lungs are continued in unceasing operation, without the necessity of mental control, is so obvious that we need not dwell upon it. The nervous power that controls them seems to be stored
cellular substance of the spinal marrow, just as the power that moves the wheels and hands of a watch is stored up in the coil of the main-spring. In winding up the watch we use a certain amount of force, and this force we transmit to the mainspring, where it remains coiled up, to be given off as needed in moving the wheels and hands of the watch. So, when this infinitely more perfect machine of the muscles of involuntary motion, made by the Great Architect, is kept properly wound up by a due supply of appropriate nourishment and pure air, and by a due observance of all the other conditions of healthy action, it continues in motion until the power stored up in it
This is a side view of the left side of the spinal cord. † Fig. 7 shows a portion of the spinal cord surrounded by its envelopes, and showing the origin of the anterior and posterior roots. Thus, at 1, 1 are shown the posterior roots of the spinal nerves, and 2, 2, the anterior roots of the same nerves; at 3, 4, and below them, the anterior and posterior roots are cut. In the upper portion of the engraving the sheath or envelope of the spinal cord is preserved; and at 6 are shown the two roots united, and projecting from the sheath; at 7 is shown a vertical section of the two roots cut close to the sheath, and showing the vertical line which divides one root from the
has been exhausted. It has been left to us to avail ourselves of the proper means of continuing for a while a supply of this force, although we can not originate it; and judicious care will enable us for a long time to keep the machine of life in motion, although it will finally wear out.
5. We have said that ordinarily the will has no control over the nerves connected with the involuntary muscles-receiving no sensations from them, and conveying no messages to them. But Infinite Wisdom, which plans all things well, has made some exceptions here. Ordinarily the action of the lungs in respiration is wholly unnoticed by the mind; but when there is embarrassment in the lungs, occasioned, for example, by the presence of some irritating substance, or by disease, the quiet process carried on by the agency of the spinal marrow alone is not adequate to meet the exigency. The act of breathing is now accompanied with positive sensations which the brain takes notice of, that the individual may, if possible, provide a remedy. By a mental effort the will can quicken the action of the lungs, if necessary. Not so, however, with the movements of the stomach in digestion; no effort of the will can quicken or retard the action of this muscle. The will can not directly influence the motions of the heart, though it can do it indirectly by so directing the thoughts as to awaken emotions calculated to produce this effect.
6. Thus it has been seen, in the mysteries of the pervous system, that there are two kinds of nerves of common sensation, one conveying impressions to the brain, and the other kind transmitting impressions that are unnoticed by the mind to other centres of nervous power and influence. It has been seen, also, that there are, corresponding to these, two different kinds of nerves of motion, one acting under mental control, and the other not. Still another important principle of nervous influence we have to notice in this connection, and that is, that there are different nerves for different kinds of sensation. The nerves of feeling are spread all over and throughout the body; but, in addition to these, there are nerves of hearing, seeing, smelling, and tasting, each entirely different in its functions from all the others.
7. Thus the nerves of hearing convey to the mind impressions that we call sound ; the optic nerve transmits impressions of another kind, and the nerves of smelling and tasting impressions of still different kinds. Each kind has its own duty to perform, and it can perform no other. Thus the optic nerve, which is only subject to the influence of light, can con
vey no impression of hearing, nor of smelling, nor of tasting; nor can it convey any impression of pain. If the
eye jured, a nerve of ordinary sensation is required to convey the intelligence to the mind. So in the nose; the nerve that takes notice of odors is a different one from that by which irritation on the same membrane is felt. The snuff-taker smells the snuff with one nerve, and feels its tingling with another.
8. Thus we have briefly explained the leading parts and principles of action of the Nervous System. In one part of this system we have found one set of nerves—the nerves of feeling, as they are called, whose office is to convey to the mind impressions of ordinary sensation from the surrounding world; and a still different set, called nerves of motion, to convey the commands of the mind to the numerous voluntary muscles. In another part of this system we have also found two sets of nerves, but different from the former, running to and from the involuntary muscles, and regulating their motions. And we have also found still different nerves, sometimes called nerves of special sense, conveying to the mind those impressions which give us a knowledge of the objects of taste and of sounds, of shades and colors, and of odors. Some mysterious power presides over all of them, and keeps them in harmonious action, until accident, or disease, or age seriously mars the beautiful mechanism, and then we die. No, not we! It is only the body—the machine that is broken or that is worn out, while we, the spirit-mind, shall exist forever.
LESSON V.-SPIRIT, THE MOTIVE POWER OF THE BODY.
1. A machine is a combination of parts composed of material substances, solid or fluid, or both, as the case may be; it possesses not its own principle of motion; it can not urge its own levers, or stretch its own cords, or turn its own wheels, or put its own fluids into circulation. The efficient cause of its motion, which is altogether distinct from the machine itself, is called the prime mover.
2. The point on which I desire now to fix your attention is, that this prime mover is altogether distinct from the machine, and independent of it; that it possesses, or at least may possess, no property in common with it; and that its existence or non-existence is not decided by the existence or non-existence of the machine.
3. The machine may be broken, destroyed, worn by age, or otherwise disabled, and yet the prime mover may still retain its original energy. Thus a steam-engine is moved by fire, a mill by wind or water; the steam-engine may be worn out, and the mill be broken by accident; and yet the fire, and the wind, and the water will still preserve their powers.
4. These observations, which correctly describe a machine, may with propriety be applied to the human body. This body is also a combination of parts, composed of material substances, solid and fluid, having certain definite forms and arrangements, possessing certain capabilities of motion and force, destined and admirably adapted to obey the dictation of its prime mover, the living principle, the immaterial spirit.
5. So long as it pleases the Great Engineer who constructed this body to permit its connection with that intellectual spirit, so long will it obey the impulses which it receives; nor does the decay in this bodily machine infer any corresponding decay of the moving spirit any more than the wear and tear of a steam-engine proves the destruction of the principle of heat which gives it motion.
6. Neither are we to infer, because this bodily machine, in its obedience to the vital spirit, acts mechanically, and is adapted to all the ordinary properties and laws of matter, that therefore the spirit which moves it partakes of the nature of matter, or is answerable to its laws, any more than we should infer that the levers, wheels, pumps, chains, cords, and valves of a steam-engine are regulated by the laws which govern heat. On the contrary, I submit it to the candor of the most skeptical materialist whether the whole tendency of anal'ogy+ does not directly overthrow the hypoth'esis that the principle of life is organic.6
7. We are assured in the Scriptures that in the first instance “God formed man of the dust of the ground;" that is to say, He created that curious and beautiful machine, the organized human body; but that body was still an inert? structure, without the principle of self-motion. A more noble work remained to be performed; the immaterial spirit, the divine essence, the prime mover of this machine, was to be applied; and, accordingly, we learn that God “breathed into his nostrils the breath of life;" and then, and not till then,“man became a living soul.”—LARDNER.
“Is, then, the being who such rule maintains
Is all that acts, contrives, obeys, commands,
Moulder in kindred dust, and move no more' ? 9. “No': powers sublimer far that frame inspire,
And warm with energy of nobler fire,
Fill all the vast capacity of mind.”
6 OR-GĂN'-1€. Organic bodies are such as · SKĚP'-TI€-AL, doubting.
possess organs, on the action of which de3 MA-TĒ-BI-AL-IST, one who denies any spir-) pend their growth and perfection.
itual existence apart from matter. 7 IN-ÉRT', without power to move. 4 A-NĂL'-0-GY, remote likeness or similarity
between different objects.
LES. VI.-VARIOUS PHENOMENA OF THE NERVOUS SYSTEM.
(Adapted from Hooker and other writers.)
I. WHAT IS NECESSARY TO SENSATION AND VOLUNTARY
MOTION. 1. The nerves, branching out to all parts of the body, do not terminate in sharp points, but usually in loops, where impressions from external things are first received; and it is found that if the organ on which the nerve is thus expanded is seriously injured, the nerve will not receive the impression. If the eye be so injured in its textures that the impression of light can not be made on the optic nerve, there can be no vision. So, too, of the other senses. Taste and smell are often impaired, sometimes even destroyed for a time, by an inflammation of the mucous membrane,2 on which the nerves devoted to these senses are expanded. This is sometimes the case in a common cold. The trunk of a nerve must also be in a proper condition. If the nerve of vision be pressed upon by a tumor, 3 no impression will be transmitted from the images formed in the eye. So, too, if the nerve going to any part of the body be cut off, there can be no transmission of impressions to the brain from that part.
2. Again, it is necessary to sensation that the brain should be in a state to communicate the impression to the mind. If the brain be pressed upon strongly by a depression of the skull from violence, or by effusion of blood by the ruptures of an artery, as sometimes occurs in apoplexy, there can be no sensation. Excitement of mind, too, sometimes prevents