Oldalképek
PDF
ePub

and the mind have no knowledge of it! And why? Because no notice of the injury could be sent from the finger up to the brain.

8. The following fact illustrates the use of the nerves of feeling in preventing injury. A man who had lost all sensibility in the right hand, on account of an injury to the bundle of nerves of feeling, while the nerves of motion were still perfect, lifted the cover of a pan when it was burning hot. Although he could feel no pain, the consequence was the loss of the skin of the fingers and of the palm of the hand, laying bare the muscles and tendons. If the nerves of feeling had not been injured, the warning of pain would have been instantly given to the brain, and orders would have been sent to the muscles to relax their grasp of the cover; and so rapid would have been these messages, through the nerves of feel. ing, to the brain, and back through the nerves of motion, that the cover would have been dropped soon enough to prevent any great amount of injury from being done.

9. In the foregoing explanation we have a general view of the functions of the nervous system, which consists, principal. ly, of the brain and the spinal marrow, and numerous sets of two kinds of nerves running from them to all parts of the body. The nerves of feeling and the nerves of motion are, so far as we can discover, the same in structure and in composition; but as the offices which they perform are entirely

[ocr errors]

al 23

Fig. 5.

Fig. 5 represents an internal INTERNAL SIDE VIEW OF THE BRAIN.

side view of the right half of the brain - the brain being cut or split downward from the white body in the centre. At 1 is shown the half of the medulla oblongata; 4 points to what is called the arbor vitae, or "tree of life," of the cerebellum; 20 points to the origin of one of the nerves that move the eyes; and 21 to the origin of the optic nerve, which is seen proceeding toward the eye; 26 points to what here appears as a crescent-shaped white substance, called the corpus callosum. It appears to be the peculiar office of the cerebellum to direct, combine, and control muscular mo. tions; and those animals which have it the most fully developed excel in their powers of motion, and are distinguished by the complication of their movements. If removed by degrees, in success

[graphic]

ive slices, the motions of the ani. mal become irregular, and, finally, it loses all power of walking or of maintaining its equis librium.

16 15 12 is 20 19 3

different, there is something about them which we do not yet understand. Nor can we understand how the mind receives impressions through one set, and sends out messages and causes motion through another, for this would be to understand how mind acts upon matter, and how the spiritual is connected with the material world.

10. But there is one thing more about the brain which we may explain here. We have said that the central part of it is of a white color, and composed of the beginnings of the minute nerve tubes which we have described. But all around this white inner part is a thick layer of gray substance, thickly lining the interior of all the convolutions or folds of the brain; and this gray substance is composed of minute cells, intermingled with which are exceedingly minute and numerous blood-vessels, which supply the cells with their requisite nourishment. This cellular substance of the brain is acknowledged by physiologists to be the seat or dwellingplace of the mind—of the intellect itself; and the mind—the ruling power within us—is believed to act directly upon this gray matter, while the white substance serves only to transmit messages to the muscular fibres, and bring back impressions. It is found, in examining the brains of animals, that, the greater the intelligence, the more abundant is the gray substance; and in man it is especially abundant, constituting much the largest proportion of the brain. 1 CĚe'-E-BRUM.

CĚR-E-BĚL'-LUM.

Fig. 6.

Fig. 6. At A is represented a collection of nerve-cells, nerve fil res, and blood-vessels from the human brain, greatly magnified. This is from that part of the brain called the optic thalamus. At a is shown one of these nerve-cells still more highly magnified. The branching tube in A is a blood-vessel showing the circular blood-cells floating in it. (See Fourth Reader, p. 32.) At B, B are represented some of the nerve-cells found in the gray substance of the brain. These cells, which have a nucleus, or central particle, are originally globular, but many often assume various shapes, and often shoot out in branches. While the nervous fibres conduct external impressions to the brain, and transmit nervous influences from it, the nerve-cells are supposed to be the various centres which receive the impressions and originate the nervous influ. ences, under the directing power of the mental principle. A collection or bundle of these cells is called a vesicle, which may be regarded as a temporary magazine of nerv. ous power, with its many cell-like divisions, each of which has some particular duty to perform. Thus the form of the large cell at B (highly magnified) would indicate that it may receive nervous influence from two directions, and then transmit it, as occasion demands, in four or five direc. tions.

[graphic][merged small]

LESSON IV.-OTHER FORMS OF NERVOUS ACTION. 1. In the preceding lesson we contemplated nervous action in only two of its forms—as producing sensation and voluntary motion, in which the mind is the recipient in one case, and the active agent in the other. But much of the muscular motion of the body is produced without the agency of the will or mind, and sometimes even in opposition to it. It would not answer to intrust the circulation of the blood, and the acts of breathing and digestion, to the control of the mind; for the mind might slumber or be forgetful; or the brain, which is its organ, might be diseased, and then the pulsations of the heart would stop, the lungs and the stomach would cease their labors, and the body would die. But by a most wonderful provision the heart beats on, even when the mind takes no notice of it; and the stomach performs the labor of digestion, and the lungs that of respiration, independently of the will. By what agency, then, is it that these and many other involuntary motions are produced'? The answer to this question will open a new view of the wonders of the nervous system.

2. We have stated that the spinal marrow, proceeding from the brain, extends downward through the spinal column or back-bone. Its substance and structure are similar to those of the brain, except that the nervous tubes—the white matter-compose the outer portions of it, and the gray cellular matter the inner parts. Along the outer portion run the two kinds of nerves from the brain, those of motion and those of feeling, branching off here and there to various parts of the body.* But, in addition to these nerves which it transmits along its channel, the spinal marrow sends off nerves of its own to the heart, lungs, stomach, etc., and other internal organs. It has long been known that all the nerves from the spinal marrow are sent off in pairs through the two furrows on each side of the back-bone, and that each pair has two roots, one root coming from the back portion, and the other from the front. These two roots unite as soon as they have fairly left the spinal cord, after which their fibres branch off to the several places of their destination.

* Whether these nerves are continuous all the way from the brain, as was formerly sup. posed, or not, is now doubted. If not, impressions must be transmitted from the brain first to the cellular substance of the spinal marrow, and then sent forward by some nerv. ous force which has been stored up in the spinal marrow for this purpose. Impression. would be returned from the lower extremities to the brain in the same way.

Fig. 7.*

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. 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 ob vious that we need not dwell upon it. The nervous power that controls them seems to be stored up in the gray 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 other.

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 ef fort 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 nervous 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

« ElőzőTovább »