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back-bone. In the engraving at the foot of page 91 is represented an internal side view of the right half of the brain.
5. The brain is composed of a soft jelly-like substance, very much like the marrow which is found in bones. The interior portion, which is of a white color, is composed of exceedingly minute tubes, which are the beginnings of the nerves. These little nerve tubes are exceedingly minute; but where they start from the brain a bundle of them is generally inclosed in a sheath; then this bundle is divided and subdivided, branching out in smaller and smaller divisions, until each little nerve tube is connected with some one muscular fibre or some one sensitive point in the body. Each of these little tubes, although sometimes too small to be distinctly seen by the microscope, is supposed to be entirely separate from its fellow nerves, and unconnected with them from its beginning to its termination.
6. We will now explain the uses of these little nerve tubes, of which there are millions in the body, and we will do it by supposing a particular case in which they are used. If the mind wishes to move the right hand it sends a message downward from the brain along the course of the spinal cord in the neck, and thence down along the arm through a bundle of the nerves that run to the band. If the mind wishes the
Fig. 3 represents the base of the brain as seen from below, together with some of the double sets of nerves branching from it. Here the figures 1, 1, represent the anterior or front lobes, and 3, 3, the lobes of the cerebellum, which lies at the base of the back part of the skull. From 4 to 18 is the line of the median fissure. Figure 13 points to the medulla oblongata, or severed portion of the upper part of the spinal marrow; 22, to one of the olfactory nerves, or nerves of smell ; 24, to the beginning of the pair of optic nerves, or nerves of sight; while 25 and 28 point to one, each, of pairs of oculo
motor nerves, or nerves that direct the Pia motions of the eyes; 27 and 29 to
nerves that move the muscles of the
face, and 30 to the auditory nerves, or 14 nerves of hearing. The doubleness of
the nerves is here well illustrated; and this is the characteristic of the whole nervous system, the cranial and spinal nerves coming forth by pairs to their distribution on the right and left sides of the body. The object of this arrangement is evidently to increase the precision of nervous action, and to compensate readily for any incidental defects.
whole hand to move, it sends the message through all the nerves that run there, and one tells the little finger, another the fore-finger, and another the thumb, etc., what to do. If the mind wishes the fingers to strike the keys of the piano, it tells each finger, through its own nerves, what key to strike. These nerves, through which the mind sends out its commands to the fingers, and to every muscle in every part of the body, telling each when and how to act, are called motor nerves, or nerves of motion. If the bundle of motor nerves running to the hand should be cut in two, the hand would not move! And why? Because it could receive no command from the brain; and it will not move without orders from head-quarters.
7. But there is another set of nerves running to the hand besides the nerves of motion. What if the fore-finger had been directed, in the dark, to touch a certain key of the piano, and had chanced to press upon the sharp point of a needle or the blade of a knife! How could the mind be informed of the danger'? Not by the nerves of motion, for their only office is to convey commands outward from the brain. Another set of nerves is needed, and such has been provided in the nerves of feeling. As soon as the finger presses upon the needle's point, some of these numerous and minute nerves of feeling are pierced, and instantaneously they convey the intelligence to the brain. As quick as thought, a command is then sent down, through the nerves of motion, to withdraw the finger, if possible, before any serious injury is received. If the bundle of nerves of feeling running to the hand should be severed, the finger might be pierced through, or cut off,
Fig. 4 represents an external side view of the right half of the brain. At 1 is seen the medulla oblongata, or beginning of the spinal marrow; at 3, a side view of the cerebellum, or little brain. The chief office of the cerebellum is believed to be that of combining, regulating, and directing all the muscular movements. From experiments on animals, it is found that when it is cut it gives rise to neither motion nor sensation, but when it is removed in slices the animal gradually loses all power of regulating its motions. The weight of the cerebellum is about one eighth of that of the cerebrum. The average weight of the entire brain in man is about fifty ounces; in females about forty-five.
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 band, 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 feeling, 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, principally, 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
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 14
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 arborvitae, or “ tree of life," of the cerebellum; 20 points to
the origin of one of the nerves that 34 move the eyes; and 21 to the or1 igin 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, com
bine, and control muscular mo33 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
ive slices, the motions of the ani. mal become irregular, and, finally, it loses all power of walking or of maintaining its equilibrium.
16 15 17 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 sụpply 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.
Fig. 6. At A is represente a collection of nerve-cells, nerve fibres, 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 suistance 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 influences, 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 nervous 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 directions.
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 supposed, 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. Impressione would be returned from the lower extremities to the brain in the same way.