Читать книгу The Mind and Its Education - George Herbert Betts - Страница 25

Divisions of the Nervous System.—The nervous system may be considered in two divisions: (1) The central system, which consists of the brain and spinal cord, and (2) the peripheral system, which comprises the sensory and motor neurones connecting the periphery and the internal organs with the central system and the specialized end-organs of the senses. The sympathetic system, which is found as a double chain of nerve connections joining the roots of sensory and motor nerves just outside the spinal column, does not seem to be directly related to consciousness and so will not be discussed here. A brief description of the nervous system will help us better to understand how its parts all work together in so wonderful a way to accomplish their great result.

The Central System.—In the brain we easily distinguish three major divisions—the cerebrum, the cerebellum and the medulla oblongata. The medulla is but the enlarged upper part of the cord where it connects with the brain. It is about an inch and a quarter long, and is composed of both medullated and unmedullated fibers—that is of both "white" and "gray" matter. In the medulla, the unmedullated neurones which comprise the center of the cord are passing to the outside, and the medullated to the inside, thus taking the positions they occupy in the cerebrum. Here also the neurones are crossing, or changing sides, so that those which pass up the right side of the cord finally connect with the left side of the brain, and vice versa.

The Cerebellum.—Lying just back of the medulla and at the rear part of the base of the cerebrum is the cerebellum, or "little brain," approximately as large as the fist, and composed of a complex arrangement of white and gray matter. Fibers from the spinal cord enter this mass, and others emerge and pass on into the cerebrum, while its two halves also are connected with each other by means of cross fibers.

Fig. 8.—View of the under side of the brain. B, basis of the crura; P, pons; Mo, medulla oblongata; Ce, cerebellum; Sc, spinal cord.

The Cerebrum.—The cerebrum occupies all the upper part of the skull from the front to the rear. It is divided symmetrically into two hemispheres, the right and the left. These hemispheres are connected with each other by a small bridge of fibers called the corpus callosum. Each hemisphere is furrowed and ridged with convolutions, an arrangement which allows greater surface for the distribution of the gray cellular matter over it. Besides these irregularities of surface, each hemisphere is marked also by two deep clefts or fissures—the fissure of Rolando, extending from the middle upper part of the hemisphere downward and forward, passing a little in front of the ear and stopping on a level with the upper part of it; and the fissure of Sylvius, beginning at the base of the brain somewhat in front of the ear and extending upward and backward at an acute angle with the base of the hemisphere.

Fig. 9.—Diagrammatic side view of brain, showing cerebellum (CB) and medulla oblongata (MO). F' F'' F''' are placed on the first, second, and third frontal convolutions, respectively; AF, on the ascending frontal; AP, on the ascending parietal; M, on the marginal; A, on the angular. T' T'' T''' are placed on the first, second, and third temporal convolutions. R-R marks the fissure of Rolando; S-S, the fissure of Sylvius; PO, the parieto-occipital fissure.

The surface of each hemisphere may be thought of as mapped out into four lobes: The frontal lobe, which includes the front part of the hemisphere and extends back to the fissure of Rolando and down to the fissure of Sylvius; the parietal lobe, which lies back of the fissure of Rolando and above that of Sylvius and extends back to the occipital lobe; the occipital lobe, which includes the extreme rear portion of the hemisphere; and the temporal lobe, which lies below the fissure of Sylvius and extends back to the occipital lobe.

The Cortex.—The gray matter of the hemispheres, unlike that of the cord, lies on the surface. This gray exterior portion of the cerebrum is called the cortex, and varies from one-twelfth to one-eighth of an inch in thickness. The cortex is the seat of all consciousness and of the control of voluntary movement.

Fig. 10.—Different aspects of sections of the spinal cord and of the roots of the spinal nerves from the cervical region: 1, different views of anterior median fissure; 2, posterior fissure; 3, anterior lateral depression for anterior roots; 4, posterior lateral depression for posterior roots; 5 and 6, anterior and posterior roots, respectively; 7, complete spinal nerve, formed by the union of the anterior and posterior roots.

The Spinal Cord.—The spinal cord proceeds from the base of the brain downward about eighteen inches through a canal provided for it in the vertebræ of the spinal column. It is composed of white matter on the outside, and gray matter within. A deep fissure on the anterior side and another on the posterior cleave the cord nearly in twain, resembling the brain in this particular. The gray matter on the interior is in the form of two crescents connected by a narrow bar.

The peripheral nervous system consists of thirty-one pairs of nerves, with their end-organs, branching off from the cord, and twelve pairs that have their roots in the brain. Branches of these forty-three pairs of nerves reach to every part of the periphery of the body and to all the internal organs.

Fig. 11.—The projection fibers of the brain. I-IX, the first nine pairs of cranial nerves.

It will help in understanding the peripheral system to remember that a nerve consists of a bundle of neurone fibers each wrapped in its medullary sheath and sheath of Schwann. Around this bundle of neurones, that is around the nerve, is still another wrapping, silvery-white, called the neurilemma. The number of fibers going to make up a nerve varies from about 5,000 to 100,000. Nerves can easily be identified in a piece of lean beef, or even at the edge of a serious gash in one's own flesh!

Bundles of sensory fibers constituting a sensory nerve root enter the spinal cord on the posterior side through holes in the vertebræ. Similar bundles of motor fibers in the form of a motor nerve root emerge from the cord at the same level. Soon after their emergence from the cord, these two nerves are wrapped together in the same sheath and proceed in this way to the periphery of the body, where the sensory nerve usually ends in a specialized end-organ fitted to respond to some certain stimulus from the outside world. The motor nerve ends in minute filaments in the muscular organ which it governs. Both sensory and motor nerves connect with fibers of like kind in the cord and these in turn with the cortex, thus giving every part of the periphery direct connection with the cortex.

Fig. 12.—Schematic diagram showing association fibers connecting cortical centers with each other.—After James and Starr.

The end-organs of the sensory nerves are nerve masses, some of them, as the taste buds of the tongue, relatively simple; and others, as the eye or ear, very complex. They are all alike in one particular; namely, that each is fitted for its own particular work and can do no other. Thus the eye is the end-organ of sight, and is a wonderfully complex arrangement of nerve structure combined with refracting media, and arranged to respond to the rapid ether waves of light. The ear has for its essential part the specialized endings of the auditory nerve, and is fitted to respond to the waves carried to it in the air, giving the sensation of sound. The end-organs of touch, found in greatest perfection in the finger tips, are of several kinds, all very complicated in structure. And so on with each of the senses. Each particular sense has some form of end-organ specially adapted to respond to the kind of stimulus upon which its sensation depends, and each is insensible to the stimuli of the others, much as the receiver of a telephone will respond to the tones of our voice, but not to the touch of our fingers as will the telegraph instrument, and vice versa. Thus the eye is not affected by sounds, nor touch by light. Yet by means of all the senses together we are able to come in contact with the material world in a variety of ways.