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 pa.s.sing 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 pa.s.s 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 ma.s.s, and others emerge and pa.s.s on into the cerebrum, while its two halves also are connected with each other by means of cross fibers.
[Ill.u.s.tration: 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, pa.s.sing 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 ca.n.a.l provided for it in the vertebrae 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.
[Ill.u.s.tration: 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 const.i.tuting a sensory nerve root enter the spinal cord on the posterior side through holes in the vertebrae. 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.
[Ill.u.s.tration: FIG. 12.--Schematic diagram showing a.s.sociation fibers connecting cortical centers with each other.--After JAMES and STARR.]
The _end-organs_ of the sensory nerves are nerve ma.s.ses, 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.
5. LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM
DIVISION OF LABOR.--Division of labor is the law in the organic world as in the industrial. Animals of the lowest type, such as the amoeba, do not have separate organs for respiration, digestion, a.s.similation, elimination, etc., the one tissue performing all of these functions. But in the higher forms each organ not only has its own specific work, but even within the same organ each part has its own particular function a.s.signed. Thus we have seen that the two parts of the neurone probably perform different functions, the cells generating energy and the fibers transmitting it.
It will not seem strange, then, that there is also a division of labor in the cellular matter itself in the nervous system. For example, the little ma.s.ses of ganglia which are distributed at intervals along the nerves are probably for the purpose of reenforcing the nerve current, much as the battery cells in the local telegraph office reenforce the current from the central office. The cellular matter in the spinal cord and lower parts of the brain has a very important work to perform in receiving messages from the senses and responding to them in directing the simpler reflex acts and movements which we learn to execute without our consciousness being called upon, thus leaving the mind free from these petty things to busy itself in higher ways. The cellular matter of the cortex performs the highest functions of all, for through its activity we have consciousness.
[Ill.u.s.tration: FIG. 13.--Side view of left hemisphere of human brain, showing the princ.i.p.al localized areas.]
The gray matter of the cerebellum, the medulla, and the cord may receive impressions from the senses and respond to them with movements, but their response is in all cases wholly automatic and unconscious. A person whose hemispheres had been injured in such a way as to interfere with the activity of the cortex might still continue to perform most if not all of the habitual movements of his life, but they would be mechanical and not intelligent. He would lack all higher consciousness.
It is through the activity of this thin covering of cellular matter of the cerebrum, the _cortex_, that our minds operate; here are received stimuli from the different senses, and here sensations are experienced.
Here all our movements which are consciously directed have their origin.
And here all our thinking, feeling, and willing are done.
DIVISION OF LABOR IN THE CORTEX.--Nor does the division of labor in the nervous system end with this a.s.signment of work. The cortex itself probably works essentially as a unit, yet it is through a shifting of tensions from one area to another that it acts, now giving us a sensation, now directing a movement, and now thinking a thought or feeling an emotion. Localization of function is the rule here also.
Certain areas of the cortex are devoted chiefly to sensations, others to motor impulses, and others to higher thought activities, yet in such a way that all work together in perfect harmony, each reenforcing the other and making its work significant. Thus the front portion of the cortex seems to be devoted to the higher thought activities; the region on both sides of the fissure of Rolando, to motor activities; and the rear and lower parts to sensory activities; and all are bound together and made to work together by the a.s.sociation fibers of the brain.
In the case of the higher thought activities, it is not probable that one section of the frontal lobes of the cortex is set apart for thinking, one for feeling, and one for willing, etc., but rather that the whole frontal part of the cortex is concerned in each. In the motor and sensory areas, however, the case is different; for here a still further division of labor occurs. For example, in the motor region one small area seems connected with movements of the head, one with the arm, one with the leg, one with the face, and another with the organs of speech; likewise in the sensory region, one area is devoted to vision, one to hearing, one to taste and smell, and one to touch, etc. We must bear in mind, however, that these regions are not mapped out as accurately as are the boundaries of our states--that no part of the brain is restricted wholly to either sensory or motor nerves, and that no part works by itself independently of the rest of the brain. We name a tract from the predominance of nerves which end there, or from the chief functions which the area performs. The motor localization seems to be the most perfect. Indeed, experimentation on the brains of monkeys has been successful in mapping out motor areas so accurately that such small centers as those connected with the bending of one particular leg or the flexing of a thumb have been located. Yet each area of the cortex is so connected with every other area by the millions of a.s.sociation fibers that the whole brain is capable of working together as a unit, thus unifying and harmonizing our thoughts, emotions, and acts.
6. FORMS OF SENSORY STIMULI
Let us next inquire how this mechanism of the nervous system is acted upon in such a way as to give us sensations. In order to understand this, we must first know that all forms of matter are composed of minute atoms which are in constant motion, and by imparting this motion to the air or the ether which surrounds them, are constantly radiating energy in the form of minute waves throughout s.p.a.ce. These waves, or radiations, are incredibly rapid in some instances and rather slow in others. In sending out its energy in the form of these waves, the physical world is doing its part to permit us to form its acquaintance.
The end-organs of the sensory nerves must meet this advance half-way, and be so constructed as to be affected by the different forms of energy which are constantly beating upon them.
[Ill.u.s.tration: FIG. 14.--The prism"s a.n.a.lysis of a bundle of light rays.
On the right are shown the relation of vibration rates to temperature stimuli, to light and to chemical stimuli. The rates are given in billions per second.--After WITMER.]
THE END-ORGANS AND THEIR RESPONSE TO STIMULI.--Thus the radiations of ether from the sun, our chief source of light, are so rapid that billions of them enter the eye in a second of time, and the retina is of such a nature that its nerve cells are thrown into activity by these waves; the impulse is carried over the optic nerve to the occipital lobe of the cortex, and the sensation of sight is the result. The different colors also, from the red of the spectrum to the violet, are the result of different vibration rates in the waves of ether which strike the retina; and in order to perceive color, the retina must be able to respond to the particular vibration rate which represents each color.
Likewise in the sense of touch the end-organs are fitted to respond to very rapid vibrations, and it is possible that the different qualities of touch are produced by different vibration rates in the atoms of the object we are touching. When we reach the ear, we have the organ which responds to the lowest vibration rate of all, for we can detect a sound made by an object which is vibrating from twenty to thirty times a second. The highest vibration rate which will affect the ear is some forty thousand per second.
Thus it is seen that there are great gaps in the different rates to which our senses are fitted to respond--a sudden drop from billions in the case of the eye to millions in touch, and to thousands or even tens in hearing. This makes one wonder whether there are not many things in nature which man has never discovered simply because he has not the sense mechanism enabling him to become conscious of their existence.
There are undoubtedly "more things in heaven and earth than are dreamt of in our philosophy."
DEPENDENCE OF THE MIND ON THE SENSES.--Only as the senses bring in the material, has the mind anything with which to build. Thus have the senses to act as messengers between the great outside world and the brain; to be the servants who shall stand at the doorways of the body--the eyes, the ears, the finger tips--each ready to receive its particular kind of impulse from nature and send it along the right path to the part of the cortex where it belongs, so that the mind can say, "A sight," "A sound," or "A touch." Thus does the mind come to know the universe of the senses. Thus does it get the material out of which memory, imagination, and thought begin. Thus and only thus does the mind secure the crude material from which the finished superstructure is finally built.
CHAPTER IV
MENTAL DEVELOPMENT AND MOTOR TRAINING
Education was long looked upon as affecting the mind only; the body was either left out of account or neglected. Later science has shown, however, that the mind cannot be trained _except as the nervous system is trained and developed_. For not sensation and the simpler mental processes alone, but memory, imagination, judgment, reasoning and every other act of the mind are dependent on the nervous system finally for their efficiency. The little child gets its first mental experiences in connection with certain movements or acts set up reflexly by the pre-organized nervous system. From this time on movement and idea are so inextricably bound together that they cannot be separated. The mind and the brain are so vitally related that it is impossible to educate one without performing a like office for the other; and it is likewise impossible to neglect the one without causing the other to suffer in its development.
1. FACTORS DETERMINING THE EFFICIENCY OF THE NERVOUS SYSTEM
DEVELOPMENT AND NUTRITION.--Ignoring the native differences in nervous systems through the influence of heredity, the efficiency of a nervous system is largely dependent on two factors: (1) The development of the cells and fibers of which it is composed, and (2) its general tone of health and vigor. The actual number of cells in the nervous system increases but little if at all after birth. Indeed, it is doubtful whether Edison"s brain and nervous system has a greater number of cells in it than yours or mine. The difference between the brain of a genius and that of an ordinary man is not in the _number_ of cells which it contains, but rather in the development of the cells and fibers which are present, potentially, at least, in every nervous system. The histologist tells us that in the nervous system of every child there are tens of thousands of cells which are so immature and undeveloped that they are useless; indeed, this is the case to some degree in every adult person"s nervous system as well. Thus each individual has inherent in his nervous system potentialities of which he has never taken advantage, the utilizing of which may make him a genius and the neglecting of which will certainly leave him on the plane of mediocrity. The first problem in education, then, is to take the unripe and inefficient nervous system and so develop it in connection with the growing mind that the possibilities which nature has stored in it shall become actualities.
UNDEVELOPED CELLS.--Professor Donaldson tells us on this point that: "At birth, and for a long time after, many [nervous] systems contain cell elements which are more or less immature, not forming a functional part of the tissue, and yet under some conditions capable of further development.... For the cells which are continually appearing in the developing cortex no other source is known than the nuclei or granules found there in its earliest stages. These elements are metamorphosed neuroblasts--that is, elementary cells out of which the nervous matter is developed--which have shrunken to a volume less than that which they had at first, and which remain small until, in the subsequent process of enlargement necessary for their full development, they expand into well-marked cells. Elements intermediate between these granules and the fully developed cells are always found, even in mature brains, and therefore it is inferred that the latter are derived from the former.
The appearances there also lead to the conclusion that many elements which might possibly develop in any given case are far beyond the number that actually does so.... The possible number of cells latent and functional in the central system is early fixed. At any age this number is accordingly represented by the granules as well as by the cells which have already undergone further development. During growth the proportion of developed cells increases, and sometimes, owing to the failure to recognize potential nerve cells in the granules, the impression is carried away that this increase implies the formation of new elements.
As has been shown, such is not the case."[1]
DEVELOPMENT OF NERVE FIBERS.--The nerve _fibers_, no less than the cells, must go through a process of development. It has already been shown that the fibers are the result of a branching of cells. At birth many of the cells have not yet thrown out branches, and hence the fibers are lacking; while many of those which are already grown out are not sufficiently developed to transmit impulses accurately. Thus it has been found that most children at birth are able to support the weight of the body for several seconds by clasping the fingers around a small rod, but it takes about a year for the child to become able to stand. It is evident that it requires more actual strength to cling to a rod than to stand; hence the conclusion is that the difference is in the earlier development of the nerve centers which have to do with clasping than of those concerned in standing. Likewise the child"s first attempts to feed himself or do any one of the thousand little things about which he is so awkward, are partial failures not so much because he has not had practice as because his nervous machinery connected with those movements is not yet developed sufficiently to enable him to be accurate. His brain is in a condition which Flechsig calls "unripe." How, then, shall the undeveloped cells and system ripen? How shall the undeveloped cells and fibers grow to full maturity and efficiency?
2. DEVELOPMENT OF NERVOUS SYSTEM THROUGH USE
IMPORTANCE OF STIMULUS AND RESPONSE.--Like all other tissues of the body, the nerve cells and fibers are developed by judicious use. The sensory and a.s.sociation centers require the constant stimulus of nerve currents running in from the various end-organs, and the motor centers require the constant stimulus of currents running from them out to the muscles. In other words, the conditions upon which both motor and sensory development depend are: (1) A rich environment of sights and sounds and tastes and smells, and everything else which serves as proper stimulus to the sense organs, and to every form of intellectual and social interest; and (2) no less important, an opportunity for the freest and most complete forms of response and motor activity.
[Ill.u.s.tration: FIG. 15.--Schematic transverse section of the human brain showing the projection of the motor fibers, their crossing in the neighborhood of the medulla, and their termination in the different areas of localized function in the cortex. S, fissure of Sylvius; M, the medulla; VII, the roots of the facial nerves.]
An ill.u.s.tration of the effects of the lack of sensory stimuli on the cortex is well shown in the case of Laura Bridgman, whose brain was studied by Professor Donaldson after her death. Laura Bridgman was born a normal child, and developed as other children do up to the age of nearly three years. At this time, through an attack of scarlet fever, she lost her hearing completely and also the sight of her left eye. Her right eye was so badly affected that she could see but little; and it, too, became entirely blind when she was eight. She lived in this condition until she was sixty years old, when she died. Professor Donaldson submitted the cortex of her brain to a most careful examination, also comparing the corresponding areas on the two hemispheres with each other. He found that as a whole the cortex was thinner than in the case of normal individuals. He found also that the cortical area connected with the left eye--namely, the right occipital region--was much thinner than that for the right eye, which had retained its sight longer than the other. He says: "It is interesting to notice that those parts of the cortex which, according to the current view, were a.s.sociated with the defective sense organs were also particularly thin. The cause of this thinness was found to be due, at least in part, to the small size of the nerve cells there present. Not only were the large and medium-sized cells smaller, but the impression made on the observer was that they were also less numerous than in the normal cortex."
EFFECT OF SENSORY STIMULI.--No doubt if we could examine the brain of a person who has grown up in an environment rich in stimuli to the eye, where nature, earth, and sky have presented a changing panorama of color and form to attract the eye; where all the sounds of nature, from the chirp of the insect to the roar of the waves and the murmur of the breeze, and from the softest tones of the voice to the mightiest sweep of the great orchestra, have challenged the ear; where many and varied odors and perfumes have a.s.sailed the nostrils; where a great range of tastes have tempted the palate; where many varieties of touch and temperature sensations have been experienced--no doubt if we could examine such a brain we should find the sensory areas of the cortex excelling in thickness because its cells were well developed and full sized from the currents which had been pouring into them from the outside world. On the other hand, if we could examine a cortex which had lacked any one of these stimuli, we should find some area in it undeveloped because of this deficiency. Its owner therefore possesses but the fraction of a brain, and would in a corresponding degree find his mind incomplete.