a brain is modified by a lesson in arithmetic, Latin, or psychology, yet we are absolutely confident that some modification has taken place and that it will be conserved. If we had microscopes powerful enough and

propriate stimuli. We should note a difference between the growth of some children mentally starving for want of appropriate stimuli and others forging ahead because abundantly supplied, just as we see the differences between the pale faces and emaciated bodies of some and the ruddy complexions and robust forms of others.

The facts of aphasia have contributed much to our knowledge of the intimate relation between the development of nerve elements and mental growth. They also show how experienceeducation-has to build neural connections between different centres. Most of our percepts and memories are exceedingly complex and may be aroused through numerous channels. In aphasia, which is merely loss of memory of a special type, it frequently happens that elements which have once served as stimuli to awaken the entire chain of relations fail to serve in this capacity. For example, it may occur that a man is unable to write his name when he sees it written while he may still be able to write it if it is pronounced. He may be unable to pronounce his name if he hears it, but may be perfectly able to do so if it is written. He may be unable to speak or write the word bell if

the bell is seen, though he can do so if the bell is rung. The explanation is that the association tracts between some of the different elements have become functionally deranged through disease or pressure and the transmission of impulses is inhibited. Sometimes it is very temporary, caused by fatigue, and sometimes much more serious, when lesions have been produced.

Effects of Use and Disuse.-Just as the efficiency of physical and mental action is destroyed by injuries to the central nervous system and the various interconnecting pathways, so the brain may fail to become an efficient instrument by lack of development of the cell elements and the various connecting fibres. Neurologists inform us that the number of cells is probably as great at birth as at maturity. But all except the lowest levels controlling the vital functions are "unripe." Experience—education-must determine the number that come to functional maturity. Through experience medullation begins to take place and connections to be established. Stimulations from the outside world begin to pour in through the senses and development proceeds. At first very simple sensory experiences and motor reactions are established. Later the association fibres become so well established and so complex in character that real thinking and deliberation may take place. These are not possible in a simple system where energy is diffused as is the case in lower animals and immature children. The association fibres in the human brain make their best development during adolescence, though growth does not cease until maturity-probably later. Coincident with this neurological development the power of thought proceeds. It is futile for teachers to expect thought power to manifest itself until the anatomical substructure is established. Since the anatomical development is contingent upon nutrition, sleep, rest, and various hygienic factors, it is preposterous to expect good mental development regardless of them (see the chapter on "Fatigue").

Importance of the Plastic Period. If nerve cells are ever developed to functional maturity and efficiency, it must be accomplished during the plastic period of childhood and youth.

During these periods the nervous system is responsive to education. It is a notable fact that restitution of function may occur when brain injuries occur in childhood and youth, but seldom later. In man the paralysis of a limb caused by brain injury is usually permanent, but in a child not frequently so. Injury to one hemisphere occurring in youth may be compensated by special development and transference of function to the other, but in adults this is no longer possible.

"The intensity with which any form of exercise is carried on during the growing period leaves its trace, and the absence of it at the proper time is for the most part irremediable. We should hardly expect much appreciation of color in a person brought up in the dark, however good his natural endowments in this direction. Thus any lack of early experience may leave a spot permanently undeveloped in the central system-a condition of much significance, for each locality in the cerebrum is not only a place at which reactions, using the word in a narrow sense, may occur, but by way of it pass fibres having more distant connections, and its lack of development probably reduces the associative value of these also." 1

2

Donaldson further says: "It has been made probable that by the cultivating processes of school training the formed structures tend to be strengthened, dormant elements roused to better growth and organization, and made more perfect in this or that direction according to the nature of the exercise. By strengthening the formed cells their powers of differential reaction, of organic memory, and resistance to fatigue are increased. By associating given sets of muscular reactions with given sense impressions habits are formed. in consequence of further organizations among the nerve elements, and finally nutritive rhythms associated with the periods of activity and rest are established, with the result of economizing the bodily energy, and rendering its expenditure more effective."

Correlation of Nervous, Muscular, and Mental Actions.-The only means we have of knowing mind is through muscular 'Donaldson, Growth of the Brain, p. 348. 2 Op. cit., p. 344.

responses. If we are to interpret psychic processes correctly, then there must be accurately co-ordinated sensations and muscular reactions. What one says orally or writes, what one paints, models, moulds, or makes, how one uses his various muscles, indicate what his mind is doing. Even in examining students to determine their grades we are obliged to rely upon some of these manifestations. If they speak or write incorrectly we judge that their thoughts have been inaccurate. From impression to expression is a law of psycho-physics. It is exceedingly important in education. It will be shown later that impressions and expressions react reciprocally upon each other. Just as we may know of the healthful activity of mind and brain through muscular actions, we may also discern signs of mental disease. Warner says: "The general condition of the nerve-system is expressed by motor signs-freshness, fatigue, irritability, all may be indicated to us by movements of the child, the absence of movements, or by the attitude or posture of the body, which depend upon motor action."

1

It thus becomes perfectly apparent that the problem of education is as much concerned with the education of the nervous system as of the mind. Later discussions will also go to show how much it is concerned with muscular adjustments and the co-ordination of mental and muscular activities. The brain is the great co-ordinating organ, making possible higher forms of choice, inhibition, and volition. To succeed in improving or systematizing the child's expressions means that corresponding improvement has taken place in his brain and nervous system. Every controlled and co-ordinated movement means the correlation of well-defined brain tracts and association areas. Conversely improvement of the brain and the establishment of organized pathways of nervous discharge are necessary for improved mental action.

Evolution of Nervous System Means Education. It is readily noted that the nervous system becomes more and more complex with the ascending scale of life. In the lowest forms of life,

1 The Study of Children, p. 40.

with simple needs and activities, no nervous structure is ever visible. Their organic structures are as undifferentiated as their functions and activities. With the appearance of differentiated functions arise specialized structures to fulfil the varied functions. We may go even further and assert simply a biological law, viz., that the specialized structures arise through the exercise of special forms of activities. That is, the experiences functioning in a particular way accentuate organs and cause their development in harmony with the actions. The individuals and even nervous organs of the individuals become accustomed to acting in specialized ways because these modes are found advantageous. Each activity tends to develop the function and structure still further. Habits are engendered which tend to be conserved, and in this way the future conduct is determined. As before stated, whatever biases the individual, or an organ, toward a particular mode of conduct is educative.

Thus the whole development and specialization of the nervous system in the ascending orders of life represent a process of education. The type of growth and the manner of functioning at any given stage represent the resultant of all previous experiences-education. As fast as activities have been experienced nature has recorded the effects indelibly in the nervous system. Thus, while the nervous system of any organism represents the kind and degree of possibilities of further experiences, it reciprocally indicates the kind and degree of experiences which have been received. While it is true, for example, that the brain of the bird is not fitted for a very high order of thinking, it is equally true that the birds and their ancestors have never indulged in very complex mental gymnastics. The most fundamental life processes, physical co-ordinations, relatively simple perceptions of sight, sound, and smell, have sufficed for their preservation. In the more sagacious animals like the dog, ape, and elephant, we find much more complicated brain structures both as a cause and as an effect of their increased intelligence. The cerebellum, the lobes of sight, smell, and hearing, are no smaller, and there is a noticeable increase of cerebrum.