neurone. The popular term for any connection, or chain of connections, between neurones which act in succession is "path." Anything which gives an impulse to any neurone-light, sound, touch in those specialized neurones contained in the sense organs, or the impulse from another neurone in the chain-is called a "stimulus." The response to a stimulus is always a "discharge" into another cell. When the impulse gets to the end of its nerve "path" it must discharge into some muscle, as in winking, talking, or running. The path between the stimulus and the "motor discharge" may be very simple, or very complex and long delayed, but to every stimulus there will ultimately be some sort of motor response. If the stimulus to the organ of hearing is a sudden loud noise, the motor discharge may be a violent and immediate contraction of many of the large muscles; it "makes us jump." If the stimulus is a talk by Dr. Grenfell, the motor response in one person may be the act, an hour or a day later, of writing a check for the work of the hospital ship on the Labrador coast; another person may decide to go to college and medical school, and become a medical missionary. In this case the final motor discharge is in going wherever the need is greatest when preparation is completed.

"Sensori-Motor Paths" Illustrated. We have been speaking as though a "stimulus" were one simple thing, and the "motor response" another equally simple. But just try to think of all the separate steps involved in so simple a "path" as that between the stimulus "pin prick" and the baby's action, "jerk hand and cry." Then imagine how many more there are between the stimuli involved in a specially skillful

b", auditory center in the brain.

Central Re-direction.

b, b', b'', b'', centers in brain receiving sensory stimuli and sending on the stimulus in discharges converging on bb, motor center in brain.

(z represents images aroused by areas of the brain also stimulated by all these sensory stimuli, such as the memory of previous successful strokes and the feeling of the muscles that made them, the idea of triumph or defeat in the contest. These also affect bb, and modify the "orders" sent to bbb.)

bb, Motor center in brain cortex, discharging into

bbb, Motor center in lower brain which distributes the impulses along the various motor paths.

C. Motor Response.

c, c', c", "", Groups of muscles whose complex but coordinated contraction results in keeping the ball in sight while running, stooping for a back-handed blow, and regulating the force applied to the ball.

serve by your opponents at tennis and your successful

return.

Instinct-Acts. In the illustration just given, the baby's cry differs from the tennis game not only in being simpler, but also in that it did not have to be learned. So, if you put your finger into a tiny baby's palm its hand closes tight; often a baby can cling so as to support its whole weight for several seconds. The first time anything is put into the baby's mouth there is a motor response of sucking. The first bright light makes the eyes blink. That is, a path between a senseimpression and a particular motor activity was all ready-made. The actions due to ready-made paths are called instinct-acts, and the paths most important for keeping one alive are ready-made at birth.

Other paths are not finished for weeks, or months, or years, and many can be completed in several different directions. One of these directions will be a little (it may be ever so little) nearer ready than another, depending on general health, or on what other connections are acting at the time the stimulus is given. When a stimulus appears, however, there must be some action; if the stimulus is entirely new the response must be through some ready-made path. If no nerve path is ready and no response takes place, we simply say that the organism was not stimulated. A kitten is not stimulated by light till its eyes are open; a chicken does not peck for a certain number of hours after it is hatched, and the most beautiful girl does not arouse romantic emotion in a boy till he reaches a certain age. These incomplete paths, which are not stimulated until the organism reaches a certain stage in its growth, are the source of what are

known as the delayed instincts. A child "learns" to walk and talk as soon as certain paths, unfinished at birth, are completed and ready to make the sensorimotor connection. Adolescence is but a period in which an important group of nerve paths become completed and the delayed race instincts ripen.

But if a child can do only what her repertory of instinct-acts will permit, how does she differ from a little automaton? The hope of developing a free personality depends upon the

Possibility of Varied Responses. It is useful to gather up all the stimuli, both sense impressions and memories, which may be affecting the individual at any one instant into the word "situation," and to call the complex of impulses, movements, feelings and ideas which it calls forth, the "response."3 What happens when a situation produces a response? Usually, along with the motor discharge there is an accompanying awareness of what is happening. The child is "conscious" of it, or "perceives" it. Most things bring a "feeling" along with the perception, and so are at the same time perceived as "agreeable" or "disagreeable," "pleasant" or "unpleasant" or "painful." These words are used by everyone with different meanings at different times, and even in the same connections they have different meanings for different people; so it will be less confusing to say that the response is "satisfying" or "annoying." 3 "Satisfying" means that the individual seeks to continue the situation or does nothing to avoid it. "Annoying" means that he tries to avoid the situation or ceases doing what would continue it. In this way we can judge of the feeling of any living being by

This is Professor Thorndike's usage.

watching his behavior, whether it is a kitten or a baby that cannot report its inner states, or an adult human being who wishes to conceal them.

Now, the first response to any totally new situation must always be instinctive. But if the first instinctive response is not satisfying, or fails to remove the cause of annoyance, another of the nearly-ready paths will be tried, and so on until a successful or satisfying response has been made. Watch a sleeping child who has become uncovered and is cold. The "cuddling" movements, and finally the outcry, show successive responses to the annoying stimulus of the cold and efforts to secure satisfying contact with the covering. Also, if a new nerve-path is ready to connect, it may respond to a stimulus to which the individual has heretofore responded by a different nerve-connection. A two-year-old may have been accustomed to give her arms to be lifted up by any adult who reached to take her. Suddenly an unaccountable shyness awakens, and she runs instead to hide her face in mother's lap; and if the adult person persists, she may even burst into a storm of sobs. Adolescence will show many instances of the response to a familiar situation by a new instinct-act; when that critical period comes we need to realize that this variable behavior is akin to that which all through the child's growth has developed her knowledge and power by making new experiences possible.

The Instinctive Basis of Habit. The first response to any totally new situation is instinctive, but any connection which has once been made is easier to make again. This fact is the basis of habit formation. Other things being equal, the second time a sense-stimulus