The simplest behavior system, known as a reflex arc consists of a sensory neurone and a motor neurone meeting at a junction point called the synapse. The following diagram. (Fig. 1), which is greatly simplified, will make this clearer:

FIG. 1. DIAGRAM OF A SIMPLE REFLEX ARC, OF SITUATION-RESPONSE UNIT CONSISTING OF A SENSE ORGAN, SENSORY NEURONE, SYNAPSE, A MOTOR NEURONE, AND A MUSCLE.

The arrows show the direction of the nerve impulse. In the diagram, A represents the fine sensory nerve endings in and about the sense organ. A nerve current is initiated when the sense organ is stimulated. Action results when the impulse reaches the effector (F).

Here A will be the receiving end of the sensory neurone (ABC), B is its conducting part, C its discharging end. CD forms a junction point or synapse (here a reflex center). DEF is the motor neurone, with D its receiving end, E its conducting part and F its discharging end. In this case the branchings of F will be joined with a muscle (or system of muscles). The child touching a hot stove will afford an illustration. The diagram (Fig. 1) represents the nervous machinery for removing the hand. The hot stove stimulates the sensory receiving end at A. The disturbance will be conducted along B to its discharging end at C. Here it will pass through the synapse to the receiving end D and thence along E to the motor-discharging ends at F, which are appropriately connected with certain muscles. These muscles upon receiving the stimulation from F contract and produce the movements necessary for withdrawing the hand.

The neural machinery by which the movement takes place may be called a mechanism. There are as many mechanisms as there are specific things to do, a mechanism for sneezing, another for focusing the eye, another for walking, and so on. Whenever a mechanism is set into action there is a liberation of vital energy of some sort that was in some way stored up in

the discharging endings with their connections. When the nerve impulse initiated by touching the hot stove was received, the mechanism for producing the response was set in action. In other words, a drive was communicated to the mechanism.

Woodworth, following Sherrington, uses the terms preparatory and consummatory reactions. To these may be added those of readiness, and mind-set-to-an-end. The eating of a dinner when hungry is a consummatory reaction, but all the steps necessary for getting the food is preparatory to the consummatory reactions. A mind-set that looks forward to the consummatory reaction as its end, may be called a mind-set-to-an-end. Readiness is a condition or state of a mechanism wherein it is more easily put in drive.

In the S-R symbolism, learning means that R is joined more surely to the S, or less surely in negative cases. This may be interpreted as meaning that the change is produced at the synapse. Exercise under favorable conditions strengthens the synaptic connection so that nervous impulses pass more readily and surely from the sensory neurone to the responding neurone. Woodworth (Psychology, p. 415) thus pictures four states of a synapse as it is strengthened by use and a succeeding fifth after it has been weakened by disuse (Fig. 2).

Psychologists have spent much study upon the conditions under which learning takes place. A careful statement of these conditions is to be found in formulations called the Laws of Learning.

It often happens that if one response does not succeed, another one is tried, and if need be a third, and so on until either success or failure attends. This is called multiple response and is very important in discussions of how learning practically takes place. The simplest case of this may be schematically diagrammed as in Figure 3.

Here the sensory neurone (A) has three branches B, C, and D, which form synapses with the three different motor responses E, F, and G. We may suppose that the stimulation along A finds its most open outlet along BE; while CF would be next open, and DG least. If now the response E is not satisfying and the stimulation continues, the second CF will be tried, and if need be finally DG.

Matters may be anticipated by saying that under the Law of Effect the annoyance attending the failures of the response E and F will make the synapses BE and CF less disposed to "conduct" the next time A is stimulated, while the satisfaction

FIG. 2. DIAGRAM ILLUSTRATING THE PHYSIOLOGY OF THE LAW OF EXERCISE.

The assumption is that use or exercise strengthens the bond by weakening the synaptic resistance. The theory is that every time a nervous current passes over the synapse, the nerve fibers involved are exercised, and the after effect of this exercise is growth. This means that each repetition makes the connection easier, until we have a condition as shown in the fourth drawing. The fifth drawing from the top shows the effect of disuse. (After Woodworth.

FIG. 3. DIAGRAM SHOWING AFFERENT (SENSORY) NEURONE WHICH INFLUENCES DIRECTLY THREE EFFERENT (MOTOR) NEURONES.

The arrows show the direction of the impulse when the sense organ (SO) is stimulated, Movement is effected when the impulse reaches the muscles (M).

attending the response G will analogously increase the readiness. of synapse DG for conducting. If like results be repeated often and consistently enough, eventually the stimulation will choose immediately the path ADG in preference to any other and the response G will then come promptly and surely.

5. Secondary Neurone Connections

[THORNDIKE, E. L., Educational Psychology, Vol. I, pp. 141-142; 307308. New York, Teachers College, Columbia University, 1913.] (Adapted.)

Mental activity is satisfying to man; mental emptiness is annoying. The brain of man contains millions of neurones which crave stimulation. They are in "readiness to conduct," though no immediate gratification of any more practical want follows their action.

Native satisfiers and annoyers referred to above as well as many others belonging to such original behavior series as have to do with food-getting, food-seeking, danger-avoiding, and the like, involve a direct bond of neurone connection between the suitable stimulating situation and its appropriate response. Thus, the presence of suitable food to the mouth stimulates through its own direct bond the masticating response. Acquired bonds may also crave exercise.

In additions to direct bonds, whether original or acquired, between situations and responses, there are original tendencies to make and enjoy making secondary connections. For example, a child likes to see and hear a rattle. Direct bonds are at work. But to handle and shake the object himself so as to produce the noise is still greater satisfaction. Hearing the rattle when he shakes it is an example of a secondary connection. It represents the action of the neurones concerned in the child's manipulations, those concerned in his sensations and those concerned in connecting the latter with the former. To do something and have something happen is, other things equal, instinctively satisfying. There are many concrete forms of secondary connections. Making plans and getting conclusions, making imaginary people and thereby getting further imaginations of how they would act, as well as making movements and thereby getting sensations, are satisfying. They are originally satisfying in the sense that nature gives satisfyingness to the connection as soon as training and education give the ability to make the plan or form the image, etc.

Secondary connections have great significance for both improving and satisfying human wants. Those bonds or connections in which the sensory situation is replaced by an abstract plan, and the immediate muscular response by a contemplated action, tried out on the level of thought only, will, in the long run, do most for satisfying human wants. Each secondary connection can do the work of thousands of gross concrete behavior-series, providing for situations. before they are met, for elements of situations never encountered by themselves, and for groups of situations whose essential similarity the more animal-like connections could never reveal. "Furthermore, these tendencies to secondary, or higher,' connections may rise free from the appetites of the single creature who exercises them and deal with the world in the interests of all men. Work and play with 'ideas' . . . can be impersonal and ideal to an extent and to a degree that would never be attained by direct responses to the concrete situations themselves."