in the cord or in the medulla, the essential nature of the sensori-motor circuit involved is the same. The connection between the sensory neurones which bring the impulse in to the center and the motor neurones which carry the impulse out to the muscles is quite direct, the redirection at the center is rather immediate, and the mode of reaction is relatively simple. It has been proved by experiments upon lower animals, in which the brain has been removed or extirpated, that this lower circuit through the spinal cord provides a mechanism adequate to the performance of all the elementary muscular movements which are involved in all the activities of the animal. But when these acts are not influenced by any discharges of nervous impulse from the brain downward into the cord, in other words when they are purely reflex, they are totally lacking in spontaneity, or voluntariness, and they are as purely mechanical and necessary in character as the movements of a machine which are released by a spring or which are set free by the pushing of a button. Indeed, the figure of speech, "press the button and the machine will do the rest," would not be inappropriate to apply to acts which involve only the first type of sensori-motor circuit.

(4) Place of consciousness in this circuit.

Properly speaking, consciousness has no place within the first type of sensori-motor circuit. In reflex action, as we might expect from the immediacy of central redirection, consciousness does not intervene between stimulus and response. Hence it cannot modify or control the reaction in any respect. If consciousness does accompany the act at all, it may be explained in either or both of two ways. The nervous impulses set up by the movements of the muscles while the action is taking place may travel to the brain and we may feel the movements. That is, we may be conscious of the fact that we are winking or that we are withdrawing the hand from the prick of a pin, but at the same time consciousness has had nothing to do with the

production of these movements. Or it may be that some of the original sensory impulse responsible for the reflex act has spread upward to the brain and resulted in awareness of the stimulus, but before this has happened a much larger portion of the current has been redirected from some center in the cord and is already out on its way to produce the muscular contraction. Thus, while we know that we have been pricked by a pin, the consciousness of this fact was not responsible for the movement; for it did not come soon enough. The central redirection into a motor channel took place before the nervous current reached the centers of consciousness in the brain.

6. SECOND TYPE OF SENSORI-MOTOR CIRCUIT.

(SEE DIAGRAM II).

(1) Definition.

At

This circuit is by way of the lower brain centers. various lower levels of the brain there are masses of gray matter, among which are the optic thalami, the striate bodies, and ganglia in the cerebellum, the pons, and the medulla. These centers lie outside of the first circuit, but through interconnections of neurones they receive impulses from the sensory side of the first circuit and transfer them to its motor side. This longer route for nervous impulses by way of the lower brain centers through a loop, as it were, added to the first circuit, is what we mean by the second type of sensori-motor circuit.

(2) Reference to the diagram.

A stimulus originating at AI may generate an impulse which reaches the cord at the level of Br. Instead of discharging at this level, it may travel upward to the centers at X. Now from X, pathways run down to all the levels of the cord. Hence there is this added possibility not offered by the first type of circuit, namely, that impulses may discharge from the centers at X simultaneously to touch off the responses C1, C2, C3 all at the same time,

thus combining them into one complex act. Or, the impulses may be discharged from the centers at X successively to touch off the responses C1, C2, C3, etc., one after another in some given order, thus producing a complex reaction process made up of a series of interrelated acts no one of which may necessarily have any significance in itself and yet the whole series be admirably adapted to the attainment of some useful end or the performance of some important function.

(3) Significance, mechanism for complex, organized, mechanical reactions.

The lower brain centers, according to prevailing theory, control complex and coördinated activities of various sorts, such as the instinctive and the habitual. For the performance of this function they seem admirably adapted. From the fact that the lower brain centers may discharge into all the different centers of the cord and thus touch off several elementary motor processes at the same time or in succession, the coördination and organization of activity is made possible. But these lower brain centers may also receive sensory impulses from every sensory surface, hence we have the possibility of the most delicate coördination of every sort of sensory impression with every sort of motor response. And muscular activity may be coördinated not alone with the activities of a single sense organ, say the eye, but also with the activities of all the sense organs,— those of touch, hearing, etc.,-all at the same time or within the compass of the same situation. All the sensory impressions that are relevant in a given situation, and all the muscular movements that are useful, may work together in one large organization of activity in which they are properly adjusted to one another to make the whole method of reaction one which is very highly adaptive.

Experiments upon animals, in which the higher brain centers have been removed or extirpated, confirm the view that the control of coördinated and organized motor

processes is through the activities of the lower ters which are brought into play in the seco sensori-motor circuit. Mr. James cites experi upon frogs in which the frog deprived of the higher brain centers could perform every com which the normal frog was capable. He c jump, turn over from his back, swim, croak, et same experiments show that the acts of this perfectly mechanical. They occurred under and only when the stimulus was given. There w spontaneous or voluntary about them. The frog ing but a complex machine. "Touch the right b a certain act would occur inevitably. The act very complex, but the complexity was one which already built up and established; the organizat response had been previously perfected and h a part of the mechanism of the animal. Such a very highly adaptive, but they are nevertheless In the lives both of lower animals and of man a number of the complex acts of ordinary life adapted to ends are of this mechanical sort pr currents of nervous energy which take the cou second type of sensori-motor circuit. They are reaction which are instinctive and determined by or which are habitual, having been built up and in their organization in the lifetime of the indivi

(4) Place of consciousness in this circuit.

In man, both instinctive and habitual acts, are dominantly under the control of the lower ters and are thus mechanical in character, are no be entirely free from the determining influen sciousness. But in many cases, as will be show in a later chapter,2 consciousness enters only t part of the most organic and automatic aspect 'James, Psychology, Briefer Course, pp. 92-96. * See Chapter VII,

perception and associative memory. In a sort of organic fashion, as distinguished from ideational, the sense perception processes are brought into very delicate accord with motor processes in a way that is very significant for the process of adjustment between organism and environment. 7. THE THIRD TYPE OF SENSORI-MOTOR CIRCUIT.

(SEE DIAGRAM III).

(1) Definition.

This circuit is by way of the higher, or cortical, brain centers. These are the centers which are known to function in connection with conscious processes and voluntary action. The connection of the cortical centers with the lower brain centers forms an additional "loop." Sensory impulses transferred from centers in the cord to lower brain centers may pass upward from these to the cortex, and motor impulses may travel downward from the cortex either directly to the lower brain centers or directly to centers in the cord. When impulses originating in a stimulus to some sense organ ultimately pass through the cortical centers before they produce their muscular response, no matter how many intervening transfers of nervous energy may have been made, their route is that of the third type of sensori-motor circuit.

(2) Reference to the diagram.

In the diagram, to avoid too great complexity in one figure, we have not represented the full "loop" from X to Z and back from Z to X or to some B. We have represented only the downward, or motor, pathways. On the motor side of this circuit, it is very significant that there are two types of connection of the cortical centers with lower centers. The impulse from Z, whether it originates in other brain activities or results from a sensory impulse due to excitation of a sense organ, may discharge directly down into the centers at X, or it may discharge directly down into some center B in the spinal cord without being