THE SENSORI-MOTOR CIRCUIT

If consciousness is to play any part in the conc the individual, if it is to have anything to do w ment, then it must have some place within th process. From the biological point of view we determine just where consciousness comes into of reaction; also under what conditions and with of function. This cannot be done without taking some important principles of nervous action.

2. THE REACTION PROCESS IN TERMS OF THE MOTOR CIRCUIT.

We shall call the course which a nervous im from the time that it originates in some sort affecting a sense organ until it results in so muscular movement, a sensori-motor circuit. T process properly includes the whole set of activiti in the completion of a sensori-motor circuit. sensory phase, including stimulus and ingoin impulse; its phase of central redirection in the or the brain; and its motor phase, including out ous impulse and muscular movement, or resp sciousness, if it comes in at all to modify reactio only in the phase of central redirection, and here this redirection takes place in the higher cen brain known as the cortex.

3. NUMBER OF TYPES OF SENSORI-MOTOR CIRCUIT.

The most casual student of the nervous system must know how exceedingly complex it is and how intricately its minute elementary structures, the neurones, connect with one another. It must be expected, then, that in the varied reactions of the complex human organism innumerable sensori-motor circuits are involved. However, we may roughly reduce them to three general types. These may not be adequate in the explanation of all the details of reaction, but they will help us to reduce to some sort of intelligible system the bewildering complexities and intricacies of nervous action.

4. THE USE OF DIAGRAMS.

(1) Cautions against their misinterpretation.

In explaining the three general types of sensori-motor circuit, we shall be greatly aided by the use of diagrams. But we must keep constantly in mind that the diagrams employed in the explanation of the activities of the nervous system cannot represent facts of detail; they can only schematize the most general principles. The diagrams which follow are highly schematic, and they are drawn purposely in such a way as to leave no room for supposing that they are at all pictorial in character, not even in the matter of conformity to the shape of the brain. No reference is made to the sympathetic system and its relation to the cerebro-spinal system. No attempt is made to represent the shape or the number of the neurones involved in a reaction process, nor the precise manner of their interconnection. For these facts, important as they are, the student should consult some standard text in physiology or neurology rather than expect them to be represented and discussed here. It would take us beyond the compass and purpose of this brief treatise to enter into the minute details of the structure of the nervous system. Yet certain general ideas of its method of action are necessary to

an understanding of the function of consciousness in the life of the organism. The diagrams given here will be helpful if it is constantly kept in mind that they are not intended to represent details of anatomy, but that they are intended to represent in a schematic way only certain typical pathways, together with certain critical points of transfer of nervous impulses, in the course of the complete sensorimotor circuit.

(2) The diagrams and their terminology.

....."

Z represents any, or all, of the cortical brain centers. X represents any, or all, of the lower brain centers. B1, B2, etc., represent different levels of the spinal cord. AI, ... An represent stimuli affecting sense organs and setting up impulses which reach spinal cord or brain. AI-BI represents an afferent impulse traveling to the cord as a center; An-X represents an afferent impulse traveling from eye, ear, or other higher sense organ to lower brain center without going by way of the cord.

BI-C1, B2-C2, etc., represent efferent impulses traveling out to muscles.

C1, C2, etc., represent muscular responses.

The dotted cross lines are for schematic convenience only. 5. FIRST TYPE OF SENSORI-MOTOR CIRCUIT.

(SEE DIAGRAM I).

(1) Definition.

In this type of sensori-motor circuit the transition from sensory to motor phase, or the central redirection, is effected in the spinal cord or in the medulla, an enlargement of the spinal cord at its upper extremity. The first type of circuit represents, then, the pathway of a nervous impulse from sensory excitation to motor response by way of the spinal cord or the medulla. The characteristic method of reaction corresponding to this is commonly called reflex. Illustrations of reflex action with the center of redirection

in the cord are to be found in such cases as the su drawal of the hand from the prick of a pin and ment of the foot of a sleeping person when it The medulla is the center of redirection for th "higher" reflexes, such as winking, sneezing, vomiting, swallowing, etc. These are not to be with automatic actions, such as breathing and bea heart, which are also controlled by the medulla part.

(2) Reference to the diagram.

In the diagram we have illustrated only that cas action in which the center of redirection is to be the spinal cord. AI-BI-CI represents the sen circuit ordinarily involved in such a case. pose that it is the response to the tickling of the motor discharge would normally take place at level in the cord as that at which the sensory received, and it would normally go out on the that is, the foot which has been tickled would drawn.

It is possible, however, when there is some in with the more primary method of response, or wh for the nervous impulse to discharge from the sp upon the opposite side. In this case, the other be used to rid one's self of the irritant. Or th might even travel up in the cord to a higher level be discharged into a motor channel,-reflex act C3. For example, one might with his arm swe fly from his bare foot, if he were in a position su could not dislodge the irritant by a movement itself. In cases so complex as this, however, it to suppose that the act is wholly reflex, parti human beings.

(3) Significance,-mechanism for simple

Whether redirection of the nervous impulse t