Seeing that the response, in reflex action, is usually made by a muscle or gland lying at some distance from the sense organ that receives the stimulus--as, in the case of the flexion reflex, the stimulus is applied to the skin of the hand (or foot), while the response is made by muscles of the limb generally--we have to ask what sort of connection exists between the stimulated organ and the responding organ, and we turn to physiology and anatomy for our answer. The answer is that the nerves provide the connection. Strands of nerve extend from the sense organ to the muscle.

But the surprising fact is that the nerves do not run directly from the one to the other. There is no instance in the human body of a direct connection between any sense organ and any muscle or gland. The nerve path from sense organ to muscle always leads through a nerve center. One {27} nerve, called the sensory nerve, runs from the sense organ to the nerve center, and another, the motor nerve, runs from the center to the muscle; and the only connection between the sense organ and the muscle is this roundabout path through the nerve center. The path consists of three parts, sensory nerve, center, and motor nerve, but, taken as a whole, it is called the reflex arc, both the words, "reflex" and "arc", being suggested by the indirectness of the connection.

Fig. 1.--The connection from the back of the hand, which is receiving a stimulus, and the arm muscle which makes the response. The nerve center is indicated by the dotted lines.

The nervous system resembles a city telephone system. What passes along the nerve is akin to the electricity that {28} passes along the telephone wire; it is called the "nerve current", and is electrical and chemical in nature.

Fig. 2.--(From Martin's "Human Body.") General view of the nervous system, showing brain, cord, and nerves.

All nerve connections, like the great majority of telephone connections, are effected through the centers, called "centrals" in {29} the case of the telephone. Telephone A is connected directly with the central, telephone B likewise, and A and B are indirectly connected, through the central switchboard. That is the way it is in the nervous system, with "nerve center" substituted for "central", and "sense organ" and "muscle or gland" for "telephones A and B."

Fig. 3.--Location of the cord, cerebrum and cerebellum. The brain stem continues the cord upward into the skull cavity. (Figure text: cerebrum, cerebellum, cord, tongue)

The advantage of the centralized system is that it is a system, affording connections between any part and any other, and unifying the whole complex organism.

The nerve centers are located in the brain and spinal cord. The brain lies in the skull and the cord extends from the brain down through a tube in the middle of the {30} backbone. Of the brain many parts can be named, but for the present it is enough to divide it into the "brain stem", a continuation of the spinal cord up along the base of the skull cavity, and the two great outgrowths of the brain stem, called "cerebrum" and "cerebellum". The spinal cord and brain stem contain the lower or reflex centers, while the cerebellum, and especially the cerebrum, contain the "higher centers". The lower centers are directly connected by nerves with the sense organs, glands and muscles, while the higher centers have direct connections with the lower and only through them with the sense organs, glands and muscles. In other words, the sensory nerves run into the cord or brain stem, and the motor nerves run out of these same, while interconnecting nerve strands extend between the lower centers in the cord and brain stem and the higher centers in the cerebrum and cerebellum.

The spinal cord contains the reflex centers for the limbs and part of the trunk, and is connected by sensory and motor nerves with the limbs and trunk. The brain stem contains the reflex centers for the head and also for part of the interior of the trunk, including the heart and lungs, and is connected with them by sensory and motor nerves. The nerve center that takes part in the flexion reflex of the foot is situated in the lower part of the cord, that for the similar reflex of the hand lies in the upper part of the cord, that for breathing lies in the lower or rear part of the brain stem, and that for winking lies further forward in the brain stem.

Big movements, such as the combined action of all four legs of an animal in walking, require cord and brain stem to work together, and throw into relief what is really true even of simpler reflexes, namely that a reflex is a coordinated movement, in the sense that different muscles cooperate in its execution.{31}