Most reflexes can be seen to be useful to the organism. A large proportion of them are protective in one way or another, while others might be called regulative, in that they adjust the organism to the conditions affecting it.
Now comparing the reflex with the simple reaction, we see first that the reflex is more deep-seated in the organism, and more essential to its welfare. The reflex is typically quicker than the simple reaction. The reflex machinery does not need a "Ready" signal, nor any preparation, but is always ready for business. (The subject in a simple reaction experiment would not make the particular finger movement that he makes unless he had made ready for that movement.) The attachment of a certain response to a certain stimulus, rather arbitrary and temporary in the simple reaction, is inherent and permanent in the reflex. Reflex action is involuntary and often entirely unconscious.
Reflexes, we said, are permanent. That is because they [{26}] are native or inherent in the organism. You can observe them in the new-born child. The reflex connection between stimulus and response is something the child brings with him into the world, as distinguished from what he has to acquire through training and experience. He does acquire, as he grows up, a tremendous number of habitual responds that become automatic and almost unconscious, and these "secondary automatic" reactions resemble reflexes pretty closely. Grasping for your hat when you feel the wind taking it from your head is an example. These acquired reactions never reach the extreme speed of the quickest reflexes, but at best may have about the speed of the simple reaction. Though often useful enough, they are not so fundamentally necessary as the reflexes. The reflex connection of stimulus and response is something essential, native, closely knit, and always ready for action.
The Nerves in Reflex Action
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.