When an afferent impulse arrives at the cells of the posterior column, it is communicated to the cells in the same segment, and reaching motor cells it gives rise to a movement of the muscles of the neighborhood from which the impulse first started. At the same time impulses travel to the brain, and there give rise to a consciousness of the various events taking place, i. e., a local stimulation and a local movement. The action of the cells of the cord takes place without the aid of the will, and occurs before the mind is conscious of it. These movements, being a turning back of the impulse, are called reflex acts.
Reflex action forms the most ordinary function of the cells of the spinal cord. Even the gentlest stimulation may give rise to a complex movement, the execution of which requires many muscles to act together, as it were, with a common object. An unexpected touch to the finger causes a person to withdraw the hand quickly. If greater or more prolonged stimulus be applied, more extensive movements occur; by the well-arranged cooperation of many muscles, a forcible, definite and familiar action is performed. For example, if the burning head of a match adhere under the thumb nail, more than a mere withdrawal of the hand takes place. The entire arm is violently shaken, before the will has time to come into operation. We have here a complex form of purposeful muscular movement, the immediate result of an impulse coming from a single point of the skin, owing to the spreading of the impulse to the cells of the segments in the vicinity. The movement is regular, performed with a definite purpose, as if it were the result of thought, but since there is no consciousness, it cannot be mental.
If the degree of stimulation be carefully regulated, it will be found that the results obtained by peripheral stimulation depend on (a) the strength of the stimulus, and the length of time for which it is applied; (b) the degree of excitability of the cells, and the readiness with which the impulses pass along the thin, conducting channels to the gray matter, and (V) the functional, activity of the muscles which act as indicators of the reflex effects.
All these points may be easily studied on a frog decapitated about an hour beforehand. If the animal be suspended by the lower jaw and the toe touched, the foot is gently withdrawn. If the toe be smartly pinched, the entire limb is forcibly raised; with intense or prolonged stimulus both legs are violently moved. If a fragment of blotting paper, moistened in weak acid, be placed on the belly, in a position not easily reached by the foot, a complex series of movements follows. The muscular action is both elaborate and purposeful, and the movements of the headless animal might almost be called ingenious.