Diagram 46.—Evolution of a Spinal Ganglion Cell.
Diagram 47.—Scheme of the Central Nervous System.
→ shows the path taken by an impulse in reflex action.
↣ shows the path for a voluntary action.
The difference between reflex and voluntary movement is, as may be seen from the above instances, very much a matter of degree; but we had better leave a comparison between them, and any discussion as to the extent to which the manifestations of consciousness are automatic, until we have finished describing reflex movement, and set forth the little we know about voluntary movement.
Time and space forbid a complete list of reflex movements. The following are, however, a few typical examples of how the body is automatically made to perform such acts as are necessary, and of how such as do not require deliberation are brought about without taxing the intellect.
A reflex action which is unpleasantly familiar is the cough, also the somewhat similar phenomenon of the sneeze. In this case, a foreign body which obstructs the windpipe, or causes irritation to the membrane lining the nose, is, on being reported at the spinal cord, incontinently blown out by an explosive blast of air from the lungs.
An organ which is very important, and at the same time very sensitive—viz., the eye—has many protective reflexes. The external surface of the eye is covered by a very delicate membrane, which must be kept moist and scrupulously clean. Whenever this membrane gets in the least dry, or any dust falls on it, the eyelids are closed for a moment, thereby bathing it with the secretion of the tear glands. Few people are aware, I think, that they blink their eyes on an average twice every minute. The eyes are also closed quite involuntarily by a reflex when any danger threatens them—for instance, a sudden dazzling light, a strong wind, or a blow aimed at the face; and if any foreign substance—say a fly—does get into one of them, the secretion of the tear glands is enormously increased to wash it out.