Page:Popular Science Monthly Volume 52.djvu/454

436 on living creatures, and these it must pursue if it is to thrive. It is not the sensitive nerve tips which are to move; it is the whole creature. By the division of labor, the whole body of the compound organism can not be given over to sensation. Hence, the development of sense organs different in character, one stimulated by waves of light, another by waves of sound; one sensitive to odor, another to taste; still others to contact, temperature, muscular strain, and pain. These sense organs through their nerve fibers must report to a sensorium, which is distinct from each one of them. And in the process of specialization the sensorium itself is subdivided into higher and lower nerve centers—centers of conscious thought and automatic transfer of impulse into motion. This transfer indicates the real nature of all forms of nerve action. All are processes of transfer of sensation into movement. The sensorium or brain has no knowedge except such as comes to it from the sense organs through the ingoing or sensory nerves. It has no power to act save by its control of the muscles through the outgoing or motor nerves. The mind has no teacher save the senses; no servants save the muscles.

The reflex action, then, is the type of all mental operations. The brain is hidden in darkness, protected from sensation, as also from injury, by a bony box or a padding of flesh. It has no ideas of its own. It can receive no information directly. But the sense organs flood it with impressions of the external world. From the body itself, by similar means, are transmitted impulses to action. Such impulses in all animals and men are transmitted from generation to generation as a part of the legacy of heredity. They are in their nature rather methods than impulses. Movements go along lines of least resistance, and such lines are part of the stock of heredity.

Many of the impressions from environment are received by the lower nerve centers alone, the sympathetic system or the spinal cord. Here they are converted at once into motion without rising into the region of consciousness. Other sensations rise to the brain itself, and are made the basis of voluntary and conscious action. And between the purely automatic actions and those distinctly conscious and voluntary there may be found every possible intermediate grade.

Moreover, a conscious action often repeated becomes in some degree reflex and automatic. By repeated action nerve connections are formed, which have been compared to the automatic switches of the electric-light plant. By these connections an action once become familiar requires no further attention. This fact is known to us as the formation of habit. That which we do to-day voluntarily and even laboriously, the force of habit will cause us to repeat to-morrow easily, involuntarily, and whether we will or not. By the