436 POPULAR SCIENCE MONTHLY. 



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 crea- 

 ture. By the division of labor, the whole body of the compound 

 organism can not be given over to sensation. Hence, the develop- 

 ment 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 pro- 

 cess 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 knowl- 

 edge 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 volun- 

 tarily and even laboriously, the force of habit will cause us to repeat 

 to-morrow easily, involuntarily, and whether we will or not. By the 



