INSTINCT AND REASON 241 
The one class, called sensory nerves, extends from the skin 
or other organ of sensation to the nerve center. The nerves 
of the other class, motor nerves, carry impulses to motion. 
127. The brain or sensorium,—The brain or other nerve 
center sits in darkness surrounded by a bony protecting 
box. To this main nerve center, or sensorium, come the 
nerves from all parts of the body that have sensation, 
the external skin as well as the special organs of sight, 
hearing, taste, smell. With these come nerves bearing sen- 
sations of pain, temperature, muscular effort—all kinds of 
sensation which the brain can receive. These nerves are 
the sole sources of knowledge to any animal organism. 
Whatever idea its brain may contain must be built up 
through these nerve impressions. The aggregate of these 
impressions constitute the world as the organism knows it. 
All sensation is related to action. If an organism is not 
to act, it can not feel, and the intensity of its feeling is 
related to its power to act. 
128. Reflex action.—These impressions brought to the 
brain by the sensory nerves represent in some degree the 
facts in the animal’s environment. They teach something 
as to its food or its safety. The power of locomotion is 
characteristic of animals. If they move, their actions must 
depend on the indications carried to the nerve center from 
the outside; if they feed on living organisms, they must 
seek their food; if, as in many cases, other living organ- 
isms prey on them, they must bestir themselves to escape. 
The impulse of hunger on the one hand and of fear on the 
other are elemental. The sensorium receives an impression 
that food exists in a certain direction. At once an impulse 
to motion is sent out from it to the muscles necessary to 
move the body in that direction. In the higher animals 
these movements are more rapid and more exact. This is 
because organs of sense, muscles, nerve fibers, and nerve 
cells are all alike highly specialized. In the star-fish the 
sensation is slow, the muscular response sluggish, but the 
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