388 ANIMAL STUDIES 



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. 



300. 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. 



301. 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 



