Instincts, Habits, and Adaptations 153 



ing sense-impressions and of translating them into impulses of 

 motion. The nerve-cells are receivers of impressions. These 

 are gathered together in nerve-masses or ganglia, the largest 

 of these being known as the brain, the ganglia in general being 

 known as nerve-centres. The nerves are of two classes. The 

 one class, called sensory nerves, extends from the skin or other 

 organ of sensation to the nerve-centre. The nerves of the other 

 class, motor nerves, carry impulses to motion. 



The Brain, or Sensorium. — The brain or other nerve-centre 

 sits in darkness, surrounded by a bony protecting box. To this 

 main nerve-centre, or seiisoriuiii, 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, and smell. With these 

 come nerves bearing sensations 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 cannot feel, 

 and the intensity of its feeling is related to its power to act. 



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 indi- 

 cations carried to the nerve-centre from the outside ; if they feed 

 on living organisms, they must seek their food; if, as in many 

 cases, other Hving organisms 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 sease, muscles, nerve-fibres, and the ner\'e- 

 cells are all alike highly speciaUzed. In the fish the sensation 

 is slow, the muscular response sluggish, but the method remains 

 the same. This is simple reflex action, an impulse from the 



