REFLEXES, IXSTIXCT, AND REASON 427 



In tlie more liiglily organized animals certain colls from the 

 primitive external layer or ectol)last of tlie omhryo are early 

 set apart to record the relations of the creature to its envircn- 

 ment. These cells are highly s])ecializ(Hl, and while some of 

 them are highly sensitive, others are adapted for carrying 

 c: transmitting the stimuli received by the sensitive cells, and 

 still others have tiie function of receiving 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 centers. 

 The nerves are of two classes. The one class, called sensory 

 nerves, extends from the skin or other organ of sensation to 

 the nerve center. The nerves of the oilier class, motor nerves, 

 carry impulses to motion. 



The brain or other nerve center sits in darkness surroimded 

 by protecting tissues or a protecting ])ox of bone. To this bndn, 

 nerve center, or sensorium come the nerves fom ill j)a:*tr> 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 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 o/ganism. 

 Whatever idea its brain may contain must be built up through 

 these nerve impressions. The aggregate of these impressions 

 constitutes 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. 



These impressions brought to the brain by the sensory 

 nerves represent in some degree the facts in the animal's en- 

 vironment. They teach something as to its food or its safety. 

 The power of locomotion is characteristic of animals. If they 

 move, their actions must dei)(Mid on the indications carrieil to 

 the nerve center from the outside; if they feed on living orga.i- 

 isms, they must seek their food; if, as in many cases, other 

 hving organisms ivrey on them, tl.cy must bestir themselves 

 to escape. Tlie im])ulsr of hunger on tlie one hand and of fear 

 on the other are elemental. The sensorium receives an im- 

 pression that f(>>d exists in r. certain direction. At once an 

 impulse to motion is .sent out from it to the muscle neces^sary 

 t:;) move the body in that direction. In the higher animals 



