CHAPTER V 



The Senses and the Nervous System 



ALL animals except the very lowest have nervous systems. 

 In all, the principal elements are the same. There are sense- 

 organs, which receive impressions from the outside world. 

 There are nerves, over which the sense-organs send reports. 

 There is a center, or group of centers — brain, spinal cord, 

 ganglia— which receive these reports. And there are other 

 nerves over which the centers send instructions to the muscles 

 as to what action to take. 



THE SENSES 



It is the popular conception that man has five senses. This, 

 like many other popular conceptions, is not true. It is based 

 on the fact that man has only five clearly visible sense-organs: 

 eyes, ears, skin, mouth, and nose. But in addition to the popu- 

 lar five senses of sight, hearing, touch, taste, and smell, he 

 has at least four others: a sense of equilibrium, which keeps 

 him right side up and going straight j of "kinesthesia," which 

 tells him whether his hand is at his side or above his head; 

 of heat and coldj and of pain. These four senses are not 

 generally recognized, partly because they seem to act through 

 the organs provided for the other senses instead of through 

 organs of their own, and partly because they act so auto- 

 matically that we are unaware of them. 



If fish had reached the same level of mental development 



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