CHAPTER IX. 



THE SPECIAL SENSES. 



WE have been considering the body as a collection of 

 organs working together to serve the brain, the mechanism 

 through which the mind operates. 



We have especially studied the muscles as the only means 

 by which the mind manifests itself to the outer world. 



But how much mind would we have if we did not receive 

 something from the outer world ? Read the story of Kaspar 

 Hauser. We are continually getting knowledge of the outer 

 world and of the condition of our own bodies through the 

 afferent nerves. We may never know fully what conscious- 

 ness and thought are, but we can understand that to the 

 brain are continually streaming nerve impulses that convey 

 messages which the brain more or less completely interprets. * 



These incoming currents pass along myriads of nerve 

 fibers. But the nerve fibers are all essentially alike. And 

 the kinds of sensations that these currents arouse in the 

 brain are but few. It is difficult to classify the senses, 

 but it will serve our convenience to divide them into two 

 groups. 



General Sensations and Special Senses. In distinc- 

 tion from the special senses, sight, hearing, etc., are the 

 general sensations already referred to, such as hunger, thirst, 

 fatigue, nausea, satiety, -faintness, etc. They are often called 

 " common sensations ; " and Martin designates them as " sen- 

 sations which we do not mentally attribute to the properties 

 of external objects, but to the conditions of our own bodies." 



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