CHAPTER II. 



SENSE ORGANS. 



SECTION I. 

 Introductory. 



A nervous system, as seen, consists essentially of a 

 center and two kinds of fibers, one carrying impres- 

 sions from the external world to the center and pro- 

 ducing changes in consciousness, which we call sensation, 

 the other carrying impulses back from the center to the 

 external world and producing changes of phenomena 

 there. But to make these exchanges more efficient there 

 must be special receptive organs of sensation (these are 

 the sense organs) and special executive organs of the will 

 (these are the muscles'). We have now to do only with 

 the sense organs. We shall speak of the muscles later. 



Relation of Special Sense to General Sensibil- 

 ity. — The sensory fibers of the conscio-voluntary sys- 

 tem terminate peripherally in external sensitive surfaces, 

 and in the lowest animals everywhere alike — i. e., in the 

 skin. This gives only the existence of an external world, 

 but not its properties. This is the case in all the lowest 

 animals, and nothing more. But as we go up the animal 

 scale certain fibers are specialized to give knowledge of 

 certain properties, and certain other fibers to give knowl- 

 edge of other properties. Thus, for example, the fibers 

 of the first pair of cranial nerves are specialized to give 

 us cognizance of odors, and nothing else, and any kind of 

 Stimulation of this nerve will produce a perception of 

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