CHAPTER XII. 



THE SENSES. 



IT is evident from preceding remarks that it is through the 

 intervention of the nervous system that we have a "sense" of 

 existence, of the existence and condition of different parts of 

 our bodies and of our relations to the external world. The knowl- 

 edge we thus obtain is based upon sensations of various kinds, 

 all of which are carried to the centers by afferent fibers. Such 

 sensations maybe what are termed (A) Common, or (B) Special, 

 including (i) Touch, (2) Smell, (3) Sight, (4) Taste, (5) Hearing. 

 It is to be remembered that the seat of sensation is in the brain, 

 and not in any organ which primarily receives or conveys the im- 

 pression. We do not in reality see with the eye or hear with the 

 ear; these are only complex organs so arranged that rays of light 

 or sound waves produce upon them such impressions as, when 

 transmitted to the sensorium, will give rise to the sensations of 

 sight or hearing. 



(A) COMMON SENSATIONS. 



As regards the uses of the fibers conveying impressions which 

 result in these sensations, they (unless it be those concerned with 

 tactile impressions) are distinct from those of special sense. 

 That is to say, the fibers of the olfactory, optic, gustatory and 

 auditory nerves do not convey general impressions; but it is al- 

 most certain that fibers conveying tactile impressions convey 

 also painful impressions and the sensation of pain is taken as 

 typical of common sensations. It is known that very painful 

 impressions sometimes overcome tactile sensibility, and that 



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