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 

 knowledge we thus obtain is based upon sensations of various 

 kinds, all of which are carried to the centers by afferent 

 fibers. Such sensations may be what are termed (A) Com- 

 mon, or (B) Special, including (i) Touch, (2) Smell, (3) 

 'Sight, (4) Taste f (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 impression. 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 trans- 

 mitted 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 con- 

 cerned 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 almost 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 over- 

 come tactile sensibility, and that very frequently tactile sen- 



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