CHAPTER XIII. 

 THE SENSES. 



HITHERTO we have been considering from a purely objective stand- 

 point the organs that compose the body, and their work. The 

 student has been assumed to be in the little world * the microcosm ' 

 of organization which he has been studying, but not of it. He 

 has listened to the sounds of the heart, seen its contraction, felt 

 it hardening under his fingers ; but we have not inquired as to the 

 meaning or the mechanism of this hearing, seeing, and feeling. We 

 have now to recognise that all our knowledge of external things 

 comes to us by the channels of the senses, and, like the light that 

 falls through coloured windows on the floor of a church, is tinged, 

 and perhaps distorted, in the act of reaching us. 



The Senses in General. The old and orthodox enumeration 

 of ' the five senses ' of sight, hearing, touch, taste and smell 

 must be augmented by at least two more, the senses oi 

 pressure and temperature. The power of appreciating the 

 amount of a muscular effort ; the power of localizing the 

 various portions of the body in space ; the sensations of 

 pain, tickling, itching, hunger, and thirst; the sensations 

 accompanying the generative act, etc., have also been looked 

 upon by some as separate senses subserved by special nerves 

 and connected with definite centres. In the development 

 of a simple sensation we may distinguish three stages : the 

 stimulation of a peripheral end-organ, the propagation of 

 the impulses thus set up along an afferent nerve, and their 

 reception and elaboration in a central organ. 



We do not know in what manner a series of transverse vibrations 

 in the ether when it falls upon the eye, or a series of longitudinal 

 vibrations in the air when it strikes the ear, excites a sensation of 

 light or sound. We can trace the ray of light through the refractive 



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