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 of pressure 

 and temperature. The so-called temperature sensations are 

 themselves divisible into two groups of quite distinctive quality, 

 sensations of warmth and sensations of cold. The power of appre- 

 ciating 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., can certainly be no longer lumped 

 together in the omnium gatherum of ' common sensibility/ They 

 are more appropriately regarded as separate senses subserved by 

 special nerves, and perhaps connected with definite centres. In 

 the development of a simple sensation we may distinguish three 

 stages : the stimulation of a peripheral end-organ, the propaga- 

 tion 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 

 media of the eyeball, see it focussed on the retina, lead off the 

 current of action set up in that membrane, which, doubtless, gives 

 token of the passage of nervous impulses into and up the optic nerve. 

 We can even follow the nervous impulses to a definite portion of the 

 cortex of the occipital lobe, and determine that if this is removed 

 no sensation of sight will result from any excitation of retina or optic 



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