CHAPTER XVIII 

 THE SENSES 



HITHERTO we have been considering from a purely objective standpoint 

 the organs that compose the body, and their work. The student has 

 been assumed to be in the little world the ' microcosm ' of organiza- 

 tion 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 recognize 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 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., 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 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 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 



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