260 Dr. E. Yung on the E: 



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a fortiori, always to be so by persons wbo have not learned to 

 see and feel — that is to say, who do not know how to check 

 by one another the data furnished by their sense-organs. 



In fact we cannot scrutinize scientifically the mysteries of 

 the external world except by the intermediation of the organs 

 of the senses, and we only know of them what the latter con- 

 vey to us. It is consequently of the first importance to us 

 that these organs should not deceive us. Nevertheless two 

 kinds of error are frequent, namely illusion and hallucination. 



To make clear what is to follow, we may briefly recapitulate 

 in what that complex phenomenon that we call a sensation 

 actually consists. In every sensation we may distinguish 

 three phases: — 



1. The impression, which consists in the direct or indirect 

 contact of the object with the receptive apparatus, or the peri- 

 pheric extremity of our sensitive nerves. In order to fix our 

 ideas let us assume (and this is in accordance with the recent 

 data of physiology) that from the contact in question there 

 results a movement, a shock, a vibration, which modifies the 

 receptive apparatus. 



2. The transmission of the impression (that is to say, of this 

 movement, or vibration) through special organic conductors, 

 the nervous tubes. 



3. Perception, which is produced for the observer by a 

 movement of reaction effected in the cellular nervous mass. 

 Perception is conscious or unconscious, according as the move- 

 ment has been transmitted to the cerebral cells, or only to 

 those of the spinal marrow. 



The nerves of special sense are unfitted to convey other 

 impressions than those for which they have been specialized. 

 Any mechanical, physical, or chemical disturbance of the 

 optic nerve, for example, will show itself not by a perception 

 of pain, but by a luminous perception. Under similar circum- 

 stances the auditory nerve will transmit a sonorous sensation 

 &c. A special nerve may therefore transmit a sensation inde- 

 pendently of the agent which normally excites it. In the 

 most complete darkness we may experience luminous sensa- 

 tions; for this, as every one knows, it is sufficient to compress 

 rather strongly the globe of the eye, and thus to agitate the 

 retina. 



We do not know the localities in the brain in which the move- 

 ments communicated by luminous or sonorous vibrations be- 

 come transformed into conscious sensations of light or sound; 

 but it is probable that all the cells of the brain are not in- 

 differently fitted for this purpose. It is very probable that 

 there is in this case a division of physiological labour, in 



