54 CLASSIFICATION 



imbedded in the deeper layer of the skin (dermis) are to be seen 

 rows of touch-corpuscles. The sense of touch proper gives us 

 information about bodies which actually come into contact with 

 the skin, and how extensive such information may be is seen in 

 the case of blind persons. The sequence of events before we feel 

 a sensation when, say, a finger-tip touches any object, is some- 

 what as follows: — The first effect of contact is to cause some 

 kind of to-and-fro movement in the minute particles (molecules) 

 of which the touch -corpuscles are made up, or, to use more 

 technical language, a " molecular vibration ". The corpuscles 

 may be called the " end -organs of touch ", because they form 

 the external end of the set of structures concerned with touch. 

 Next follows a " nerve impulse " (which is another kind of 

 molecular vibration) along the sensory nerve to the spinal cord 

 and thence to the brain. Lastly, a part of the grey matter 

 composing the external coat or cortex of a brain hemisphere 

 is affected, a third variety of molecular vibration being set up in 

 it. Then, and not till then, is a sensation of touch experienced, 

 by which we know that something has happened in th^ finger-tip. 

 What that something may be is judged by comparison with past 

 happenings of a like nature. The sequence of events here very 

 roughly described for touch applies, mutatis mutandis, to any 

 other sense, and therefore will not be spoken of again; but it 

 is important to bear in mind that all the various sensations — 

 touch, taste, smell, hearing, and sight — are, so to speak, manu- 

 factured in the brain, and it is only as the result of long practice 

 that we learn to recognize them as resulting from changes set up 

 in end-organs by different external agents (stimuli), enabling us 

 to form judgments about what is happening outside the body 

 altogether. 



The skin is not merely concerned with touch proper, but 

 is also the organ of the temperature-sense, by which we learn 

 something about the condition of external bodies as regards 

 their condition with reference to heat or cold. 



The Sense of Taste (fig. 30). — The object of this sense being 

 to give us information about food, it is not surprising to find its 

 end-organs limited to the lining of the mouth-cavity. The most 

 obvious of these are contained in the mucous membrane of the 

 tongue, upon the upper side of which organ, at the back, are to be 

 noted a number of small projections or papillce, in the sides of 



