32 AN INTRODUCTION TO SCIENCE 



tions of smell and taste are limited to the orbits of the 

 molecules of odorous and sapid substances. Those which 

 stimulate the organs of vision and hearing have an unlimited 

 progression in space, the waves of light being ' ' up and 

 down ' ' vibrations, which follow one another at the rate of 

 hundreds of billions to the second, whereas sound is con- 

 veyed in the form of "to and fro " pulsations, which are not 

 appreciated by the ear if they are more rapid than 40,000 to 

 the second. An analysis of the several kinds of stimuli 

 which affect the sense-organs of the skin would take up more 

 space than we have to spare. 



Light was first thrown upon the mode of working of the 

 cortex by the discovery that by stimulating it with an electric 

 current definite movements can be invariably evoked. This 

 is commonly expressed by saying that it contains centres of 

 movement. The discovery of its allocation among the 

 several senses was made later. The question of the relation 

 as cause and effect of the sensations which are received in 

 the cortex and the movements which are originated by it is 

 one of great complexity. Nevertheless, taking the most 

 general view of the cortex as the organ of the mind, we may 

 safely say that it is the nerve-tissue in which sensations are 

 received and become conscious perceptions, and from which 

 nerve-impulses for the evoking of muscular actions are 

 despatched. In sleep and some other unconscious condi- 

 tions these two terminals are placed in connection ; sensa- 

 tions flow over into movement by reflex action. During the 

 waking state the mind intervenes. Sensations become per- 

 ceptions, and the mind, taking cognizance of these presenta- 

 tions of sense, decides whether they shall flow over at once 

 into action or whether they shall be stored as memories for 

 future use ; whether, flowing over with very little reinforce- 

 ment, they shall produce an obviously correlated action, or 

 whether, by combination with dormant perceptions, they shall 

 be expressed in a sequence of movements which seems, 

 until it is minutely analysed, to be too complicated to result 



