382 THE POPULAR SCIENCE MONTHLY. 



brain, and that the sensations excited through these organs con- 

 stitute the raw material of the mental life — touch, hearing and 

 sight being recognized as par excellence intellectual senses. Now, 

 it is a most significant fact, from the point of view of my 

 third thesis, that the activities of these three senses in particular 

 involve muscular co-operation as an essential accessory ; and the 

 profound relations which exist between many of the mental pro- 

 cesses and muscular action are at least adumbrated in certain 

 experimental observations by Wundt upon the eye. He has 

 shown, for example, that vertical distances appear greater than 

 equal horizontal distances in the proportion of 4'8 to 4, and that 

 the same proportion exists between the muscular forces which 

 move the eye vertically and those which move it horizontally ; 

 that the minimum of movement of the eye capable of exciting 

 consciousness of contraction and the smallest perceptible dis- 

 tance are in exact agreement, both answering to an angle of one 

 sixtieth of a degree ; that we are able to distinguish a difference 

 in length of two lines if it amount to one fiftieth of the entire 

 length of the shorter one — the difference in movement of the 

 eyes in this case being also one fiftieth of their entire linear 

 movement.* 



These relations can not be mere coincidences. Ideas of the 

 size and distance of objects are also attributable in part to the 

 degree of muscular action involved ; for the nearer an object to 

 the eye, the greater the muscular exertion required in converging 

 the axes of the balls upon the object, and the greater the tax upon 

 the muscles of accommodation ; and it is not the visual sensation 

 alone which gives the idea of distance, although the degree of 

 distinctness, no doubt, has a marked influence, but the muscular 

 sensations excited by the movements of accommodation and con- 

 vergence must also contribute to the result. A mere allusion to 

 the immense importance of visual perceptions in our mental fur- 

 nishings will sufficiently indicate the bearings of these facts on 

 the relations of muscular activity to mental activity and growth. 

 To the significance of the muscles as organs of the muscular 

 sense must then be added that which is due to the existence of a 

 muscular element in other sense-organs. 



Since movements, no less than sensations, play a conspicuous 

 part in the acquisition of knowledge of the external world, it fol- 

 lows that ideas are a revival of ideal movements as well as of 

 ideal sensations. My fourth thesis is, therefore, that ideas have 

 no special separate centers in the brain, but result from the excita- 

 tion of those areas which have taken part in the original acquisi- 

 tion of ideas, viz., the sensory and motor centers. These two kinds 

 of centers, with the inhibitory centers and numerous connecting 



* See " German Psychology of To-day," Ribot. 



