10 WILLIAM JAMES ON 



conditioned by the actual fact of muscular contraction. That it is dependent on 

 centripetal impressions generated by the act of contraction, I have already endeavored to 

 show. When the paths of the centripetal impressions, or the cerebral centres of the same, 

 . are destroyed, there is no vestige of a muscular sense. That the central organs for the 

 apprehension of the impressions originating from muscular contraction, are different from 

 those which send out the motor impulse, has already been established. But when Wundt 

 argues that this cannot be so, because then the sensation would always keep pace with the 

 energy of muscular contraction, he overlooks the important factor of the fixation of the 

 respiratory muscles, which is the basis of the general sense of effort in all its varying 

 degrees." 



To these remarks of Terrier's I have nothing to add. Any one may verify them, and 

 they prove conclusively that the consciousness of muscular exertion, being impossible 

 without movement effected somewhere, must be an afferent and not an efferent sensation, 

 a consequence, and not an antecedent of the movement itself. An idea of the amount of 

 muscular exertion requisite to perform a certain movement can consequently be nothing 

 other than an anticipatory image of the movement's sensible effects. 



Driven thus from the body at large, where shall the circumstantial evidence for the 

 feeling of innervation lodge itself? Where but in the muscles of the eye, from which 

 last small retreat it judges itself inexpugnable. And, to say the truth, it may well be 

 excused for its confidence ; for Ferrier alone, so far as I know, has ventured to attack it 

 there, and his attack must be deemed a very weak failure. Nevertheless, that fastness 

 too must fall, and by the lightest of bombardments. But, before trjdng the bombard- 

 ment, let us examine the position with a little care, laying down first a few general prin- 

 ciples about optical vertigo, or illusory appearance of movement in objects. 



We judge that an object moves under two distinct sets of circumstances : 



1. When its image moves on the retina, and we know that the eye is still. 



2. When its image is stationary on the retina, and we know that the eye is moving. 

 In this case we feel that y^e, follow the object. 



In either of these cases a mistaken judgment about the state of the eye will produce 

 optical vertigo. 



If in case 1, we think our eye is still when it is really moving, we shall get a move- 

 ment of the retinal image which we shall judge to be due to a real outward motion of 

 the object. This is what happens after looking at rushing water, or through the windows 

 of a moving railroad car, or after turning on one's heel to giddiness. The eyes, without 

 our intending to move them, go through a series of involuntary rotations, continuing 

 those they were previously obliged to make to keep objects in view. If the objects had 

 been whirling by to our right, our eyes when turned to stationary objects will still move 

 slowly towards the right. The retinal image upon them will then move like that of an 

 object passing to the left. We then try to catch it by voluntarily and rapidly rotating the 

 eyes to the left, when the involuntary impulse again rotates the eyes to the right, 

 continuing the apparent motion, and so the game goes on. 



