ii SENSIBILITY OF THE INTERNAL ORGANS 103 



of weights. If a comparatively long series of tests is made with a 

 rather heavy weight, and the experimenter suddenly has to lift a 

 lighter weight, it will seem excessively light. If, on the contrary, 

 the weight is made heavier, it appears much heavier than it really 

 is. This also shows that we usually predetermine the volitional 

 impulse, and measure it according to previous experience for the 

 weight we are about to lift, and that these errors of judgment 

 depend on the disproportion between the energy employed and 

 the mass actually raised. 



Treves adduces another familiar experience in support of the 

 same point. If in coming down a staircase one step is higher 

 or lower than the rest, we are apt to fall or stumble, because the 

 foot is moving at a rate corresponding to the rhythm of the 

 previous descent, and is not adapted to the unequal step. 



(e) To prove the existence of a sense of innervation, Treves 

 adds some ingenious remarks on the education of the volitional 

 impulse. He points out that the less the niotor impulse (which 

 results directly from the volitional impulse) is sufficient, i.e. 

 adequate to the mechanical task imposed, the greater will be the 

 sensation of effort. The physical basis of this sensation may be 

 numerically expressed by the reciprocal value of the product of 

 the resistance into the square of the velocity imparted. But if 

 the sense of effort is mainly based on the degree of tension given 

 to the muscle, and on the time this tension lasts till the desired 

 aim is reached, it follows that the education of the volitional 

 impulse which serves to reduce the sense of effort to its minimum 

 must be the result of previous experience : this cannot be explained 

 unless we admit the sense of innervation, by which we are able to 

 graduate the volitional impulse, and with it the motor impulse, 

 and adapt it to the desired end. This idea of Treves agrees with 

 Mach's proposition : that what we term will is no more than the 

 sum of those states associated with the previsions of the effect 

 that precede a movement, of which we are partially conscious. 

 This sum must be something more than the mere mnemonic 

 ideation or representation of movement, and something other 

 than the sense of effort that accompanies the actual move- 

 ment ; both these in fact are not seldom opposed to the 

 mechanical effects foreseen and actually obtained, as for instance 

 in Delboeufs experiment quoted above, in the so-called "cramps" 

 of different professions, the ataxic movements of diabetes, and 

 the like. The sum of the central conditions antecedent to 

 the movement must, as Mach points out, form the content 

 of the sense of innervation, directly perceived as such, and 

 thus constitute the initial factor in every voluntary movement, 

 even when by long practice it has passed into the region of the 

 unconscious. 



Mach, whose definition of a voluntary act has just been cited, 



