106 PHYSIOLOGY CHAP. 



accompany the movement. In our habitual movements we are 

 not aware of overcoming resistance, so long as it is confined to 

 the weight of our limbs. But this does not prevent us from 

 regulating the impulses in voluntary acts, so that they perfectly 

 fulfil their purpose. In regard to the inner vation of the eye- 

 muscles Mach remarks : " Thanks to the organic arrangement and 

 long practice we straightway employ the innervation necessary 

 to fixate any object, of which the image falls upon our retina. 

 Innervation is only disturbed when the external motor forces 

 are not associated with the voluntarily measured innervation." 



It is a matter of common knowledge that the sensations 

 originally present in our acts become less and less vivid with 

 practice, till at last, as they pass into the region of the unconscious, 

 they become mechanical or automatic, as they are usually 

 termed (an ambiguous and unfortunate expression). So that 

 the absence of any clearly perceived sensation of the act of 

 innervation is not sufficient to justify the statement that it was 

 not originally more or less conscious. Such are the delicate 

 mechanical movements by which the artist performs a musical 

 piece on different instruments, as contrasted with the long and 

 tedious practice required before the piano or violin can be 

 mastered. 



Again, while it is fully proved that the motor disturbances 

 in ataxy produced by disease of the dorsal roots are due exclusively 

 to the diminution or loss of the muscular sense, it would be a 

 bold assertion to declare that all cases of disturbance of voluntary 

 motility can be explained without the assumption of the inner- 

 vation sense. This would lead, as Treves pointed out, to the 

 conclusion that we can never foresee the external consequences of 

 our voluntary acts, and never avail ourselves of the most favour- 

 able conditions, in order to reduce the sense of effort to its lowest 

 degree. G. E. Miiller and Schumann who deny the sense of 

 innervation speak of a voluntary adaptation to resistance which 

 they attribute to the tendency of motor and sensory activities 

 of certain intensities and rhythm to become automatic by habit. 

 This differs little, as Treves rightly points out, from the idea of 

 an education of the impulse and accompanying conscious and 

 primitive gradation of innervation, which these authors expressly 

 denied. 



The logical conclusion from the whole of this discussion is 

 that voluntary acts are normally regulated by sensations of 

 peripheral origin, which we have considered under the head of 

 muscular sensations, and by those of central origin, which are 

 known as innervation sensations. 



VIII. In the last chapter when discussing tactile or pressure 

 sensibility we were unable to bring out its full importance from 

 the psychological point of view, because the perceptions and ideas 



