12 THEORETICAL BIOLOGY 



muscular sensations are feeble in comparison with these sensa- 

 tions in the joints ; consequently, we feel the finger-move- 

 ments within the finger itself, although the muscles that 

 produce the movements lie in the forearm.. The muscular 

 sensations become distinct only when we clench the fist ; but, 

 in that case, there are no sensations of direction. 



This sort of thing indicates that the direction-signs are not 

 indissolubly bound up with muscular sensations, but with 

 the impulses that initiate movement. I see further evidence 

 of this in the fact that when we swing our arms to and fro 

 there is a summation of the sensations in the two limbs, but 

 a subtraction of the direction-signs ; and this leads us to the 

 fact that we can distinguish two different kinds of innerva- 

 tion — one for muscular action and one for movement. When 

 we seize an object, we are innervating a muscular action ; 

 when we move it to another place, we are innervating a move- 

 ment. In somewhat more complicated movements which 

 we perform with ease, we should be greatly embarrassed if 

 we consciously ordered the individual muscular actions to 

 follow one after the other. It is easy for us to write the 

 numeral 2 on a slate, with our eyes shut ; but we have no 

 idea what muscular action we set a-going in order to do it. 



It was Helmholtz who pointed out that we must have 

 innervation-sensations for the impulses belonging to a central 

 sense-organ. As soon as we bring them into relation with 

 the impulses, the inner direction-signs appear in quite a new 

 light. We innervate muscular actions either by a single 

 impulse, as in clenching the fist (in which case we ignore the 

 direction-signs), or else we innervate them by an impulse- 

 sequence, as in writing (in which case we are conscious only 

 of the direction-signs). 



The following experiment gives us some information as to 

 the role played by the direction-signs in innervation. First 

 with the right hand and then with the left write slowly in the 



