Bauhaus-Universität Weimar

84 
Walter W. Davis, 
tion in connection with Wissler’s theory of the diffusion of the mot0 
impulse. For if there is a diffusion of impulse to symmetrical muscles 
the traveling of the impulse to the muscles will wear the paths deeper 
and deeper and so make a clear way for the impulse when it is voluntarily 
sent this way at the final test. 
The fact that S.’s grip for the right hand had weakened by an accident 
directly affecting only the left raises the question whether the reverse of 
cross-education is true; whether a mal-condition of the governing ap¬ 
paratus for the one side is generally transferred to the other side of the 
brain, and so causes a weakening of the other side of the body. Speak¬ 
ing theoretically it seems reasonable to suppose such to be the case. It 
would be interesting in this connection to study pathological cases where, 
for any reason, one member has fallen into disuse, to determine whether 
the symmetrical member has been weakened. The difficulty would be to 
find cases that had not used the well arm more in consequence of the 
other’s loss of power. Such conditions would of course nullify any 
results secured. 
It must not be understood that any explanation of the causes of cross¬ 
education can be applied to all individuals or even absolutely to any in¬ 
dividual. Even provided that all the conditions of practice could be 
made precisely the same, no two individuals would give the same results 
in development. 
Again, any results of practice are modified by the kind of test that is 
made. Tests of endurance differ very materially from tests of strength. 
The development of the factors of practice may be required in entirely 
different proportions in the two kinds of tests. The summary we present 
here holds only for tests of strength and, even more specifically, only 
for practice with the dynamometer. 
The central effects of practice are emphasized as most prominent in this 
investigation. It is quite probable that the peripheral effects are more 
necessary of development in tests of endurance than in tests of strength- 
In the same test we believe it quite possible that the central and periph 
eral effects may vary in different individuals. 
This investigation gives prominence to the following factors of cross 
education. . -s 
(a) Volition or will power.—-When developed for one act it 
efficacious for similar acts done by symmetrical or closely re 
muscles ; in a lesser degree it is possibly developed for all acts. 
{b') Neuro-muscular coordination or “ knack."—When developed 
one set of muscles it is transferred across to the symmetrical set on 
other side of the body.
        

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