270 rnYsmi.oiiv OF MTSCLKS AM> NERVES.. 



Those In Binning to play the piano find it difficult to 

 move individual lingers apart ['mm the others, though 



by practice they soon learn to do this. \Vhene\er 

 :ui intended contraction of a muscle i- accompmied 

 by another unintended and simultaneous, the latter 

 is called a co-relative movement. Such co-relative 

 movements sometimes accompany illness. Stammerer-. 

 for instance, when they speak, twitch the face muscles 

 or even those of the arm. It has also been observe I 

 that in the case of injuries, after blood has lieeii lost 

 from the brain, movements of the injured limbs not 

 voluntarily possible occur involuntarily as co-relative 

 motions. Some co-relative movements are natural in 

 the organism; for instance, when the eye is turned 

 inward, the pupil simultaneously decreases in size, and 

 a contraction of the adjusting muscle occurs, l>y which 

 the eye is enabled to see at a short distance. This 

 co-relative motion has been regarded as a case of the 

 transmission of the excitement from one nerve-fibre to 

 another; but it seems to me that this is incorrect. 

 For there is nothing to show that the excitement 

 originated in one fibre and was then <ran>ferred to 

 other fibres, and it is more simple to assume that the 

 various fibres were excited simultaneously by the will, 

 either because isolated excitement of these fibres sepa- 

 rately i.- really impossible on account of the anatomical 

 structure of the nerve, or because of an insufficient 

 specialisation of the influence of the will, resulting 

 from want of e\eivi>e that is, it is due to un>kilful- 



oti the part of t he will. 



If it is asked how the \obmtary excitement of the 

 nerve-fibres is caused in the nerve-cells, an answer 

 is yet to be sought in physiology. Into the quest ion 



