270 . niYSIOLOGY OF MUSCLES AND KERVES. 



Those beginning to play the piano find it difficult to 

 move individual fingers apart from the others, though 

 by practice they soon learn to do this. Whenever 

 an intended contraction of a muscle is accompanied 

 by another unintended and simultaneous, the latter 

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

 movements sometimes accompany illness. Stammerers, 

 for instance, when they speak, twitch the face muscles 

 or even those of the arm. It has also been observed 

 that in the case of injuries, after blood has been 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, by 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 transferred 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 is 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 exercise — that is, it is due to unskilful- 

 ness on the part of the will. 



If it is asked how the voluntary excitement of the 

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

 is yet to be sought in physiology. Into the question 



