TRANSMISSIBILITY OF FUXCTIONAL MODIFICATIONS 71 



voluntary actions which are often repeated gradually acquire the 

 character of instinctive actions. The individual movements pertaining 

 to the particular action are no longer each guided by the will, but 

 a single exercise of will is enough to liberate the whole complex 

 action, such as writing, speaking, walking, or the playing of a whole 

 piece of music ; frequently the will-impulse may be absent altogether, 

 and the action be set going simply by an adequate external stimulus, 

 as in the case of sleep-walking, which is observed in fatigued children 

 and soldiers, and in somnambulists. The external stimulus is trans- 

 mitted to the proper group of muscles as unfailingly as in the case 

 of true instincts, and this happens not only in regard to actions which, 

 like walking, are essential to the life of the species, but also in regard 

 to those which have arisen from chance habits or exercises. Often 

 a short practice is sufficient to make an action in this sense instinctive, 

 and the complexity of the instinct-mechanism gained by such practice 

 is often astounding. Under some circumstances a person may play 

 a piece on the piano from the score, and yet be thinking intently 

 of other things, and be quite unconscious of what is played. In 

 the same waj^ it may happen that a person dominated by violent 

 emotion, when trying to free himself from it by reading, may read 

 a whole page, line by line, without understanding in the least what 

 has been read. In the last case it is not directly demonstrable that 

 the reader has made all the complex delicate eye-movements which 

 would be liberated by the sight of the words, but in the case of 

 playing, the listeners can perceive that the piece is correctly played, 

 and thus that the stimulus exercised by each note on the retina 

 of the eye is translated into the complex muscular movement of 

 arm and finger, corresponding both to the pitch and the duration 

 of the note, and to the simultaneousness of several notes. 



In all these cases it is probably not always quite new paths 

 which are established in the brain, but use is made of particular 

 tracks in the innumerable nerve-paths already existing in the nerve- 

 cells (neurons) which are 'more thoroughly trodden' by practice, 

 so that the distribution of the nerve-current takes place more easily 

 along them than along others \ This much-used metaphor does 

 not indicate the actual structural changes which have taken place, 

 but it serves at least to indicate that we have to do with material 

 changes in the ultimate living elements of the nerve-substance (nerve- 

 biophors) whether these changes be in position or in quality. Xow, 



' Thi^, liowever, is by no means intended to cast doubt on the possibility that quite 

 new paths may arise during the individual life, as is made probable by the recent 

 investigations of Apathy, Bethe, and others. 



