220 NERVOUS SYSTEM OF VERTEBRATES. 



are widely scattered through the cord, some of them may receive 

 impulses from one source, some from another. Their neurites 

 enter the lateral tracts of the cord and run forward or backward 

 for a longer or shorter distance, ending in relation with motor 

 cells. There has been observed anatomically no order or system 

 about these cells and their fibers. They seem to offer opportunities 

 for the wide spread of all kinds of impulses from segment to 

 segment of the cord. The heterolateral cells add the possibility 

 of impulses reaching the opposite side of the cord. Whether 

 any regularity in the relations of these neurones is constant in the 

 species and is inherited from generation to generation is unknown. 

 It seems more probable that these neurones offer a relatively 

 indifferent material in the embryo, providing for the diffusion 

 of impulses from segment to segment and from one side to the 

 other, and that definite paths for impulses are set up chiefly as the 

 result of the experience of the individual. If impulses traveling 

 through certain cells and fibers serve for the performance of an 

 act efficiently, ihe success attending the act will lead the young 

 child to repeat the attempt. The repetition will render the 

 impulse-pathway more easy for succeeding impulses to follow. 

 Thus a habitual pathway is set up, while other possible pathways 

 offered by the indifferent tissue of the embryonic nervous system 

 become after a time unavailable through lack of use. So in the 

 early life of the child it is probable that certain orderly sets of 

 connections are established by way of these indifferent tract cells 

 by means of which complex reflexes are carried out, and the actions 

 of two divisions of the nervous system correlated. It is indeed 

 just this development of orderly connections in the central nervous 

 system which is going on during and as a result of the aimless 

 movements of the infant in the first few months of its life. 



In the brain the same kind of processes have been at work but 

 the indifferent material is proportionately greater in amount 

 and in certain regions special brain centers have been formed. 

 In order to understand these it is necessary to look at them from 

 the genetic point of view, especially as regards their relations to 

 the functional divisions. Strictly speaking, no fast line of division 

 can be drawn between the elements of the functional divisions 



