EXPERIMENTAL PSYCHOLOGY 



being altogether wanting in the white substance which 

 makes up the chief mass of the brain. Even in the 

 gray matter, though sometimes thickly distributed, the 

 ganglion cells are never in actual contact one with an- 

 other ; they always lie embedded in intercellular tissues, 

 which came to be known, following Virchow, as the 

 neuroglia. 



Each ganglion cell was seen to be irregular in con- 

 tour, and to have jutting out from it two sets of mi- 

 nute fibres, one set relatively short, indefinitely numer- 

 ous, and branching in every direction; the other set 

 limited in number, sometimes even single, and starting 

 out directly from the cell as if bent on a longer journey. 

 The numerous filaments came to be known as proto- 

 plasmic processes; the other fibre was named, after its 

 discoverer, the axis cylinder of Deiters. It was a nat- 

 ural inference, though not clearly demonstrable in the 

 sections, that these filamentous processes are the con- 

 necting links between the different nerve cells and also 

 the channels of communication between nerve cells and 

 the periphery of the body. The white substance of 

 brain and cord, apparently, is made up of such con- 

 necting fibres, thus bringing the different ganglion cells 

 everywhere into communication one with another. 



In the attempt to trace the connecting nerve tracts 

 through this white substance by either macroscopical or 

 microscopical methods, most important aid is given by 

 a method originated by Waller in 1852. Earlier than 

 that, in 1839, Nasse had discovered that a severed 

 nerve cord degenerates in its peripheral portions. 

 Waller discovered that every nerve fibre, sensory or 

 motor, has a nerve cell to or from which it leads, which 

 TOL. XT. 19 279 



