MINUTE STRUCTURE OF THE NERVOUS TISSUES 29 



The property involved has been spoken of as a "valve- 

 action" (Fig. 5). 



Most of the processes which spring from nerve-cells 

 are of the branching character which has been described. 

 They are called dendrites or dendrons. But certain proc- 

 esses are found to be of an entirely different order. These 

 can be followed often for long distances from the cells 

 which give rise to them, and are presently found to 

 acquire the sheaths previously mentioned as enveloping 

 the axis-cylinders of nerve-fibers. Just here we reach 

 a most fundamental conclusion as regards the constitu- 

 tion of the nervous system: that each nerve-fiber or, 



Fig. 5. To illustrate a synapse (S), which is the functional 

 junction between the terminal arborizations i. e., fine branchings 

 of the left-hand neuron and certain dendrites of the one to the right. 

 Effects are secured only in the direction of the arrows. 



more accurately, its central core is the outgrowth of a 

 nerve-cell. This defines the relations existing between 

 the gray and the white matter. In the gray matter are 

 the cell-bodies; in the white matter run their long proc- 

 esses, the conductors of energy in the form which we have 

 called nerve-impulses. However far nerve-fibers may 

 stretch from the situation of their presiding cells, their 

 axons are to be recognized as continuous with the proto- 

 plasm of those microscopic elements. The axon in a 

 long fiber may contain many times as much substance as 

 the cell-body in which it originates. 



A question of terminology now arises. If the axis- 

 cylinder is part and parcel of the structural unit which 



