CENTRAL NERVOUS SYSTEM. 621 



produces outgoing impulses in the dorsal nerve-roots except when the stimulus 

 is applied to the neurons which are outgrowths of the cells of the dorsal ganglia. 



Arrangement in the Central System. As will be shown later on, there 

 is reason to picture the passage of the nerve-impulse through the central 

 system as accomplished by a series of relays in which each cell-body is roused 

 to discharge its own impulse as the consequence of an impulse received from 

 some other cell. 



When therefore an impulse is brought by one neuron to a cell-body, and 

 passed on by way of it to another neuron which is a part of the stimulated cell, 

 there is no escape from the conclusion that, if in this case the cell-body is 

 physiologically significant, it rather originates the impulse which traverses 

 the second neuron than acts merely as the conductor for it. 



This is suggested by the changes caused in the cell-body as the result of 

 stimulating it. At the same time there is an appreciable delay (0.036 second) 

 in the passage of the nerve-impulse through the cell -body in the case of those 

 cells which form the spinal ganglion. 1 



Double Pathways. If the view is correct, that in passing through the 

 spinal ganglion the impulse enters the cell-body, then the nerve-impulse passes 

 to and fro along the common stem which joins the cell-body with the two 

 neurons (vide Fig. 148). In such a case, the impulse going toward the cell 

 must travel either through the entire stem, or through a part of it only. This 

 stem is conceived as homologous with the bases of the two neurons which 

 originally arose from the dineuric cell, thus morphologically representing a 

 double pathway, although in the mature cell there is, from the histological 

 side, absolutely no trace of this duplicity. 



The same arrangement must exist in the case of cells like those represented 

 in Figure 152, in which the neuron arises from the base of a dendron at some 



FIG. 152. Showing the relations between the terminal branches of the dendrons (D) and of the 

 neurons (N') of the optic fibres where they come together in the superficial layer of the optic lobe of 

 the chick ; also showing the origin of the neuron (N) from a dendron (van Gehuchten). 



distance from the cell-body, and in which nerve-impulses arriving over the 

 dendron and leaving by the neuron must follow the portion of the cell-branch 

 which is common to both, passing along it first in one direction and then in 

 the other. It appears not improbable, therefore, that some outgrowths of the 

 cell-body which morphologically are simple, really contain more than one 

 physiological pathway. 



1 Gad and Joseph : Archiv fur Anatomic und Physiologic, 1889. 



