88 ELECTRO-PHYSIOLOGY CHAP. 



cell, lint from the protoplasmic processes that interlace in a close 

 network. 



The physiological connection of the central motor elements is 

 still, therefore, an open question, while the nature of their relations 

 with the afferent (sensory) fibres may be regarded as histologically 

 established. 



The posterior root-fibres, which must be regarded as processes 

 from the cells of the spinal ganglia, bifurcate (according to Ramon 

 y Cajal and Kolliker), as they enter the cord, into two branches, 

 running respectively upwards and downwards in the mass of the 

 posterior column. These longitudinal fibres give off ("collateral") 

 branches mostly at a right angle which enter the gray matter, 

 and terminate in free dendritic ramifications (Kdlliker's " arborisa- 

 tions"). The similarity of these bouquets to the terminal expansion 

 of the axis-cylinder in the striated muscles of vertebrates at once 

 suggests a similar relation with the motor cells of the spinal cord. 

 Kolliker, indeed, maintains that the twigs of an arborisation (which 

 usually carry a small swelling) are closely interlaced with the 

 ganglion-cells, but never really anastomose with these or their 

 processes. Contiguity is, however, indispensable to transmission 

 of excitation. " Radiation " (Anstralilung} from a free nerve- 

 ending to another that merely lies near it (as in the olfactory 

 giomeruli), or to a cell (as in the simple reflex arc), is an assump- 

 tion the less justifiable since there is no sufficient histological 

 evidence for a hypothesis so divergent from all prevailing views on 

 conductivity of excitation. 



Since each nerve-fibre represents the process of a cell, and 

 forms with the same a physiological unit (neuron, neurodendron), 

 it is intelligible that a nerve-fibre separated from its parent-cell 

 should sooner or later undergo degeneration. Each nerve-cell is 

 thus the " trophic " centre of the outgoing nerve-fibre, and the 

 normal connection between them is one of the most essential 

 conditions of the permanent preservation of conductivity and 

 excitability in the nerve-fibre. In view of the facts above cited, 

 we can hardly doubt that this trophic influence is largely due to 

 the action of the nucleus, as follows, indeed, by analogy with other 

 cells. 



Seeing the extraordinary instability of the central ganglion- 

 cells, the resistance which in peripheral mednUntc<l nerve-fibres 

 preserves the fundamental vital properties (provided the nerves are 



