622 MANUAL OF PHYSIOLOGY. 



direction, have the effect of inhibiting the action of its reflecting 

 cells. 



The theory of reflex action lies at the bottom of all nervous 

 activities, and it is therefore useful to attempt to work out the 

 details of the mechanisms by means of which it is carried on. 

 The simplest scheme of the channels traversed by the impulses is 

 given in the diagram (Fig. 241), in which the arrow-heads show 

 the direction of the afferent impulse passing along the posterior 

 root of the cord to reach the cell in the posterior gray column, 

 thence through the fine gray network, it passes to a cell in the 

 anterior column, to reach the efferent fibre, and through the an- 

 terior motor root of the nerve on its way to the muscle. It has 

 been suggested that the impulse meets with considerable resistance 

 in passing through the protoplasm of the cells, and that owing to 

 this resistance the effect of a slight stimulus remains localized, 

 while a more powerful irritation gives rise to impulses that can 

 overcome the resistance, and thus spread to a greater number of 

 cells, even reaching those situated in a remote district. Thus the 

 coordination in the cord would be simply dependent on the ina- 

 bility of the impulses to affect cells other than those in their im- 

 mediate neighborhood, and the relation between the strength of 

 stimulus and the effect would in this manner be easy of explana- 

 tion. It has also been suggested that this resistance is increased 

 by impulses arriving at the cells from a different direction, and 

 the checking or inhibitory action of the higher centres, or intense 

 peripheral excitation of another part, impedes the spreading of 

 the impulses to be reflected, and a lesser result is obtained. 



But this view of resistance to and interference with the trans- 

 mission of impulses in the nerve-cells hardly explains all the phe- 

 nomena observed even in the reflex action of the spinal cord and 

 the various modifications it can undergo with varying conditions. 

 It will, however, help us in formulating the mechanism if we 

 suppose the resistance in the gray part of the nerve centres to 

 be much greater than in the ordinary nerve channels, and that 

 throughout it the ways are so infinitely numerous that we can 

 imagine every individual nerve-cell to be in communication with 

 every other nerve-cell by some path possible of being traversed. 



