622 MANUAL OF PHYSIOLOGY. 



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 anterior 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 re- 

 mains 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 inability of the impulses to affect cells other 

 than those in their immediate neighborhood, and the relation 

 between the strength of stimulus and the effect would in this 

 manner be easy of explanation. 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 

 phenomena observed even in the reflex action of the spinal cord 

 and the various modifications it can undergo with varying con- 

 ditions. 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. 

 But these paths have to be made passable by use ; the oftener an 

 impulse traverses a given route the more adapted such a route 

 becomes for future traffic. Thus, by practice or the habit of using 

 certain sets of muscles, we constantly freshen certain channels of 



