ARRANGEMENT OF STRUCTURAL ELEMENTS. 227 



From these main branches are given off smaller ones, 

 which penetrate the grey matter. These collateral 

 branches, as they are called, may be very numerous, 

 and we desire to know whether in each case the impulse 

 that comes in by the main stem is distributed in the cord 

 through all these branching pathways. Certainly diffe- 

 rent reactions follow the stimuli which must arrive at 

 the cord by the same fibres, and it is difficult, therefore, 

 on the one hand, to see how there should be a choice of 

 pathways open to the impulse ; and, on the other hand, 

 how, if such choice does not exist, successive reactions 

 should not be more alike. 



Possibly some basis for an explanation can be found 

 in the fact that the series of cells to which these colla- 

 terals deliver the impulse may not on the several occa- 

 sions be in the same condition, or that the impulse may 

 be here and there blocked on reaching the interval 

 separating the cell branches, and so at different times 

 different groups of cells respond with variations both in 

 order and intensity. The following example may serve 

 as an illustration of this hypothesis. There is reason to 

 think that a given branch or collateral may terminate in 

 the neighbourhood of several cells in such a manner that 

 it is able to deliver an impulse to all of them. If the 

 next step in the physiological process depends upon the 

 condition of the receiving cells, the greater their num- 

 ber and the more open they are to stimuli from other 

 sources the more diverse would be the responses. It 

 thus happens that the machine-like reactions of the 

 nervous system would be best carried out by a small 

 number of nerve elements holding a highly constant 

 relation to one another and acted on by a minimal 

 number of modifying stimuli, while a larger number of 

 elements and more numerous avenues for afferent im- 

 pulses would permit of greater variations in the reac- 



