84 ELECTRO-PHYSIOLOGY 



lumbar swelling, is then absolutely ineffective. If these experi- 

 ments establish the presence of directly excitable motor elements 

 in the ventral half of the frog's spinal cord, it is on the other 

 hand undeniable that there are important differences as regards 

 both excitatory conditions and the nature and mode of the 

 reaction, according as the muscle is stimulated by excitation of its 

 motor nerve, or through the spinal cord. The comparative in- 

 efficacy of mechanical and of single electrical stimuli, as well 

 as the total failure of chemical excitation, must be remembered. 

 Nor is this surprising, since the motor fibres of the cord 

 are not, like the peripheral motor nerves, in direct connection 

 with the muscle, but are interrrupted by ganglion -cells (as 

 proved, more especially by the observations of Birge, for the frog). 

 This view is supported by the far-reaching analogies which "exist 

 with regard no;-, merely to time - relations, and distribution, 

 in direct (i.e. with excitation of the motor elements of the cord) 

 and in reflex muscular movements, but also to the conditions of 

 discharge in the two cases. 



With reference, first, to time -relations, we have already 

 seen that the transmission of the excitatory process from sensory 

 to motor fibres, via nerve-cells, takes up a considerably longer 

 period than the simple conduction of excitation through a corre- 

 sponding length of nerve. Mendelssohn (37) has recently found 

 that the reaction-time of the ventral half of the frog's spinal cord, 

 i.e. the interval between the moment of excitation and the appear- 

 ance of the gastrocnemius twitch on one side, is shorter than the 

 reaction- time of the dorsal half. In other words, excitation of the 

 ventral half of the spinal cord produces movement of the limbs 

 more rapidly than when the same stimulus is sent into the corre- 

 sponding point of the dorsal segment. The difference, according 

 to Mendelssohn, amounts to O'Ol 0*025 sec. This reaction 

 indicates that (in accordance with theoretical presumptions) the 

 muscular contraction due to direct excitation of the anterior 

 column makes its appearance earlier than the reflex contraction 

 discharged from the posterior column, the cause of the delay in 

 the last case being the larger mass of interpolated gray matter. 



The most significant factor in judging of the differences that 

 result from stimulation of the spinal cord, and direct excitation of 

 the peripheral motor nerves, is the fundamental difference in the 

 physiological properties of nerve-cells and nerve-fibres, the former 



