vni CONDUCTIVITY AXD EXCITABILITY OF NERYK 83 



duction currents, that the divided anterior columns of the frog's 

 spinal cord gave apart from quantitative differences a reaction 

 precisely similar to that of any peripheral motor nerve. If, with 

 descending direction of the " break " shock, the electrodes con- 

 nected with the secondary coil are (at not too great a distance) 

 so placed that one is at first actually applied to the section, 

 and are then moved farther and farther away, the initially 

 marked effect of excitation diminishes rapidly, and soon dis- 

 appears altogether. The first effect of stimulating with currents 

 which, if directly applied to the free surface of the muscle, 

 produce no visible excitation, and which are not felt by the 

 tongue, invariably consists in a more or less pronounced tetanic 

 disturbance of the muscles of the two posterior extremities, often 

 amounting to regular tetanus. Stronger excitation often evokes 

 co-ordinated movements. When the just effective stimulus at 

 the transverse section of the anterior column has been determined, 

 the electrodes (with descending direction of break shock, which 

 alone acts at first) can usually be shifted into the im- 

 mediate proximity of the lumbar enlargement thereby greatly 

 increasing the danger of direct or reflex excitation of the anterior 

 roots without producing any trace of excitatory phenomena in 

 the muscles of the hind limbs. This, however, is only the case 

 if the electrodes are shifted along the ventral surface of the 

 anterior column. If they are applied to the inner surface, i.e. 

 the cut section of the ventral half of the medulla, in direct 

 contact with the exposed gray matter, there is never a perceptible 

 difference of excitability at points near the cross-section, as com- 

 pared with the deeper parts. It is doubtful whether this experi- 

 ment by itself justifies the conclusion that in the last case the 

 gray matter is directly excited, while in the former there is cer- 

 tainly excitation of the longitudinal nerve-fibres (in the anterior 

 columns). 



By using proper precautions we can thus excite the spinal 

 cord of the frog (divided below the medulla oblongata, but 

 not otherwise injured), without fear of complication by reflexes. 

 It is sufficient to shift the exciting electrodes along the ventral 

 surface of the cord, after determining that distance of coil at 

 which the descending break shock takes effect in the immediate 

 proximity of an artificial cross-section, at any point. The ex- 

 citation of any other point along the cord, even close to the 



