SAMUEL ERNEST POND 825 



accompanying a reduction process; after the excited region has 

 reached a maximum negativity (electrically negative with respect 

 to adjacent regions) the reverse or oxidative process automatically 

 follows, with the effect of reestablishing the passive or resting state. 

 According to this hypothesis the free oxygen enters directly into only 

 one part of the local stimulating process; but this oxidation must 

 be repeated at each successive region during the propagation of the 

 excitation state along the tissue. 



If transmission in the living tissue is in fact dependent upon sec- 

 ondary electric stimulation of the resting region by the local bioelec- 

 tric current between that region and the active region adjoining, 

 there should be a direct proportionality between the electrical con- 

 ductivity of the medium and the rate at which the state of activity 

 spreads from region to region. The electrical conductivity of the 

 first local stimulating circuit, other conditions being equal, deter- 

 mines the intensity of the current at any point in the circuit; and if 

 the conductivity is uniform throughout the tissue and medium the 

 rate of propagation will be uniform throughout the tissue. Differ- 

 ences in the electrical conductivity of the medium will be associated 

 with differences in the rate of propagation because of the effect upon 

 the intensity of the local bioelectric current traversing the tissue at 

 any point adjacent to the active (electrically negative) area. The 

 greater the conductivity of the medium, the greater will be the 

 distance (from the already active region) at which the current trav- 

 ersing the resting tissue is sufficiently intense to effect the stimu- 

 lating reaction. It is on the basis of such general considerations that 

 Lillie believes we may provisionally disregard the special chemical 

 nature of the local reaction (at the cell-medium boundary), on the 

 ground that at present we know very little about it. Whatever the 

 nature of the local stimulation process may happen to be, if it is 

 initiated electrically by the current of the local bioelectric circuit 

 it should spread from region to region at a rate proportional to the 

 electrical conductivity of the circuit. This conductivity depends 

 chiefly on the conductivity of the external medium. 



In the preliminary work before any. oxygen had been employed 

 experimentally and when conditions were constant (as with air 

 bubbled through the solutions or with fresh solutions) it seemed 



