490 SCIENCE PROGRESS 



pressure. Every sudden change in physical environment is 

 liable to stimulate nerve. Efficient chemical means are all 

 illusory. Only such chemical reagents as have a capacity for 

 indirectly producing physical change are capable of stimulating. 

 Their effects are then in proportion to the physical change 

 which they are capable of producing, as has been well shown by 

 Matthews. Solutions of electrolytes produce electrical effects. 

 Solutions of varied concentration cause variations in the water 

 content of the nerve. Concentrated solutions by concentrating 

 the electrol3'tes in the interior of the nerve give rise to self- 

 stimulation. The transference of the excited state from point 

 to point of the nerve can therefore be assigned with certainty 

 to ph3^sical change, and this no matter whether the disturbance 

 at each point thus affected involves chemical change, such as 

 the decomposition of a carbohydrate or a fatty side chain, or 

 physico-chemical change such as the disruption of some adsorp- 

 tion complex, or purel}' physical change such as the conferring 

 of new motion upon matter in solution. There is, in fact, no 

 experimental datum which would enable us to think of any 

 other factor than physical change as responsible for the trans- 

 mission of the excited state from point to point. We are, 

 therefore, debarred from accepting in this case many of the 

 mechanisms which have been suggested, such as, for example, 

 the transference of an ox^-gen atom from side chain to side 

 chain of an excited giant molecule. 



Compelled thus to retreat to physical change alone, there 

 can be little doubt as to the nature of the phj'sical change 

 which is responsible. Most careful experimental inquiry has 

 failed to obtain any evidence of heat-production such as occurs 

 in the train of gunpowder. As to the occurrence of an electrical 

 change of very considerable magnitude, such as has been most 

 beautifully measured by Gotch and Burch, there is no question. 

 This in fact is the onl}' kind of change which has been found 

 to accompany the transit of a nervous impulse. The point, 

 therefore, which has seemed long to Hermann and others of 

 capital importance should be stated boldly. No matter what may 

 be the complete change at each excited pointy its transference from 

 point to point is accomplished by electrical agency. To this state- 

 ment the absence of any temperature change enables us to 

 make an important addition. This excited state which is 

 propagated by electrical change is certainly not a chemical 



