STIMULATION AND TRANSMISSION 269 



motor nerve, is extreme; the frog's sciatic nen-c may 

 be stimulated by a current of .000001 ampere or less; 

 it is well known that the neuro-muscular ai)paratus of 

 the frog was used by Galvani as the most sensiti\e 

 means known to hmi by detecting variations in the 

 electrical state of bodies; in fact it is to this property of 

 living tissues that we owe the discovery of current 

 electricity by Volta. Galvani's experiments also showed 

 ^although their significance was disputed at the time — 

 that electric currents are produced in the activity of 

 living tissues. Thus two of the most fundamental 

 properties of living matter, its electrical sensitivity, and 

 its production of electrical currents during activity, 

 were early observed; and many of the chief problems 

 of general physiology at the present time relate to the 

 physico-chemical conditions and physiological signifi- 

 cance of these properties. That they are among the 

 chief factors controlling normal cell-processes seems 

 certain. 



NATURE OF THE LOCAL CHANGE 



It is remarkable that complete stimulation, involving 

 a change in the activity of the entire cell, is produced 

 in many if not all irritable elements by agents which 

 affect directly only the surface-layer of i)rotoj)lasm. 

 Some local modification of surface conditions seems to 

 be all that is required to set in motion the whole complex 

 process of stimulation. This is best shown in the 

 mechanical stimulation of single cells; thus in a ciliated 

 protozoon like Paramoeciiim a slight IoikH is suflicient 

 to call forth the characteristic motor reaction, involving 

 a reversal of the direction of ciliary activity over the 

 whole surface of the organism. The cxtraordinar>' 



