THE NERVOUS IMPULSE 99 



the cathode it is increased (catelectrotonus). The conductivity is 

 diminished. 



By these effects of electricity and of the agents mentioned before, 

 it is clear that while conduction is the specialized function of nerve, 

 the tissue still retains the irritability common to all protoplasm, and 

 the two may vary independently of each other. The conductivity 

 of the nerve, then, whatever its specific process, is something different 

 from the irritability of other tissues. The nervous impulse is readily 

 actuated not only by electricity, but also by rapid changes of tem- 

 perature either way, by mechanical impact, or by irritating chemical 

 substances. However set in movement, the nervous impulse is accom- 

 panied or preceded by currents which are truly electrical, as may be 

 readily shown by instruments for the indication and measurement only 

 of electricity. It has never been proved yet that nerves originate the 

 impulses which pass over them. The energy necessary for starting the 

 impulse comes probably either from the bodily tissues (mechanically, 

 thermally, chemically, or electrically acting) or else from outside the 

 organism altogether. This must not be stated, however, too abso- 

 lutely, for the nerve-tissue has metabolism as well as the other tissues, 

 and an amount of energy proportional to the mass of the nervous sys- 

 tem may be generated within it. Probably this energy or part of it 

 goes to the origination of nervous impulses as well as to supporting them. 

 Although so important, this question has as yet no sort of certain 

 answer. 



