TRANSMISSION AND CONDUCTION 229 



cell body in an axon never affects another neuron. 1 Con- 

 ceivably such an impulse may be unable for some 

 reason to pass the cell body, or it may be unable to 

 pass through the dendrite, or finally it may be unable to 

 pass the synapse. Current views regard the synapse 

 as chiefly responsible for this functional irreversibility 

 of direction of conduction from one neuron to another. 

 Moreover, blocking, delay and the phenomena of sum- 

 mation of impulses in the reflex arc are ordinarily 

 attributed to the synapse. According to Lucas (191 7) 

 the junction between motor nerve and muscle is a 

 region in which the nervous impulse apparently under- 

 goes a decrement, and he suggests that conditions at 

 this junction may throw some light on the problem of 

 the synapse. To account for the delay and for summa- 

 tion, it is commonly assumed that the synapse differs in 

 some way from other parts of the nervous system as 

 regards the excitation process in it. Either it is not 

 readily excitable or the intensity or height of the excita- 

 tion wave in it is less than in other parts. Consequently 

 it constitutes to a greater or less extent an obstacle in 

 the way of the impulse, but continued or repeated 

 excitation may alter its condition so that the impulse is 

 able to pass. Passage, however, is apparently in one 

 direction only, from axon to dendrite, and some further 

 assumption is necessary to account for the irreversibility 

 of direction. Membranes, more or less irreversible as 

 regards direction with respect to passage of chemical 

 substances, do occur in other organs, but the physico- 

 chemical basis of this characteristic is not known with 



1 If the peripheral processes of the spinal ganglion neuron are axons 

 (see p. 196) these cells constitute an exception to this general rule. 



