TRANSMISSION AND CONDUCTION 231 



as a conducting path and not merely the synapse itself, 

 may be concerned deserve attention. The dendrite is 

 much less highly specialized morphologically than the 

 axon and is evidently concerned in nutrition and mainte- 

 nance of the neuron as well as in conduction. We know 

 practically nothing concerning conduction in the ordi- 

 nary dendrite, but it is evident that the assumptions 

 usually made concerning the synapse may be made, 

 perhaps on even better grounds, concerning the dendrite. 

 If the dendrite is less readily excitable than the axon, 

 does not show the " all-or-none " type of reaction, and 

 conducts with a steep decrement, it will serve as well as 

 the synapse to account for the characteristics of the 

 reflex arc commonly attributed to the synapse. Assum- 

 ing for the sake of argument that the dendrite possesses 

 these characteristics of the more primitive conduction 

 path, an excitation conducted up the axon, through the 

 cell body, and down the dendrite may be so slow or so 

 weak or both, by the time it reaches the tip of the 

 dendrite that it is unable to excite the axon tip with 

 which the dendrite is in contact. The excitation wave 

 in the axon, being steeper or higher, or both, than that 

 in the dendrite and undergoing little or no decrement in 

 its passage down the axon, is able to excite the dendrite 

 or the cell body, with which the axon is in contact. 



As regards delay and summation, it may require a 

 number of impulses from the axon to bring the dendrite 

 to a physiological condition in which it is able to initiate 

 an excitation wave, high enough or intense enough to 

 travel to and through the cell body to the axon. In 

 the conduction of impulses toward the apical end of the 

 ctenophore plate row following mechanical stimulation 



