TRANSMISSION AND CONDUCTION 225 



nervous reticulum such differences have been observed 

 in various cases. In the coelenterate tentacle, for 

 example, impulses pass more readily and to greater 

 distances basipetally than in the opposite direction. 

 But the observations of Mayer (1906, 1908, 1916) and 

 Harvey (191 2) on conduction in the medusa, Cassiopea, 

 indicate that in this form the nerve net may conduct 

 both with and without a decrement. Slight stimuli 

 may produce reactions which disappear within a short 

 distance, but the conduction of the "trapped" excita- 

 tion waves in circular strips of the medusa body may 

 continue around the strip for days, or until it dies, 

 without appreciable decrement. 



It is evident from these facts that in Cassiopea some 

 submaximal excitations are conducted with a decrement, 

 while maximal excitations, or those above a certain 

 quantitative level, are conducted without appreciable 

 decrement, at least in the direction around the body. 

 Apparently in such cases as this the weaker excitations 

 do not set in action the whole of the excitatory mechan- 

 ism, perhaps do not bring about certain chemical 

 reactions or other changes, and the effects of such 

 excitations as stimuli on following points of the path also 

 give partial excitations weaker than the original. 

 Maximal or sufficiently strong excitation, however, sets 

 the whole mechanism in action and brings about the 

 same result at following points of the path; consequently 

 no decrement appears. Whether such differences in 

 conduction are characteristic of the reticular type of 

 nervous system is not yet known. 



It is generally assumed that the axon of the higher 

 animals conducts without a decrement, but it has 



