Rhythmical Pulsation in Scyphoincdusa. 



117 



More commonly, however, the two waves which arise from the stimu- 

 lated point are unequal, one being strong and the other weak. This is doubt- 

 less caused by an inequality in the transmitting power of the nervous net- 

 work on either side of the starting-point, and the stronger wave goes in 

 the direction of the least resistance, as is shown in fig. 2, A. Under these 

 conditions, when the strong contraction-wave meets the weak one (fig. 2, B), 

 it is still capable of stimulating the tissue over which the weak wave has 

 traveled, but the weak wave can not stimulate the tissue which has, only 

 the instant before, been exhausted in responding to the stimulus of the strong 



FIG. 2. Showing that when a strong wave meets a weak one it suppresses 

 the weak wave, and remains the only wave in the circuit. 



wave. Thus the weak wave is annulled, and only one (the strong) wave 

 remains to travel continuously around the ring in one direction, as is shown 

 in fig. 2, c. 



This single wave going constantly in one direction around the circuit 

 may maintain itself for days traveling at a uniform rate. The circuit must, 

 however, be long enough to allow each point to rest for an appreciable inter- 

 val of time before the return of the wave. The wave is actually " trapped ' : 

 in the circuit and must constantly drive onward through the tissue. 



The ventricle of the extirpated heart of the loggerhead turtle (Thalasso- 

 clielys caretta), if cut into ringed-shaped strips may also be caused to main- 

 tain itself in sustained pulsation in the manner described above for the scy- 

 phomedusa Cassiopca. The initial wave in the ring-shaped strip of turtle's 

 heart may be started by an electrical or a mechanical stimulus, and will con- 

 tinue to travel at a uniform rate in a single direction around the ring until 

 the dying of the tissue causes it to cease. 



It is interesting also that this wave through the strip of turtle's heart 

 passes mainly, if not wholly, through the dense, peripheral, muscular layer 

 of the heart, the inner cavernated tissue being practically inert in so far as 

 the pulsation-stimulus is concerned. The heart may be likened to a sponge 

 inclosed within a periodically contracting bag. We meet with a parallel con- 

 dition in the subumbrella tissue of Scyphomedusse, where the peripheral 

 nervous and muscular layers are all that are concerned in the pulsation ; the 

 thick gelatinous substance of the umbrella being a non-conductor and inert. 



