100 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



the isthmus serves to connect, the resulting con- 

 traction-waves will at first pa^^s freely across the 

 isthmus ; but after a time it may happen in some 

 preparations that every now and then a contraction- 

 wave fails to pass across the isthmus. When this 

 is the case, if the stimulation is still continued, a 

 greater and greater proportion of waves fail to pass 

 across the isthmus, until perhaps only one in every 

 five or six becomes propagated from the one area to 

 the other. If single induction-shocks be then sub- 

 stituted for the faradaic stimulation, it may be found 

 that by leaving an interval of four or five seconds 

 between the successive shocks, every w^ave that 

 is started in the one area will be propagated across 

 the isthmus to the other area. But if the interval 

 between the successive shocks be reduced to two or 

 three seconds, every now and then a wave will fail 

 to pass across the isthmus; and if the interval be 

 still further reduced to one second, or half a second, 

 comparatively few of the waves will pass across. 

 Now, however, if the tissue be allowed five minutes' 

 rest from stimulation, and the single shocks be 

 thrown in at one second's intervals, all the first six 

 or ten waves will pass across the isthmus, after 

 which they begin to become blocked as before. It 

 may be observed also that when the waves are thus 

 blocked, owing to exhaustion of the connecting 

 isthmus, they may again be made to force a passage 

 by increasing the intensity of the stimulation, and 

 so giving rise to stronger waves having a greater 

 power of penetration. Thus, on re-enforcing the 

 electrical stimulus with the simultaneous application 



