96 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
In normal water 
Quarter of an hour after 
exposure to weak dose ... 
Two minutes after strength- 
ening of dose roe 
Five minutes later, and just 
before blocking of wave 
Fifteen seconds later, wave 
continuing blocked 
Immediately after passage 
of wave on restoration to 
normal sea-water ... 
An hour later ... 
One second 
Fig. 20. 
From these experiments, however, it must not 
be definitely concluded that it is the anesthesiating 
property of such substances which exerts this slow- 
ing and blocking influence on contraction-waves, 
for I find that almost any foreign substance, whether 
or not an anesthetic, will do the same. That nitrite 
of amyl, caffein, ete. should do so, one would not 
be very surprised to hear ; but it might not so readily 
be expected that strychnine, for instance, should 
block contraction-waves; yet it does so, even in 
doses so small as only just to taste bitter. Nay, 
even fresh water completely blocks contraction- 
waves after the strip has been exposed to its 
influence for about half an hour, and exerts a per- 
manently slowing effect after the tissue is restored 
to sea-water. These facts show the extreme sensi- 
tiveness of the neuro-muscular tissues of the 
Medusze to any change in the character of their 
surrounding medium, a sensitiveness which we 
shall again have occasion to comment upon when 
treating of the effects of poisons. 
