THE CAUSE OF PULSATION 487
Tims the sodium oxalate which foims in the sense-organs is simply changed into ordinary table salt, which acts as a stimulus to produce pulsation.
We can prove experimentally that this suffices to explain the phe- nomenon of pulsation, for if we simply add from 1 to 5 parts of com- mon salt to 1,000 parts of sea-water, we find that this slight excess of salt acts as a powerful stimulant to the sense-organs, but produces no pulsation if placed upon other parts of the jellyfish.
It thus appears that each sense-organ normally maintains a certain excess of common salt which acts as a stimulus, and which is prevented from becoming too concentrated by the fact that being soluble it is constantly dissolving out into the surrounding sea-water.
It may trouble us for a moment to see why a recurrent pulsation should arise from a constantly present stimulus, but long ago Romanes discovered that a weak constantly present stimulus, such as a faradaic current of electricity, will cause rhythmical pulsation, the jellyfish responding to it periodically and regularly.
We see then that the natural stimulant which produces the pulsa- tion of the jellyfish is only that most familiar substance common salt !
The hearts of higher animals behave in a manner so similar to that of the pulsating jellyfish that we neecl not be surprised if it be demon- strated that here also a slight excess of sodium chloride gives rise to each and every pulsation.