122 Papers from the Marine Biological Laboratory at Tortugas. 
the end B it dies out, for it can not return over the recently stimulated 
tissue along which it has just passed. Thus we see that the index strip AB 
simply serves to catch a portion of each wave which passes its base. 
Now suppose we place the ring in a pure solution of magnesium chlo- 
ride, and allow the index strip AB to remain in natural sea-water. Then 
the contraction-wave gradually dies out in the pulsating ring, for the mag- 
nesium paralyzes the muscles; and at the end of about a quarter of an 
hour all movement will have ceased in the ring, but for from 12 to 15 
minutes after this we find that the strip AB still continues to transmit con- 
tractions at regular intervals of time. We see, then, that whenever the 
something which produced the contraction in the ring comes around to 
the point A it is still capable of setting up a 
contraction in the strip AB, although it can not 
now cause the muscles of the ring itself to 
pulsate. 
The explanation is that the stimulus which 
produces pulsation is nervous in nature, and 
travels through the nervous tissue quite indepen- 
dent of the presence or absence of the muscles. 
When, therefore, the magnesium paralyzes the 
muscles the nervous stimulus still travels around 
the ring even though the muscles can not now 
respond to it by contraction. 
The pulsation-stimulus is nervous, not epi- 
thelial, for in the exumbrella we find the epithelial 
but no nervous elements, yet the exumbrella 
(iG. 7.—Showing thatthe pul- tissue can neither pulsate nor conduct the pulsa- 
sation-stimulus ws nervous . . 7 . : . 
not muscular, in nature, #Om-Stimulus. The transmission of the stimulus 
which produces muscular contraction is therefore 
dependent upon the presence of nervous elements in the tissue. 
Bethe, 1903,’ in his important research upon the pulsation of Rhizostoma 
and Cotylorhiza, concludes that the pulsation-stimulus is nervous; for it 
readily passes over parts of the subumbrella where there are no muscles. 
Moreover he shows that the radial muscle-strands contract before the circu- 
lar muscles, although the latter lie closer to the sense-organs. This is due to 
the longer latent-period of the circular muscles, and it is evident that this 
latent-period is a property of the muscles, not of the nerves. On the other 
hand, Bethe shows that Marey’s refractory period during systole is a prop- 
erty of the nerves, not of the muscles. 
T. Brailsford Robertson, 1905,2 demonstrated that contraction may be 
abolished, and yet conduction of a peristaltic wave will take place at the 
* Allgemeine Anatomie und Physiologie des Nervensystems. 
“Trans. Roy. Soc. South Australia, vol. 29, p. 34. 
