124 Papers from the Marine Biological Laboratory at Tortugas. 
a pure 4mm MgSO, solution, the immersed portion of the ring gradually 
loses its contractibility, but it still conducts the pulsation-stimulus. After 
ten minutes’ immersion the immersed portion of the ring can not be ob- 
served to contract, even if it be viewed under a microscope, but the wn- 
immersed part still responds by normally vigorous contractions at each 
passage of the pulsation-stimulus; and it is evident that the pulsation- 
stimulus is transmitted through the non-contracting immersed part of the 
ring. Indeed, the pulsation-stimulus will usually continue to pass through 
the inert, immersed part of the ring for fully half an hour after all re- 
sponse to its presence has ceased. This experiment gives the same result 
if the ring be partially immersed beneath MgSO,, MgCl, or MgBr,. It 
is evident that magnesium chiefly affects the muscles, rendering them in- 
capable of contracting and producing a state of inert relaxation. Magne- 
sium has, however, less effect upon the pulsation-stimulus itself, which is 
nervous in nature. That it has some effect upon the nervous elements 
is, however, evident, for the immersed part of the ring, after losing its 
ability to contract, finally ceases even to conduct the pulsation-stimulus. 
Moreover, the rate at which the pulsation-wave travels around the ring 
always declines. For example, one ring partially immersed beneath 54m 
MgSO, slowly declined in rate from 67 to 57 per minute after 35 minutes 
immersion. In another case the rate declined from 93 to 78 per minute 
after 28 minutes’ immersion, and in another from 88 to 40 in 31 minutes, etc. 
Weaker solutions of magnesium, made by adding MgSO, or MgCl, 
to natural sea-water, may not cause any decline in rate, although they will 
destroy the contractibility of the muscles in the immersed part of the ring. 
The effect of these weaker solutions, such as 66.6 sea-water + 33.3 of 5g¢m 
MgSO, is about wholly confined to rendering the muscle inert, and not to 
hindering the pulsation-stimulus, which is nervous in nature. 
It is remarkable that the normal medusa, pulsating by means of stimuli 
set forth from its marginal sense-organs, can not pulsate for more than 
20 seconds in a pure 5gm MgSO, solution, whereas a ring-shaped strip of 
subumbrella tissue without marginal sense-organs can pulsate for at least 
ten minutes in the above solution. The marginal sense-organs can not 
send forth the pulsation-stimuli unless they be surrounded by calcium in 
solution, and one office of this calcium is to offset the anesthetic effects 
of the magnesium. Indeed, if magnesium be absent, calcium may also be 
absent, and the sense-organs will continue to send forth their pulsation- 
stimuli for a long time; but if magnesium be present, calcium must also be 
present if pulsation is to endure long. Calcium produces tetanus, as has 
been shown by Loeb; while magnesium produces muscular relaxation, as 
has been shown by Meltzer and Auer and by myself. In this sense calcium 
and magnesium are antagonistic in their effects and offset one the other. 
Both are necessary for maintaining that delicately balanced state which 
