MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSH4. 
185 
To my mind, then, it is an interesting fact that ganglionic tissue, where it can first 
be shown to occur in the animal kingdom, has for one of its functions the maintenance 
of muscular tonus; but it is not on this account that I now wish to draw prominent 
attention to the fact before us. Physiologists are almost unanimous in regarding 
muscular tonus as a kind of gentle tetanus due to a persistent ganglionic stimulation ; 
and against this opinion it seems impossible to urge any valid objection. But, in 
accordance with the accepted theory of ganglionic action, physiologists further suppose 
that the only reason why some muscles are thrown into a state of tonus by gang¬ 
lionic stimulation, while other muscles are thrown into a state of rhythmic action by 
the same means, is because the resistance to the passage of the stimulation from the 
ganglion to the muscle is less in the former than in the latter case. Here, be it 
remembered, we are in the domain of pure speculation ; there is no experimental 
evidence to show that such a state of differential resistance as the theory requires 
really obtains. Hence we are quite at liberty to suppose any other kind of difference 
to obtain, either to the exclusion of this one or in company with it. Such a supposition 
I now wish to suggest, and it is this : That all rhythmical action being regarded as due 
(at any rate in large part) to the alternate exhaustion and restoration of excitability 
on the part of contractile tissues, the reason why continuous ganglionic stimulation 
produces incipient tetanus in the case of some muscles and rhythmic action in the case 
of others, is either wholly or partly because the irritability of the muscles in relation 
to the intensity of the stimulation is greater in the former than in the latter case. If 
this supposition as to differential irritability be granted, my experiments on Aurelia 
prove that tetanus would result in the one case and rhythmic action in the other; for 
it will be remembered that in these experiments, if the continuous faradaic stimulation 
were of somewhat more than minimal intensity, tetanus was the result, while if such 
stimulation were but of minimal intensity, the result was rhythmic action. Now, that 
in the particular case of Sarsia the irritability of the tonically contracting polypite 
is higher than that of the rhythmically contracting bell is a matter, not of supposition, 
but of observable fact ; for not only is the polypite more irritable than the bell in 
response to direct stimulation of its own substance, but it is generally more so even 
when the stimuli are applied anywhere over the excitable tissues of the bell. And 
from this it is evident that the phenomena of muscular tonus, as they occur in Sarsia, 
tend more in favour of the exhaustion than of the resistance theory."" 
* The evidence, however, is not altogether exclusive of the resistance theory, for it is quite possible that 
m addition to the high irritability of the polypite there maybe conductile lines of low resistance connecting 
this organ with the marginal ganglia. I entertain this supposition because, as afterwards explained in the 
text, I see reason to believe that the natural swimming movements of Sarsia are probably in part due to an 
intermittent discharge of the ganglia. I think, therefore, that in this particular case the ganglia supply 
a tolerably constant stimulation to the polypite, while it is only at intervals that their energy overflows 
into the bell, and that the higher degree of irritability on the part of the polypite ensures the tonic response 
MDCCCLXXX. 2 B 
