MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDTJS2E. 
197 
The explanation which I have offered to account for the phenomena of artificial 
rhythm is, that an alternate process of exhaustion and restoration of excitability on the 
part of the contractile tissues causes the constant stimulation of minimal intensity 
alternately to fall below and to rise above the limits of adequacy. This explanation 
has been tested and supported by various experiments, which it is not necessary again 
to detail. Taking, therefore, this explanation of the artificial rhythm as satisfactory, 
it follows that it probably has an important bearing on the facts of natural rhythm. 
For if in the production of artificial rhythm the exhaustion of contractile tissues is 
found to play so essential a part, it becomes incredible that it should not likewise 
play some part in the production of natural rhythm. I am therefore led to suppose 
that in all ganglio-muscular tissues which present rhythmic movement, the rhythm 
is not exclusively due to any resistance mechanism on the part of the ganglionic 
tissues, but that it is at any rate in large measure due to this alternate rise and 
fall of excitability on the part of the muscular tissues. And I have submitted that 
this view is sustained, d priori, by the fact that rhythmic action is of the most 
frequent occurrence in lowly organized tissues where as yet there has been no segrega¬ 
tion of ganglionic structure, and, d posteriori, by the fact that in Sarsia the same 
ganglia supply at the same time a stimulus to the rhythmic action of the lowly 
excitable tissues of the bell, and a stimulus to the tonic action of the more highly 
excitable tissues of the polypite. And, as additional facts confirmatory of the same 
view, I have also cited the observations of Eckhard, Heidenhain, Foster, and Dew 
Smith on the rhythmic action of the heart-apex under the influence of the constant 
current; the observations of Dr. Foster on the rhythmic action of the same tissue 
under the influence of the faradaic current; his observations on the phenomena of 
artificial rhythm as they occur in the Snails heart; the observations of Dr. Burdon 
Sanderson and Mr. Page on the rhythmic properties of the excitable tissues of 
Dioncca; and my own observations on the rhythmical motions of the Frog’s tongue. 
But in thus suggesting some modification in the current theory of ganglionic action, 
I have been careful to state that I do not attempt to overturn, but merely to supple¬ 
ment it. I think I have shown that the theory in question is inadequate to account 
for all the facts, and that in order to render it a complete theory, we must recognise, 
not only a periodicity of stimulation on the part of the ganglionic tissues, but also an 
inherent tendency to rhythmic action on the part of the responding tissues. 
Light has been shown to act as a stimulus on the lithocysts of the covered-eyed 
Medusae. 
The polypite of Aurelia aurita has been shown to perform localising movements 
somewhat similar to those which are performed by the polypite of Tiaropsis indicans, 
though the localising movements are of a much more sluggish and uncertain character 
in the former than in the latter species. 
The effects of alternating the direction of the constant current on the excitable 
tissues of the Medusae has been shown similar to those which occur in the muscular 
