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MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
would course in the opposite direction e h, and in that direction only. On stimulating 
the line a b itself, the result might either be no response at all, a contractile wave 
running in one direction, a contractile wave running in the other direction, or contractile 
waves running in both directions. These various results are doubtless to be explained 
by the various degrees in which the current escaped from the slender line a b. 
( c ) Exploration by stimulus, then, not only shows that different tissue-tracts differ in 
their degree of irritability; but also that they further differ in the degree of their 
permeability to the stimulating influence. In the cases just cited, viz. those of complete 
blocking, we perceive a total absence of this permeability; but it must now be stated 
that, just as in the cases previously cited of differential irritability, so in those of 
differential permeability there are all the degrees of difference observable. And, in 
the one set of cases as in the other, the lines of demarcation between adjacent areas are 
exceedingly well defined. For example, returning to our former sketch, the area 
enclosed by the line bf and the angular line b g was an area the minimal stimulation of 
any part of which was followed by a local contraction of the area g bf, while minimal 
stimulation of any point outside of the lines g bf was followed by a general contraction 
of the entire strip situated outside of these lines. And of course there are other cases 
in which even the strongest stimulation of such an insular area as g b f would fail to 
elicit any thing further than a local contraction of that area. 
( d) From all this, then, we see that exploration by graduated stimuli reveals, first, 
differences in the degrees of excitability of closely contiguous tissue-areas, and, second, 
differences in the degrees of permeability to stimuli on the part of such areas — there 
being thus tissue-areas which admit of being more or less physiologically isolated from 
the rest of the tissue-mass by section. And the fact that in all these cases the lines of 
demarcation between the differentiated tissue-tracts are so sharply defined constitutes, I 
think, an additional support to the hypothesis concerning the presence of nerves, or 
“ lines of discharge”*. 
(B) Excitational Continuity. — (a) Closely related to this hypothesis are also the 
* It has been suggested to me that contractile waves are merely muscle-waves, and that their blockage in 
contractile strips is due to shock. This, of course, would he a delightfully simple solution of the difficulties 
besetting this subject, and it is naturally the first one that occurs to an experimenter. But further observation 
shows that the tissues of Medusae cannot he made to suffer shock unless the tissue-tracts affected by the section 
are exceedingly narrow, while blocking of contractile waves may take place in strips of considerable width. 
Moreover, the above results with regard to exploration by stimulus prove that even the unmutUated contractile 
tissues are far from being functionally homogenous. These results also negative the supposition which may 
possibly occur to some physiologists, viz. that blocking of the contractile waves in a spiral strip is due to a 
resistance that progressively varies with the length of the strip. 
A more plausible explanation would he that the line of blocking is determined by an accidental strain to 
which that line has been previously subjected. That this is sometimes the case there can he no doubt, as the 
tissue hears optical indications of having been strained at the fine of blocking. But that this cannot be the 
explanation in all cases is proved by the occurrence of those numerous gradations in different parts of the same 
tissue-mass, both with regard to exeitahility of tissue-areas and intimacy of physiological connexions. 
