NERVE-CONDUCTION IN CASSIOPEA XAMACHANA.
It is the author's privilege to acknowledge his indebtedness to kind friends for advice and aid: To Professor George A. Hulett, of Prince ton University, for having had prepared in his laboratory the con ductivity water used in this research; to Professor J. F. McClendon, of the University of Minnesota, for valued advice and aid, and to Professor L. R. Cary, of Princeton, for permission to make use of his recent deter minations of the rate of nerve-conduction at different temperatures. By means of the generous interest of Professor E. G. Conklin and the authorities at Princeton I have enjoyed the excellent facilities afforded by the Biological Laboratory in Guyot Hall, wherein the kymograph records taken at Tortugas were studied and the results tabulated.
Methods AND CORRECTIONS.
The object of this research was to obtain an accurate quantitative determination of the rate of nerve-conduction in natural and in diluted sea-water at constant temperature, and also to estimate the effects of various artificial sea-water solutions containing all or some of the sodium, magnesium, calcium, and potassium cations of sea-water. The effects of temperature upon nerve-conduction are also of great importance.
These studies were carried out in June and July 1916, upon Cassiopea xamachana, a rhizostomous scyphomedusa which is abundant in the salt-water moat surrounding Fort Jefferson at Tortugas, Florida, and is also common upon the bottoms of many of the shallow, semi stagnant lagoons of the West Indian region. It is thus accustomed to a considerable range both in salinity and temperature, and being infested with commensal plant cells, it is in some measure independent of the oxygen-supply of the surrounding water, and even pulsates at a nearly normal rate in sea-water which has been deprived of air by boiling. The medusa thrives in confinement in glass aquaria and can be maintained alive in the laboratory for months while experiments are being performed upon it. Thus it is one of the most favorable of marine invertebrates upon which to conduct physiological studies.
An aboral view of Cassiopea xamachana is shown in figure 1, plate 1; figure 2 is an oral view of the subumbrella with the stomach and mouth-arms removed, and in figure 3 we see an annulus made by two circular cuts, one removing the marginal sense-organs and the other cutting out the central stomach, and in addition a circular scratch cutting through the nervous and muscular layer of the subumbrella, thus separating the broad annulus of tissue into two rings. This subumbrella tissue becomes paralyzed through the removal of the marginal sense-organs, but the outer of the two annuli may then be stimulated by an induction shock until a contraction-wave going in one direction is entrapped in it, as illustrated in figure 4, and as has been described by Mayer, 1906, 1908.* Such a neurogenic contraction wave must travel con tinuously through the circuit of tissue which has entrapped it, and may maintain itself for days with but little change of rate, provided the temperature, salinity, and hydrogen ion concentration of the sea-water remain con stant. Thus we have a means of entrapping a single neurogenic stimulus which remains practically uniform in intensity and rate for any desired length of time.
Strong stimuli travel faster than weak ones, and thus if a wave stops it can not be started again at the same rate, for no two stimuli are received alike by the tissue. Such circuit waves may be stopped by counter-waves proceeding in the opposite direction against them, or blocked in the nerve net without apparent cause (fig. 5) ; or, if the tissue be exhausted, as in the absence of calcium, or by heat, cold, the wave may become irregular (fig. 13, lowest line) indicating that it is about to cease. In exhausted or weakened tissue, pulsus alter nans may be displayed by the wave, the muscles becoming capable of responding fully only to every alternate (or even every second or third) return of the nerve stimulus (fig. 6, lowest line) ; but in normal, healthy tissues the wave is a full, regular sinusoid, the intervals being almost machine-like in their rhythmic sequence.