49 
great the beats will merge into one another, and the heait 
will go into tonic contraction. 
Thus, the normal rhythm of the heart is due to the ions 
diffusing in from the blood, and the delicate adjustment of 
the threshold number to the nature and concentration of the 
salts in the blood. Almost any point in the heart is capable, 
in a greater or less degree, of initiating this rhythm, e.g., if 
the auriculo- ventricular groove be ligatured or cut through, 
a series of rhythmical contractions is initiated : this is soon 
suppressed ; subsequently a more permanent series is initi- 
ated."^' The "rhythm of excitation" is due to the kations re- 
leased by the injury due to the cut or ligature, the "rhythm 
of development" to kations diffusing in from capillary spaces. 
A permanent rhythm, such as we see in the normal 
heart, could not be maintained on a nutrient fluid whose 
stimulation efficiency was negative, unless the excess of 
anions was continually removed, for otherwise the anions 
would gradually convert most of the kation-proteid into 
anion-proteid, and contraction would become impossible. The 
solutions, however, which are generally used as circulating 
media to keep up the heartbeat have negative stimulation 
efficiencies owing to the predominance of NaCl. We should, 
therefore, expect to find, if the preceding reasoning has been 
correct, that the rhythm of the heart would be slowed by add- 
ing a little KCl to the solution (sufficient to reduce its 
stimulation efficiency without making it positive), and quick- 
ened by adding CaCl.^ so as to increase its stimulation effi- 
ciency (since CaCl.2 has a greater stimulation efficiency than 
NaCl.). This was found to be the case by Greene. f He 
found that calcium salts in isotonic solutions of NaCl stimu- 
lated a cardiac strip to increased rhythm and final permansat 
contracture. KCl in isotonic solutions of NaCi prevent- 
ed contractions and kept the ventricular strip in a state of 
relaxation. If the salts CaCL, and KCl were in the pro- 
portions of •026 per cent. CaCl, to '03 per cent. KCl, a 
few good contractions at a very slow and irregular rate mi^ht 
result. If the ratio was changed by increasing the CaCL, 
or by decreasing the KCl, then the contractions were in- 
creased in frequency ; but if the CaCl, was diminished or 
KCl increased, few contractions were developed, or none at 
all4 At first sight, these results might seem to be opposed 
* Gaskell : Schafer's Textbook of Physiology, vol. ii., page 
175. 
t C. AV. Greene : Amerioaii Journal of Physiology, 1899, 
vol. ii.. page 82. 
t Ihid., vol. ii., pages 107 and 125. 
