47 
5 -<>U V t W,,^,i ,vC«rt<w ^^ xT 
Figure 7. 
13. — Rhythmicity in Muscle and the Action of 
Inhibitory and Augmentor Nerves. 
Direct proof that the rhythm of the heart is due to the 
presence of electrolytes in the circulating medium is afforded 
by the fact Luiat if the proteids be removed from serum which 
is then circulated through the heart the rhythmic contractions 
will continue. If the salts are removed and the serum is 
circulated it is ineffective."^ The solutions generally used and 
found effective stimuli for the heart-beat have negative stimu- 
lation efficiencies, owing to the predominance of NaCl. 
Let us, therefore, consider the case of an excised heart 
through which a solution, which has a negative stimulation 
efficiency, is circulated. Assuming that the walls of the 
heart are equally permeable to both the ions in the solution — 
an assumption which, however, is not strictly permissible — 
we see that, owing to the difference of ionic concentration on 
the two sides of the muscle surface, ions are continually 
diffusing in— but at different rates — the anions more quickly 
than the kations. Since the time taken for the anions enter- 
ing the hyaloplasm to reach a gfiven number — the threshold 
number — will be inversely proportional to the velocity with 
which the anions enter, we may conclude that, other things 
being equal, the frequency of the beat is greater the greater 
the velocity of the anions in the solution. Also, since the 
driving force which causes the ions to diffuse into the muscle 
is dependent upon the difference in ionic concentrations on 
the two sides of the muscle surface, we see that, if the solu- 
* Gaskell : Schafer's Te:s?tbook of Phyedology, vol. ii., page 
226. 
