THE FROG, AND OH THE ACTIOH OF THE YAGGS HR RYE. 
1003 
sinus only, which causes the motor ganglia to send out impulses at a much quicker 
rate without increasing the excitability of the ventricle, may cause the ventricle to 
respond synchronously to every second of these quicker impulses, instead of to every 
impulse. 
Now, although this explanation may be sufficient in the case of the half-rhythm 
produced by heating the sinus and auricles only, it is clearly inefficient to explain the 
action of tightening the clamp, for in this latter case no alteration in the rate of the 
auricular contractions occurs. 
Since, therefore, the compression by means of the clamp in the sulcus does not 
produce its effects upon the ventricle by lowering its excitability, or by altering the 
rate of the impulses from the motor ganglia, it is most probable that it does act by 
weakening the strength of the impulses in the same way as suggested by v. Basch, 
in order to explain the half-rhythm caused by the use of sufficiently weak electrical 
stimuli. 
Further, the similarity between the effects of tightening the clamp and of heating 
the sinus and auricles alone, is sufficiently great to suggest that in both cases the 
effects are produced in the same way, and therefore the main reason why heating the 
sinus and auricles caused the observed alteration in the rhythm of the ventricle is to 
be found in the fact that, when the motor ganglia are heated, not only is the rate of 
discharge of the impulses quickened, but also the quicker impulses are of necessity 
weaker than before. 
To sum up the conclusions arrived at so far, we have the following propositions :— 
1. The rhythm of the heart is caused by discrete motor impulses passing to the 
muscular tissue from certain motor ganglia. 
2. Each one of these impulses produces a contraction of the ventricle only when a due 
relation exists between the strength of the impulses and the excitability of the 
ventricular muscle. 
3. When each impulse is inefficient to cause a contraction of the ventricle, the 
ventricular muscle has the power of summing up the effects of two or more of these 
inefficient impulses, and so continues to beat rhythmically, though no longer 
synchronously with every impulse. 
4. The easiest explanation of this summation process is as follows: Every impulse 
which is inefficient to produce a muscular contraction increases the excitability of the 
muscle, and therefore makes it easier for a second similar impulse to cause a contraction. 
5. The impulses can be made inefficient to produce contractions synchronous with 
them by lowering sufficiently the excitability of the ventricle, as is seen in the action 
of poisons, even although the rate and strength of the impulses remain unaltered. 
6. The impulses can also be made inefficient when the excitability of the muscle is 
unchanged by diminishing the strength of the impulse, as is seen in the effects of 
compressing the tissue between the ventricle and the motor ganglia or of heating the 
auricles and sinus without heating the ventricle. 
