1881.] 



On the Rhythm of the Heart of the Frog. 



201 



effects of two or more of these inefficient impulses, and so continues 

 to beat rhythmically though no longer synchronously with every 

 impulse. 



4. The most satisfactory 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 syn- 

 chronous 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 im- 

 pulses, 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. 



7. There is a limit to the extent to which a series of inefficient im- 

 pulses can raise the excitability of the muscle, so that the ventricle 

 can remain absolutely quiescent, even although the impulses still pass 

 to it, when those impulses are sufficiently weakened. 



In Part II the action of the vagus nerve is considered, and it is 

 shown that its stimulation produces a most marked effect upon the 

 force of the contractions, both of auricles and ventricle, entirely 

 independent of any alteration of rhythm. The curves obtained can 

 be classified under the three following types : — 



1. Complete quiescence of both ventricle and auricles, followed by 

 contractions which at first are scarcely visible, but which rapidly 

 increase in size, until at the maximum they are much greater than 

 before the stimulation of the nerve. From this maximum they very 

 gradually decrease, until the original size of contraction is again 

 reached. 



2. During the stimulation no quiescence of either ventricle or 

 auricles, but simply a diminution of the size of the contractions, 

 followed by a rapid and marked augmentation of the contractions 

 beyond the original height, and then a slow gradual diminution to the 

 size obtaining before the nerve was stimulated. 



3. No primary diminution, but from the commencement of the 

 stimulation the beats increase in size, and after a time gradually 

 return again to the original size. 



Between these three types every conceivable variation may occur, so 

 that a series of curves may be selected in which no line of demarca- 

 tion can be drawn between complete primary quiescence, or to use the 

 usual term, inhibition, on the one hand, and a simple primary aug- 

 mentation of the size of the contractions on the other. 



These curves alone show that the vagus is able to cause a standstill 



