202 



On the Rhythm of the Heart of the Frog. [Dec. 22, 



by diminishing the force of the contraction down to quiescence ; this 

 is further shown by the fact that standstill of the ventricle alone can 

 occur while the auricles are beating with accelerated or unaltered 

 rhythm, but diminished force, or even when from the commencement 

 of the stimulation the force of the auricular contractions is in- 

 creased. 



This same gradation of effect, as the result of the stimulation of the 

 nerve, from absolute standstill to a simple primary augmentation, is 

 seen more or less clearly in the course of each separate experiment ; 

 the stimulations that occnr immediately after the suspension of the 

 heart are much the most likely to produce standstill ; later ones to 

 cause primary diminution followed by augmentation, and finally aug- 

 mentation alone. 



The power of diminishing the contractions to standstill appears to 

 last longer after the heart has been suspended at some times of the 

 year than at others. 



The conclusion is drawn that the variations in the effects produced 

 by stimulation of the vagus on the force of the contractions are 

 dependent essentially upon the condition of the nutrition of the heart ; 

 and possibly for the same cause the vagus tends to lose all power of 

 producing slowing after the heart has been suspended in the apparatus, 

 for in most cases acceleration only is seen, although slowing occurred 

 on stimulation before the heart was cut out, and apparently slowing is 

 more likely to occur immediately after the suspension of the heart 

 than later. 



The action of the vagus upon the muscular tissue is not only shown 

 by its effect on the size of the contractions, but also by its influence 

 on the excitability and tonicity of the ventricular muscle. 



When by tightening the clamp the ventricle is made to beat 

 synchronously with every second auricular beat, stimulation of the 

 nerve may cause the ventricle during the stimulation to beat 

 synchronously with every third, fourth, or more auricular beats ; and 

 the same alteration in the relation between the rhythm of the two 

 parts above and below the clamp is seen in the case of the contractions 

 of the apex and base of the ventricle, when the clamp is placed midway 

 across the ventricle. 



Also, when the ventricle is beating with half-rhythm from the 

 action of the clamp, stimulation of the nerve may make it beat syn- 

 chronously with every beat of the auricles for a definite time ; and 

 when the ventricle is not beating, either in consequence of tightening 

 the clamp, or of heating the auricles and sinus, then vagus stimula- 

 tion may cause a series of contractions synchronous with those of the 

 auricles. 



These experiments are to be explained on the supposition that the 

 vagus stimulation diminishes the excitability of the ventricle at one 



