258 HUMAN PHYSIOLOGY 



Heart-block. If a thread is tied around the heart 

 of a frog or a turtle between the auricles and the single 

 ventricle, pressure may be applied in the groove until 

 the tissue at that level is seriously injured. It is then 

 usually observed that the auricles go on beating as be- 

 fore but that the ventricle is arrested and remains re- 

 laxed. The conclusion might be drawn that the ventricle 

 beats in the normal heart because of rhythmic stimula- 

 tion passed on to it from the auricles. This appears 

 to be the case. Another conclusion might suggest its 

 self: namely, that the ventricle has no automaticity. 

 This is not justified when all the facts are taken into 

 consideration. 



The ventricle will beat without the auricles if it is 

 bathed within by blood or certain solutions which may 

 be substituted therefor. Its rate when beating inde- 

 pendently is lower than that of the auricles. Reference 

 has been made to the bundle of His which conducts 

 the excitation from the auricles to the ventricles in the 

 hearts of the higher animals. This bundle may be in- 

 terrupted by a skillful operation, a hook being inserted 

 so as to pass behind it and then used to bring pressure 

 upon it. When the bundle of His has been disorgan- 

 ized, the auricles beat as before while the ventricles 

 adopt a new and slower rate. A condition has been 

 created which is known as heart-block. The experi- 

 ment confirms the inference made with the frog's heart 

 that the auricles usually impress upon the ventricle 

 their own rhythm but we see at the same time what 

 could not be made out in the other case, that the ven- 

 tricular tissue is automatic when the circumstances are 

 favorable. 



Heart-block such as can be produced by injuring the 

 bundle of His is supposed to have many points of 

 similarity with a malady known as Stokes-Adams dis- 

 ease. The victim of this disorder suffers at times 

 from inadequate circulation and examination shows that 

 the pulse at the wrist falls during the seizures to per- 



