436 Prof. C. S. Roy and Mr. J. G. Adami. [Feb. 11, 



value of observations made on the heart by " button " cardio- 

 graphs. 



Section III begins by a consideration of the relationship between 

 the circumference of a hollow spherical muscle and its cubic contents, 

 this being illustrated by a diagram, and by one or two concrete 

 examples with regard to the bearing of this subject upon the phy- 

 siology of the ventricles. 



We then state the relation between the internal circumference of a 

 hollow spherical muscle and the resistance to contraction of its 

 walls. Reference is also made to the elastic resistance which the 

 heart wall itself offers to contraction, and the bearing of this upon 

 the production of negative pressure within its cavity under certain 

 conditions. 



We then consider briefly the effect on the ventricular contractions 

 of changes in the blood pressure within the systemic and pulmonary 

 arteries, pointing out how much the heart has in common with the 

 voluntary muscles of the body, and explaining why the amount of 

 residual blood is liable to changes, concluding with a few remarks 

 upon " failure of the heart." 



In Section IV we enter upon a study of the effects of the vagus 

 nerve upon the heart. We begin with the changes in the contraction 

 volume, and point out that, at first sight, our curves seem to show 

 that, other things being equal, the volume of blood expelled at each 

 systole varies in inverse ratio to the rapidity of heart beat. We show, 

 however, that this general law does not hold good for vagus slowing 

 (if, indeed, it be exact for slowing of any kind), which is found to 

 be accompanied by a lowering of the output ; that, with moderate 

 slowing, this diminution of the output may be as much as 30 or 35 

 per cent. 



We then speak of the increase in the amount of residual blood in 

 the heart which is produced by vagus excitation, showing that this 

 does not necessarily indicate any weakening of the ventricular con- 

 tractions. 



We next analyse myocardiographic records of the action of the 

 vagus upon the heart, showing that the auricular contractions are 

 weakened or arrested, and noting that the influence of the vagus upon 

 the force of the auricular contractions bears no constant proportion ta 

 the vagus slowing. By strong vagus excitation or by muscarin the 

 auricles may be completely arrested, it may be, for hours. This com- 

 plete arrest is, in some cases, led up to by progressive weakening, but 

 sometimes arrest occurs immediately after fairly strong beats, or with 

 fairly strong beats presenting themselves at times during the arrest. 

 These latter cases may be explained by weakening of the excitations 

 which reach the auricles from the sinus, although they are possibly 

 due to diminished excitability of the auricles. 



