vii MECHANICS OF THE HEART 221 



Diastolic pressure is accordingly reduced after division of the 

 vagi to an average of 11 cm. H 2 0, i.e. to a value little more than 

 half of that developed with intact vagi, which confirms the diastolic - 

 or dilator action of the vagi upon cardiac muscle. 



If the vagus is excited after bringing the circulation to a 

 standstill by pericardial pressure, the manometer in connection 

 with the pericardial cavity shows a rise of pressure which soon falls 

 again when stimulation ceases. If these oscillations of the mano- 

 meter be recorded on a rotating cylinder, the following tracing 

 (Fig. 7*7) is obtained, which is a new and direct proof that the 

 degree of diastolic dilatation is regulated by the vagus. 



Lastly, in order better to control the theory of diastolic activity, 

 Stefani successfully employed certain poisons (atropine, digitaline, 

 and strychnine), measuring the diastolic pressure before and after 

 injection of the drug, and before and after section of the vagi. 

 He came, in a few words, to the following conclusions : that 



FIG. 77. The recording mercury manometer is connected with the pericardial cavity. Show 

 disappearance of plethysmographic oscillations in the heart, in consequence of hydrostatic 

 pressure exerted within pericardium. The vagus was stimulated in the neck at + , slightly at 

 first, afterwards more strongly. marks close of excitation. During stimulation the heart 

 dilates, and resumes its original volume at the close of excitation. (Stefani.) 



atropine lowers the diastolic pressure because it paralyses the 

 dilator action of the vagi, and that digitaline and strychnine increase 

 the diastolic pressure because they act directly upon the cardiac 

 muscle, and render it capable of active dilatation in excess of the 

 normal. 



Until these important facts determined by Stefani are con- 

 tradicted or shown to be fallacies, we shall continue to regard the 

 theory of diastolic activity (which we formulated in 1871) to be 

 well founded, both from a purely mechanical and from a physio- 

 logical point of view. 



In the ninth chapter we shall discuss certain experimental 

 data which enable us to determine, up to a certain point, the 

 nature of the internal process by which the vagus develops and 

 regulates the activity of the diastole. 



IX. After developing the mechanism of the systolic evacuation 

 and diastolic refill of the heart it is easy to deal with the question 

 of the cardiac beat or pulse, which, like the heart-sounds, con-' 

 stitutes an important external sign in the investigation of this- 



