VII 



MECHANICS OF THE HEART 



219 



FIG. 75. Plethysmogram showing that .strong ex- 

 citation of one vagus (at +) exaggerates effect 

 shown in preceding figure. 



Franck (1891), who in 1877 had pronounced in precisely the 

 opposite sense. 



It thus became necessary to determine more exactly the nature 

 of the action of the vagus, in so far as it is a diastolic nerve, capable, 

 i.e., of modifying the internal 

 physiological state of the 

 cardiac muscles. With this 

 object, Stefani undertook to 

 measure the pressure that 

 must be exerted on the outer 

 surface of the heart in order 

 to arrest the circulation of 

 the blood. This he compared 

 with the pressure simultane- 

 ously exerted upon its in- 

 ternal surface, and noted the 

 changes in the difference between the two pressures, according as 

 the vagus was excited or paralysed. From this ingenious and 

 original research Stefani obtained most important results, which 

 we may summarise in a few words. 



A pericardia 1 fistula was made on a dog, the cavity being 

 connected by a T-shaped tube, on the one hand with a pressure 

 bottle filled with 1 per cent solution of NaCl, on the other with 

 a mercury manometer. When the fluid is made to descend into 

 the cavity of the pericardium, the manometer measures the pressure 

 exerted on the outside of the heart. Another manometer, in which 



an alkaline, anti-coagulant 

 solution is substituted for 

 mercury, is connected with 

 a glass tube Introduced in- 

 to the vena cave superior, 

 in order to measure the 

 venous pressure acting 

 upon the internal surface 

 of the heart. A third 

 mercury manometer can 

 be connected with the 

 carotid in order to meas- 



, . , 



At a, a (the corre- lire the arterial pressure. 



In proportion with the 

 increase of pericardial 

 pressure, the venous pres- 

 sure increases owing to the obstruction of the flow to the heart, 

 while arterial pressure diminishes owing to the diminished out- 

 put from the heart. When the pericardial pressure is so high 

 that it completely inhibits the flow of blood to the heart and 

 circulation is arrested, the arterial pressure rises to 15-20 mm. Hg 



FIG. 7G. The upper curve represents the venous pressure 

 the lower the plethysmogram. At a, a (the corre 

 spending points on either curve) one vagus was strongly 

 excited. The two curves show that the augmentation 

 of cardiac volume corresponds to a slight fall of venous 

 blood pressure. , 



