218 



PHYSIOLOGY 



CHAP. 



From this Stefani draws the logical conclusion that both the 

 first and second periods of the plethysmograrn are the effect of 

 physiological activity while only the third can be regarded as the 

 effect of repose of the heart. 



Fi<;. 73. Pli'thysmogram showing that stimulation of oii> vagus by strong induction currents 

 has no e fleet on systolic evacuation, but conspicuously increases the period of passive refill. 



(c] If the vagus be excited, either directly or by cessation of 

 artificial respiration (which has the effect of increasing the venosity 

 of the blood), profound changes appear in the cardiac -plethys- 

 inograms, varying in form according to the degree of excitation 

 and the excitability of the nerve, but all having this in common 

 that the ascending line of the diastolic refill has a considerable 

 upward lift, which signifies that the heart acquires a greater 

 volume. (Tracings of Figs. 71, 72, 73, 74, 75.) 



This phenomenon cannot be dependent on the slowing or 

 suspension of the beats of the heart, because on the one hand the 

 increase of volume occurs rapidly and in a degree which far exceeds 

 the normal, and on the other there is a marked demarcation 

 between that part of the line of refill which corresponds with active 

 and that which corresponds with passive diastole ; and finally 

 because the heart is able on excitation of the vagus to attain its 

 maximum volume before the venous pressure has had time to rise 



to any considerable 

 extent (Fig. 76). 



Stefani considers 

 that these facts con- 

 firm the doctrine 

 we have maintained 

 since 1871, to the 



Fio. 74. Plethysrnogram in which gentle excitation of one vagus effect that the VagUS 

 (at +) makes systolic evacuation incomplete, while it increases rlioofrJin narxro 



diastolic refill. 1S a OUdSlOllC nerve, 



in the sense that 



" it actively provokes increase of volume in the heart, by modify- 

 ing the physiological condition of the cardiac muscles." 



This same theory of the mode of action of the vagus upon the 

 heart (to be discussed in detail in the next chapter) was successively 

 put forward by Eossbach (1882), Heidenhain (1882), Williams 

 (1887), Tigerstedt and Johannssohn (1889), and also by Francois- 



