THE CONTROL OF THE FUNCTION OF THE HEART 523 

 40 r 



efferent \agal activity, provided of course that the 

 observed changes are within one circulation time. 

 Figure 27 (upper) illustrates the depressant effect of 

 carotid sinus ner\e stimulation on the "a" wave of 

 the paced atrium in a dog with surgically induced 

 heart block. Two or three atrial "a" waves appear in 

 the interval between each ventricular beat. The 

 segmented panel at the left is the control tracing; the 

 middle, that during stimulation of the carotid sinus 

 ner\e; and the right is again a control tracing. The 

 long lower tracing shows the rapidity of the onset and 

 wearing off of this effect. In figure 27 (lower) the 

 blocking of the response with atropine demonstrates 

 that the response is achieved by vagal efferent fibers. 

 The upper set of four panels are from left to right : 

 control, carotid sinus nerve stimulation, control, and 

 carotid sinus ner\e stimulation. The stimulation was 

 then maintained during the 70-sec interval until the 

 beginning of the long lower panel (below) 12 sec 

 before which 4.0 mg of atropine sulfate was adminis- 

 tered intravenously. 



The depression of the atrial '"a" wave observed 

 with carotid sinus nerve stimulation is comparable to 

 changes obtainable with direct \'agal stimulation. 



THE c.-^ROTiDO-SYMPATHO-ATRiAL REFLEX. Ill experi- 

 ments on this reflex a bilateral cervical vagotomy is 



FIG. 28^4. L.^ = left atrial pressure. 



done so that the effects of changing carotid pressure 

 or of carotid sinus nerve stimulation on the contrac- 

 tion of the atrium can be attributed to a sympathetic 

 efferent pathway. The dog with surgically induced 

 heart block is used. In figure 28.-I the left panel is the 

 control tracing, and the right panel is the tracing 12 

 sec after beginning of left carotid sinus nerve stimula- 

 tion. Carotid sinus nerve stimulation depresses the 

 ""a"" wave of the paced atrium and its reflected effect 

 on left ventricular diastolic pressure. 



The interplay between sympathetic and para- 

 sympathetic influences on the atrium in a heart block 

 preparation is shown in figure 28ZJ. The atrium was 

 not paced. In the upper panel of this figure, the atrial 

 "a" wave was diminished by keeping carotid pressure 

 high. To this was added sufficient efferent vagal nerve 

 stimulation so as to all but abolish the atrial "a" 

 wave as well as produce atrial slowing (middle panel). 

 With the vagal stimulus kept constant, the carotid 

 pressure was then lowered; the increased "'a" waves 

 and re-established atrial rate are shown in the lower 

 panel. The slowing of the ventricular rate produced 

 by vagal stimulation (middle panel) in the dog with 

 heart block confirms the 1906 experiments of Er- 

 langer (35). 



