W. p. GEIS AND M. P. KAYE 
301 
entering the heart along the superior margin 
of the LA while the cardio-accelerator fibers 
and those to the RV enter the heart along other 
routes. 
Atrial Innervation: Parasympathetic 
In all experiments a decrement in contractility- 
occurred in both atria during electrical stimula- 
tion of the right and left thoracic vagus nerves, 
the right and left cervical vagosympathetic 
trunks, the craniovagal nerve and the caudo- 
vagal nerve. Stimulation of the recurrent car- 
diac nerve resulted in depression of RA and LA 
contractility in 11 of 17 experiments, v^hile 
stimulation of the innominate nerve caused the 
response in the RA in 5 animals and in the LA 
in 2 animals. 
! Right Atrium 
The parasympathetic response in the RA 
was interrupted in all experiments to stimu- 
lation of the left thoracic vagus, left cervical 
vagosympathetic trunk, craniovagal nerve, re- 
current cardiac nerve and innominate nerve 
following a combination of two procedures: 
transection of the SVC and transection of 
nerves along the root of the great vessels. In 
the majority of experiments either intervention 
resulted in depression of the magnitude of re- 
sponse, but only rarely did either of the inter- 
ventions completely abolish the response. In- 
terruption of the parasympathetic response in 
the RA to stimulation of the right cervical 
vagosympathetic trunk and caudovagal nerve 
also required transection of the nerves along 
the root of the great vessels and transection of 
the SVC ; however, in 3 experiments, dissection 
of the inter-atrial groove was necessary to com- 
j pletely abolish the response to these two nerves. 
The response to the right thoracic vagus nerve 
stimulation was not abolished by transections 
along the root of the great vessels. In contrast, 
the response was abolished in the majority of 
experiments by a combination of transection of 
the SVC and dissection of the inter-atrial 
I groove. In 3 experiments, the parasympathetic 
' response persisted until transection of the in- 
ferior margin of the LA was performed. 
Left Atrium 
The parasympathetic response in the LA 
during stimulation of the recurrent cardiac 
nerve was on all occasions interrupted after 
transection of the SVC whereas the response 
to innominate nerve stimulation was inter- 
rupted after transection of the superior LA. 
The response to stimulation of the right cervi- 
cal vagosympathetic trunk and the craniovagal 
nerve was also interrupted following transec- 
tion of the SVC in the majority of experiments. 
However, on two occasions, the response was 
not totally abolished to right vagosympathetic 
trunk stimulation until transection of nerves 
at the root of the great vessels was performed ; 
in one experiment, the response to craniovagal 
nerve stimulation was totally abolished follow- 
ing transection along the great vessels. The 
response to left cervical vagosympathetic trunk 
and left thoracic vagus nerve stimulation was 
abolished following transection of the superior 
LA in the majority of instances. In two experi- 
ments, transection at the root of the great ves- 
sels was necessary to completely abolish the 
response to left vagosympathetic trunk stim- 
ulation, and in one experiment the same pro- 
cedure was necessary to interrupt the response 
to left thoracic vagus nerve stimulation. Inter- 
ruption of the parasympathetic response in the 
LA to caudovagal nerve and right thoracic 
vagus nerve stimulation required both tran- 
section of the SVC and transection of the in- 
ferior margin of the LA. Moreover, in one ex- 
periment, transection at the root of the great 
vessels was also required to totally interrupt 
the response to caudovagal nerve stimulation. 
Note in Figure 3 that following left SVC 
transection, RA and LA contractile force (RAF 
and LAF) continue to be depressed during 
stimulation of the left cervical vagus nerve. 
However, following transection of the right 
SVC (column-right SVC), RA contractile force 
is no longer depressed, while LAF continues to 
be depressed with stimulation. After transec- 
tion along the pulmonary artery (column-left 
PA), stimulation no longer elTects depression 
of LAF. Thus, parasympathetic fibers from 
this nerve to the RA enter the heart along the 
right side of the SVC, while parasympathetic 
