Bauhaus-Universität Weimar

Handbook for the Physiological Laboratory. Text
Burdon-Sanderson, John Scott E. Klein Michael Foster T. Lauder Brunton
fingers of the right hand are placed under the animal’s back, 
while the thumb is firmly pressed upon the aorta, the beats of the 
needle having been previously counted. On making pressure, the 
frequency of the contractions of the heart is diminished, and this 
effect continues so long as the pressure lasts. 
Both vagi are now divided and the experiment repeated. The 
frequency of the pulse is still slightly diminished, but the degree of 
diminution is not to be compared with the previous effect. This 
experiment can be made with greater exactitude by applying the 
pressure to the aorta directly, at the same time connecting the 
carotid artery with Fick’s kymograph. To accomplish the first 
of these objects, the abdominal cavity is opened in a chloralized 
rabbit in exactly the same way as for excitation of the left splanch¬ 
nic nerve. It is then easy to place the thumb directly on the 
aorta as it passes between the crura of the diaphragm. Tracings 
are thus obtained which show that, during obstruction of the 
aorta, the arterial pressure is doubled or even trebled, and the 
pulse rate much diminished, the status quo being re-established 
when the thumb is removed from the aorta. After division of the 
vagi, the effect as regards pressure is of course as marked as 
before, but there is scarcely any slowing of the pulse. 
The fact that the effect of aortic obstruction in diminishing the 
frequency of the pulse is so markedly weakened by section of both 
vagi, shows that these nerves bear a large part in its production, 
and therefore that the relation between cause and consequence is 
in this case not dependent on the lengthening of the systole by 
^resistance, as supposed by Marey. The question, however, remains, 
«whether the mechanical explanation may not be accepted as regards 
jthe remainder of effect which is observed after the vagi are divided. 
iîThere are two reasons why this is not possible. One is, that here, 
ßas in other cases when the pulse rate is retarded, the retardation 
bdoes not signify that the systole is lengthened, but that the 
{(diastolic intervals are more protracted. The other reason is, that 
^ven after section of the vagi, the retardation of pulse produced by 
increased arterial pressure is postponed, whereas if it were merely 
[mechanical it wrould certainly be immediate. We must therefore 
[Burn to the nervous system for its explanation—either to some 
[influence exercised on the heart by means of accelerator nerves, 
[which after section of the vagi are the only channel by which the 
ojieart is in communication with the cerebro-spinal centres ; or to 
excitation of the inhibitory nerves in the heart itself. Considering 
jdhat in the frog the same effects are produced by exciting the ganglion 
*)>f the vagus in the cut-out heart as by exciting the vagus itself, 
>und that we have no reason to believe that increased pressure pro¬ 
duces any paralyzing influence on the accelerators, we need have 
ifttle hesitation in concluding that the effect of increased blood- 
T 2


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