THE CIRCULATION OF THE BLOOD. 275 
The effect of vagus stimulation on the blood-pressure is 
always very marked, as would be supposed. To examine-an 
extreme case, suppose the heart arrested for.a few seconds, the 
elastic recoil of the arteries continues to maintain for a time 
the blood-pressure, though there is, of course, an immediate 
and pronounced fall. And it may bé remarked, by-the-way, 
that in cases of fainting, when the heart ceases to beat, or beats 
in the feeblest man- 
ner, the importance 
of this arterial elas- 
ticity as a force, 
maintaining the 
circulation for sev- 
eral seconds at 
least, is of great 
importance. 
As seen in the 
tracing, the beats, 
when the _ heart 
i . Fic. 242.—Tracing from a rabbit, showing the influence of car- 
commences its ac diac inhibition on blood-pressure. ‘he fall in this case 
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at i was very rapid, owing to sudden cessation of the heart- 
tion agains tell ss beat. The relative eri pines of the vessels accounts for 
the comparatively the peculiar character of the curve of rising blood-pressure 
(Foster). 
slack walls of the 
arteries, distending them greatly, and this may be made evident 
by the sphygmograph as well as the manometer; indeed, may 
be evident to the finger, the pulse resembling in some features 
that following excessive loss of blood. 
If the heart has been’ merely slowed, or its pulsation weak- 
ened, the effects will of course be less marked. 
The Quantity of Blood—The blood-pressure may also be 
augmented, the cardiac frequency remaining the same, by 
the quantity of blood ejected from the ventricles, which again 
depends on the quantity entering them, a factor determined 
by the condition of the vessels, and to this we shall presently 
turn, 
In consequence of changes in different parts of the system 
by way of compensation, results follow in an animal which 
might not have been anticipated. 
Thus, bleeding, unless to a dangerous extreme, does not 
lower the blood-pressure except temporarily. It is estimated 
that the body can adapt itself to a loss of as much as 3 per 
cent of the body-weight. 
The adaptation is probably not through absorption chiefly, 
