Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea
Todd, Robert Bentley
circulation of the blood, rapid as it may be, 
takes place in this remarkably short space of 
time, and we are disposed to suspect that the 
experiments themselves are liable to several 
sources of fallacy. The tendency of the prus- 
siate of potass to permeate the textures of the 
body, more freely than any other substance 
known, has been proved by many expe¬ 
riments, and it is therefore necessary that 
Hering’s experiments should be performed with 
some other substances, before they can be re¬ 
garded as a correct means of estimating the 
rapidity of the circulation. 
The velocity of the blood is generally be¬ 
lieved to be greater in the pulmonic than in 
the systemic circulation,—an opinion founded 
chiefly on the supposed less capacity of the 
vessels belonging to the pulmonary trunks. 
Actual measurements of the velocity of the 
blood in the capillaries of the lungs of cold¬ 
blooded animals by Hales, Spallanzani, and 
others, would seem to give support to this 
view, but it must at the same time be re¬ 
collected that the course through which the 
blood passes in the pulmonary or lesser circu¬ 
lation, is considerably shorter upon the whole 
than that of the systemic or greater,—a circum¬ 
stance which must diminish to a certain extent 
the disproportion in the velocity.* 
b. Force of the blood in the arteries and force 
of the heart.—Another interesting inquiry con¬ 
nected with this subject relates to the force 
with which the blood is impelled in the arte¬ 
ries, and the calculations that have been made 
of the power of the heart itself, from the ob¬ 
servation of the force of the blood in the arte¬ 
ries. The experiments made with a view to 
discover these forces appear sufficiently simple 
in their nature ; but the calculations founded 
upon the experiments have differed so widely, 
as to have furnished a plausible pretext for 
throwing ridicule on the application of physical 
laws to the living animal functions. 
As the arteries and other vessels are kept 
distended with blood by the action of the 
heart, it follows that were they rigid tubes, 
the force of the heart would, in accordance 
with the laws of propagation of pressure 
through fluids, be transmitted without loss 
through the whole column of blood in the 
arteries at one and the same moment : but in 
consequence of their yielding to distension, 
the force of the heart operates upon the blood 
only through the elastic reaction of the coats 
of the arteries. 
When an opening is made into one of the 
larger arteries, the blood issues with force, and 
spouts to some distance, but the height to 
* In reference to the above calculations, it must 
also be kept in mind, in the first place, that the 
estimate of tue velocity of the blood in the pul¬ 
monic circulation in the frog can scarcely with 
propriety be applied to man, seeing that in the 
frog the pulmonary artery is only a branch of the 
aorta; and, in the second place, that in animals 
with a double circulation, although the quantity 
of blood which leaves both sides of the heart at 
each systole be equal, it does not necessarily follow 
that the whole blood which circulates through the 
system should in the same time pass through the 
which the blood rises when allowed to escape 
from a simple aperture in an artery varies from 
many accidental circumstances, and cannot 
therefore be taken as affording an accurate 
measure of the force with which the blood 
moves within the vessels. 
Hales seems first to have investigated this 
force in a more accurate and experimental 
manner, by observing the weight which the 
blood in one of the arteries of a living animal 
is capable of sustaining within a tube adapted 
to it. He remarked that the blood issuing 
from a simple aperture in the carotid artery of 
a horse and directed upwards did not rise 
above three feet,* but that when the blood was 
allowed to pass into a long glass tube adapted 
to the same artery it rose very quickly to a 
much greater height, as to nearly ten feet in 
some of the experiments. Hales performed 
similar experiments on the arterial flow in 
sheep, oxen, dogs, and other animals, and 
after observing for each the pressure which the 
blood in the arteries is usually capable of ex¬ 
erting, he endeavoured to compute the pres¬ 
sure of the blood in the arteries of man, by 
a comparison of the size of his whole body or 
heart and bloodvessels with those of the other 
animals. The pressure of the blood in the 
aorta of the horse being considered as eleven 
pounds, Hales estimates in the way above- 
mentioned the force of the blood in the human 
aorta at 4 lbs. 6 oz. ; seven and a half feet 
being the height to which he supposed that the 
blood would rise in a tube connected with the 
larger arteries of a man. 
These experiments of Hales shewed in a 
very clear manner, that the height to which 
the blood rises in one of the larger arteries 
affords us the means of ascertaining directly 
the amount of pressure which the stream of 
blood impelled by the heart through the arte¬ 
ries is capable of exerting at any part of the 
arterial system, or in other words it gives us a 
measure of the statical force of the heart as it 
operates through the arterial tubes.f 
According to a well-known law of physics, 
the heart must be pressed upon in every part 
of its internal surface by the column of blood 
which it has raised; so that by multiplying 
the area of the internal surface of the ventricle 
into the height of the column of blood sup¬ 
ported in the tube connected with an artery, 
we shall ascertain the pressure which acts 
backwards on the inner surface of the heart. 
Hales estimates the inner surface of the ven¬ 
tricle of the human heart at fifteen square 
inches, and multiplying the pressure of a co- 
* This experiment we have repeated with Mr. 
Dick’s assistance. 
f These experiments, as well as others subse¬ 
quently per ormed, demonstrate the importance of 
confining our researches in an inquiry of this nature 
to the estimation of the statical force operating in 
the organs of circulation, as the only useful ob¬ 
ject of such calculations,—the propriety of which 
is also sufficiently apparent from the extraordinary 
results of the attempts to estimate the dynamical 
power of the heart or the whole force generated 
in that organ by muscular contraction, by Borelli 
and Bernouilli, the first of whom calculated this 
force to equal 180,000, the second 3,000 lbs.


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