Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea
Todd, Robert Bentley
luinn of blood of seven feet and a half high 
into the area of the inner surface of the heart : 
he hence calculates the pressure on the in¬ 
ner surface of the human heart to be nearly 
51£ lbs. The pressure on the interior of the 
horse’s heart he estimates at 113 lbs. upon 
similar principles. 
As pressure applied in any direction to a 
fluid column is equally transmitted through all 
its parts, and as the blood in the arteries forms 
continuous columns which all branch off from 
the aorta, it might a pj’iori have been con¬ 
cluded that the force of the blood must be the 
same in all the arteries of any considerable 
size. Hales, though he does not state this 
proposition very explicitly, seems yet to have 
taken it for granted ; for, in estimating the 
pressure of the heart, he takes into account 
merely the height of the column without re¬ 
ference to the size of the artery. We shall 
find this proposition to be satisfactorily proved 
to be correct by direct experiments subse¬ 
quently performed. 
The experiments of Hales were liable to two 
principal objections: 1st, that the coagulation 
of the blood in the long glass tube adapted to 
the artery must have prevented its free motion ; 
and, 2nd, that the length of the tube, besides 
giving rise to the necessity of frequently re¬ 
moving it and various other inconveniences, 
must have occasioned a considerable loss of 
blood in filling from the arteries of small ani¬ 
mals. Both these sources of fallacy have been 
provided against most successfully by M. 
Poiseuille,* an ingenious ex- 
Fig. 329. perimenter of Paris, who, by 
the adoption of a simple con¬ 
trivance, has been enabled to 
measure with great accuracy 
the arterial pressure of the blocd, 
and has thus confirmed and 
extended the interesting re¬ 
searches of Hales. 
The instrument employed by 
Poiseuille, to which he gives 
the name of Hemadynamome- 
, , n, ter> (M- 329>) 
—--rggjr--^ consists of a bent 
glass tube of the 
form here repre- 
a sented, filled with mercury in 
the lower bent part (a, d, e). 
The horizontal part (6),provided 
with a brass head, is fitted into 
the artery, and a little of a solu¬ 
tion of carbonate of soda is 
{interposed between the mercury 
and the blood which is allowed 
to enter the tube for the pur¬ 
pose of preventing its coagula¬ 
tion. When the blood is al¬ 
lowed to press upon the fluid 
in the horizontal limb, the rise 
of the mercury towards (c) 
measured from the level to 
Poiseuille S He- which it has fallen towards (d) 
madynatnometer. gives the pressure under which 
the blood moves. 
* Magendie’s Journal, vols. viii. & ix. Breschet’s 
Repert. d’Anat. et de Physiol. 1826. 
One of the most important facts established 
by Poiseuille’s experiments is, that the pressure 
of the blood is within certain limits nearly the 
same in arteries of very different calibre and 
at different distances from the heart; as proved 
by the rise of the mercury of the hetnadyna- 
mometer to nearly an equal height when this 
instrument was connected with the iliac, caro¬ 
tid, radial, facial, and other arteries in some 
of the lower animals. It is hence apparent, 
that, in order to ascertain the whole amount 
of force with which the blood is propelled 
in the aorta, or the statical force of the 
heart itself, it is sufficient to measure by 
means of the tube the momentum of the 
blood in any one of the arteries. Poiseuille 
estimates the force with which the blood is 
propelled in the commencement of the aorta 
m naan at 4 lbs. 3 oz.,—a result which agrees 
remarkably with that obtained by Hales* 
Poiseuille, however, considers the pressure 
backwards within the heart to amount to 13 lbs. 
only, as he calculates this in a different way 
from that followed by Hales, viz. by multi¬ 
plying the pressure of the blood in the aorta 
into the surface of a plane passed through the 
base and apex of the left ventricle,—a mode 
of calculation which it appears that Dr. Hales 
had not lost sight of; fur, at page 21 of the 
work on Hemastatics, he proposes it as the 
“ means of estimating the force of the blood 
which the muscular fibres of the ventricle must 
Poiseuille estimates the force with which the 
blood moves in the radial artery of man at four 
Hales had remarked that the blood in the 
tube connected with an artery rose regularly a 
little way at each systole of the ventricle, and 
remained always somewhat higher during the 
straining of the animal, that is, while the 
muscles of expiration were in action. These 
phenomena, known to Haller, were demon¬ 
strated experimentally by Magendie, and re¬ 
ceive a still more decided confirmation from 
the experiments of Poiseuille made with the 
We would here remark that, it having been 
shewn by the above-mentioned experiments 
that the force of the heart is sensibly the same 
in the trunks and larger branches of the arte¬ 
ries, it is manifest that the angles of rami¬ 
fication and the friction of the blood against 
the sides of the vessels can give rise to very little 
if any diminution in the force of the heart 
transmitted by the elasticity of the arterial 
parittes. We shall afterwards see that the 
case is very different in the smaller vessels. 
VVe would also call the attention of the 
reader to an interesting application of the fact 
of the complete transmission of pressure through 
the fluid contained within the bloodvessels in 
all directions, in the immense force which the 
* The power of the heart has also been calcu¬ 
lated from the force supposed necessary to raise 
the foot of one of the legs thrown across the other 
in the pulsatory movement which is then seen to 
occur,—one of the most inaccurate methods that 
could be adopted. 
t See Part IV. of this article.


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