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 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 tlie 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.-t
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 lumn 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 511 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 priori have been con cluded that the force of the blood must be the same in all the arteries of any considerable size. IIales, 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 Ilales were liable to two principal objections : 1 st, 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 fillincr from the arteries of small ani mals. Both these sources of fallacy have been provided against most successfully by M.