CIRCULATION. 



661 



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 



1 In reference to the above calculations, it must 

 also be kept in mind, in the first place, that the 

 estimate of the 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 

 lungs. 



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 Ibs. 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. 



t These experiments, as well as others subse- 

 quently performed, 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 JJorelli 

 and Bernouilli, the first of whom calculated this 

 force to equal 180,000, the second 3,000 Ibs. 



