ON THE FWNCTieNS OF THE HEART AKD ARTERIES. ^Q 



In order to calculate the magnitude of the resistance, it Calculation of 

 is necessary to determine the dimensions of the nrterial sys- ^ ^ resistancco 

 tem, and the velocity of the blood which flows through it. 

 According to the measurements of Keill and others, we 

 may take ^ of an inch for the usual diameter of the aorta, 

 and suppose each arterial trunk to be divided into two 

 branches, the diameter of each being about |- of that of the 

 trunk, (or more accurately, I : 1'26 — 10 -^oosctj^ ^^^ ^^^^ 

 joint areas of the sections about a fourth part greater, (or 

 1-2586: 1 — 10°5='"5*). This division must be continued 

 twenty-nine times, so that the diameter of the thirtieth seg- 

 ment may be only the eleven hundredth part of an inch, 

 that is, nearly large enough to admit two globules of the 

 blood to pass at once. The length of the first segment 

 must be assumed about nine inches, that of the last, the 

 twentieth of an inch only; and supposing the lengths of 

 the intermediate segments to be a series of mean propor- 

 tionals, each of them must be about one sixth part bhorter 

 than the preceding, (or 1 : l*g6l — 10~"°''^^), the mean 

 length of the whole forty-six inches, the capacity to that of 

 the first segment as 72'71 to 1, and consequently the weight 

 of the blood contained in the arterial system about 9-7 

 pounds. It is probable that this calculation approaches Quantity of 

 sufficiently near to the truth: for the whole quantity of '^^'^^'^ '" ^^* 

 blood in the body being about 40 pounds, although some 

 have supposed it only 20, others no less than 100, there is 

 reason to believe, that half of this quantity is contained in 

 the veins of the general circulation, and that the other half 

 is divided, nearly in equal proportions, between the pulmo- 

 nary system and the remaining arteries of the body, so that 

 the arteries of the general circulation may contain about 9 

 or 10 pounds. Haller allows 50 pounds of <:irculating 

 fluid, partly serous, and partly red, and supposes 4- of this 

 to be contained in all the arteries taken together: but in a 

 determination which must be in great measure conjectural 

 we cannot expect perfect accuracy: and according to Hal- 

 ler's own account of the proportions of the sections of the 

 arteries and veins, the large trunks of the veins appear to 

 be little more than twice as capacious as those of the arte- 

 ries, and the araalier branches much more nearly equal, so 



that 



