202 CIRCULATION OF THE BLOOD 



to eleven atmospheres, the carotid of man seven to eight atmospheres (mean). 

 Since the maximum normal blood pressure in the carotid may be estimated at 

 one-quarter atmosphere, we see that arteries can be ruptured only by a pres- 

 sure twenty-eight to thirty-two times the normal. The resistance of the smaller 

 arteries is still greater. This means that arteries can never be ruptured by 

 excessive blood pressure, unless they have first been abnormally weakened (Hales, 

 Grehant and Quinquaud). 



B. METHODS FOR THE DETERMINATION OF BLOOD PRESSURE 



The mercury manometer is commonly used in determining arterial blood 

 pressure, because it gives directly the absolute value of the pressure without any 

 calculation. The elastic manometer also (page 9) finds wide application where 

 it is desired to follow exactly the variations of pressure accompanying individual 

 heart beats. 



The Hg-manometer is not adapted for following rapid variations of pressure 

 because of the inertia of the Hg column, which causes the maxima and minima 

 corresponding to systole and diastole to be incorrectly reproduced. With a 

 slow rhythm the maxima are too high, the minima too low ; while with a quicker 

 rhythm the maxima are too low, the minima too high. 



But a tolerably satisfactory value of the mean pressure for a given time can 

 be obtained by means of the Hg-manometer in the following way (v. Kries) : In 

 the figure given on page 8 the smaller oscillations on the curve represent indi- 

 vidual heart beats, the larger represent pressure variations caused by the respira- 

 tory movements ; the line a b is the line of no pressure, and the line T gives the 

 time in seconds. When the mercury moves up the free limb of the manometer, 

 it naturally falls just as much in the other limb (Fig. 3). If we neglect the 

 error due to inertia of the Hg. column, the blood pressure at any instant is 

 therefore twice the distance of the curve from the line of no pressure. Hence, 

 in order to determine the mean blood pressure e. g., during the period a to fr, 

 vertical lines are drawn from a and ~b to the curve, then the surface a b d c, 

 is measured in square millimeters and is divided by the line a b. The quotient 

 is the height of a rectangle of the same surface as a ~b d c, and with a base a b. 

 This height doubled is the mean pressure in millimeters of mercury. 



If the pressure curve presents no very great variations but runs along with 

 perfectly regular oscillations as in Fig. 8 the mean pressure can be determined 

 by simply measuring the highest and lowest points during the period, and doub- 

 ling the mean of these two. 



The mode of connection of the cannula with the artery must be borne in 

 mind in interpreting the pressure obtained. If a T-cannula be used, and the 

 unpaired limb be connected with a manometer, so that the current in the artery 

 is not interrupted, the manometer records the lateral pressure of the blood in 

 this particular artery at the place where the cannula is inserted. But as a rule 

 it is more convenient to make the cannula terminal to the central end of the 

 artery, so that the manometer records the lateral pressure of the larger artery 

 of which the one used is a branch. Thus a cannula in the central end of the 

 carotid gives the lateral pressure of the blood in the aorta. 



Several methods, which proceed upon the principle of finding the pressure 

 necessary to stop the blood flow in the artery investigated, have been devised for 

 determining the blood pressure in man. 



The sphygmomanometer of v. Basch consists of a button or plunger, bound 

 to a metal manometer by means of a rubber tube. The button is placed over 

 some superficial artery (preferably one that is supported on a solid substratum 



