378 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



times as great as that in the smaller veins of the head. As, during such an 

 experiment, the blood is free to pass from the aorta through one carotid and 

 both vertebral arteries to the head, and to return through all the veins of 

 that part, except one external jugular, to the vena cava, it is demonstrated 

 that there must be a continuous flow from the aorta, through the capillaries 

 of the head, into the veins, because the pressure in the aorta is many times as 

 great as the pressure in the veins. Obviously, such an experiment, although 

 very instructive, gives only roughly qualitative results. 



Two things will be noted, moreover, in such an experiment. One is that 

 the venous column is steady ; the other is that the arterial column is perpetu- 

 ally fluctuating in a rhythmic manner. The top of the arterial column shows 

 a regular rise and fall of perhaps a few centimeters, the rhythm of which is 

 the same as that of the breathing of the animal ; and, while the surface is thus 

 rising and falling, it is also the seat of frequent flickering fluctuations of 

 smaller extent, the rhythm of which is regular, and agrees with that of the 

 heart's beat. At no time, however, do the respiratory fluctuations of the arte- 

 rial column amount to more than a fraction of its mean height ; compared to 

 which last, again, the cardiac fluctuations are still smaller. It is clear, then, 

 that the aortic pressure changes with the movements of the chest, and with 

 the systoles and diastoles of the left ventricle. But stress is laid at present 

 upon the fact that the aortic pressure at its lowest is several times as high as 

 the pressure in the smaller veins of the head. Therefore, the occurrence of 

 incessant fluctuations in the aortic pressure cannot prevent the continuous 

 movement of the blood out of the arteries, through the capillaries, into the 

 veins. 



The upright tubes employed in the foregoing experiment are called " man- 

 ometers." l They were first applied to the measurement of the arterial and 

 venous blood-pressures by a clergyman of the Church of England, Stephen 

 Hales, rector of Farringdon in Hampshire, who experimented with them 

 upon the horse first, and afterward upon other mammals. He published his 

 method and results in 1733. 2 The height of the manometric column is a 

 true measure of the pressure which sustains it ; for the force derived from 

 gravity with which the blood in the tube presses downward at its lower open- 

 ing is exactly equal to the force with which the blood in the artery or vein is 

 pressed upward at the same opening. The downward force exerted by the 

 column of blood varies directly with the height of the column, but, by the laws 

 of fluid pressure, does not vary with the calibre of the manometer, which cali- 

 bre may therefore be settled on other grounds. It follows also that the arterial 

 and venous manometers need not be of the same calibre. Were, however, 

 another fluid than the blood itself used in the manometer to measure a given 

 intravascular pressure, as is easily possible, the height of the column would 

 diifer from that of the column of blood. For a given pressure the height 



1 From /uavds, rare. The name was given from such tubes being used to measure the tension 

 of gases. 



2 Stephen Hales : Statical Essays : containing Hcemastaticks, etc., London, 1733, vol. ii. p. 1. 



