VELOCITY AND PRESSURE OF BLOOD-FLOW. 487 



an artery is cut the blood flows out in a forcible stream and with 

 spurts corresponding to the heart beats. When a large vein is 

 wounded, on the contrary, although the blood flows out rapidly, 

 the stream has little force. Exact measurements of the hydrostatic 

 pressure under which the blood exists in the large arteries apd veins 

 were first published by Rev. Dr. Stephen Hales, an English clergy- 

 man, in his famous book entitled "Statical Essays, containing 

 Haemostaticks," 1733.* This observer measured the static pressure 

 of the blood in the arteries and veins by the simplest direct method 

 possible. After tying the femoral artery in a horse he connected 

 it to a glass tube 9 feet in length. On opening the vessel the blood 

 mounted in the tube to a height of 8 feet 3 inches, showing that 

 normally in the closed artery the blood is under a tension or pressure 

 sufficient to support the weight of a column of blood of this height. 

 A similar experiment made upon the vein showed a rise of only 12 

 inches. 



Methods of Recording Blood-pressure. — Since Hales's work 

 the chief improvements in method which have marked and caused 

 the development of this part of the subject have been the application 

 of the mercury manometer by Poiseuillef (1828), the invention of 

 the recording manometer and kymographion by LudwigJ (1847), 

 and the later numerous improvements by many physiologists, and 

 latterly the development of methods for measuring blood-pressures 

 directly in man. The Hales method of measuring arterial pressure 

 directly in terms of a column of blood is inconvenient on account 

 of the great height, large fluctuations, and rapid clotting. The 

 two former disadvantages are overcome by using a column of mer- 

 cury. Since this metal is 13.5 times as heavy as blood, the column 

 which will be supported by the blood will be correspondingly shorter 

 and all the fluctuations will be similarly reduced. Poiseuille 

 placed the mercury in a U tube of the general form shown in Fig. 

 192, M. One leg was connected with the interior of an artery by 

 appropriate tubing filled with liquid and when the clamp was 

 removed from the vessel its pressure displaced the mercary in the 

 limbs by a certain amount. The difference in height between the 

 levels of the mercury in the two limbs in each experiment gives the 

 blood pressure, which is therefore usually expressed as being equal 

 to so many millimeters of mercury. By this expression it is meant 

 that the pressure within the artery is able to support a column 

 of mercury that many millimeters in height, and by multiplying 

 this value by 13.5 the pressure can be obtained, when desirable, 



* For an account of the life and works of this physiologist see Dawson, 

 '^ rhe Johns Hopkins Hospital Bulletin," vol. xv, Nos. 159 to 161, 1904. 

 t Poiseuille, "Recherches sur la force du coeur aortique." Paris, 1828. 

 t Ludwig, "MiiUer's Archiv f. Anatomie. Physiologie. etc.." 1847, p. 242 



