THE BLOOD-PRESSURE 875 



as the pulse pressure. Thus in the dog, with a mean pressure of about 

 120 mm. Hg in the aorta, the systolic pressure may be as much as 160, 

 while the diastolic pressure is only 100 mm. In this case the pulse pressure 

 would be 60 mm. Hg. In man the systolic pressure, as measured in the 

 brachial artery, is under normal conditions about 110 mm., while the 

 diastolic pressure is only 65 to 75 mm., so that the pulse pressure is about 

 45 mm. Hg. As we pass outwards towards the periphery the pulse pressure 

 becomes less and less marked, until finally in the capillaries and veins 

 there is no pulse- wave perceptible. 



THE DETERMINATION OF THE BLOOD-PRESSURE IN MAN 



It is important for clinical purposes to be able to determine even approximately the 

 blood-pressure in the different parts of the vascular system in man, and various methods 

 have baen devised for this purpose. The determination of the systolic blood-pressure 

 in the arteries is easily carried out by the use of Riva Rocci's sphygmomanometer. 

 This apparatus (Fig. 377) consists of a leather or canvas band about 10 cm. wide, which 

 can be buckled closely round the upper arm. Inside this band is a rubber bag of the 

 same shape, which communicates by a rubber tube with a mercurial manometer and by 



UP* 



FIG. 377. Riva Rocci's sphygmomanometer. 

 (C. J. Martin's pattern. HAWKSLEY.) 



B 



FIG. 378. 



a three-way tap with a pressure bulb or bicycle pump, or with the external air. The 

 band is buckled round the arm and the fingers of the observer are placed on the radial 

 pulse. The bag is then distended with air so that it exercises a pressure on the arm, 

 the pressure being indicated on the mercurial manometer. Air is forced in until the 

 radial pulse disappears. By means of the three-way tap the air is then let slowly out 

 of the bag until the radial pulse is just perceptible. The height of the mercurial mano- 

 meter at this moment is equal to the systolic pressure in the main arterial trunk from 

 which the brachial artery takes origin. The principle of this method will be made 

 clear by reference to the diagram (Fig. 378). If we imagine A as a segment of the brachial 

 artery passing through the tissues which are surrounded by the rubber bag, we see that 

 so long as the pressure in the interior of the artery is greater than that on the exterior 

 exerted by the tissues, the artery will be patent and the pulse can pass through. If, 

 however, the pressure in the tissues becomes greater than the maximum pressure 

 inside the artery at any time of the heart-beat, the segment of artery will collapse (as 

 in B), thus stopping the transmission of blood and of the pulse-wave. If we exclude the 

 elasticity of the tissues themselves we may take the pressure in the bag as representing 

 the pressure in the tissue fluids surrounding the artery, so that- the pulse-obliterating 

 pressure in the bag will correspond to the maximum or systolic pressure in the artery. 

 By a slight modification of the apparatus it is possible to determine also the diastolic 

 pressure. For this purpose the rubber bag is connected also with a manometer of small 

 inertia, giving a true representation of the actual changes of pressure. It is evident 



