VELOCITY AND PRESSURE OF BLOOD-FLOW. 455 



by some means the pulsations of the artery under different pressures and de- 

 termining under what pressure the maximal pulsations are given. This pres- 

 sure should be equal to the diastolic pressure within the artery. The prin- 

 ciple involved may be illustrated by the accompanying figure (Fig. 189). 



Let a represent a longitudinal section of an artery distended by normal 

 diastolic arterial pressure. At each heart beat the force of the pulse will dis- 

 tend the artery still more, as represented by the dotted lines, and this in- 

 crease in size may be measured by proper transmitting apparatus. If now 

 pressure is brought to bear upon the outside of the artery its lumen 

 will be diminished as the outside pressure is increased, and when this pres- 

 sure is equal to the diastolic blood-pressure within the artery one will neu- 

 tralize the other, and the diameter of the artery will be equal to that assumed 

 when the vessel contains blood under no pressure and is kept patent only by 

 the stiffness of its walls (6). Under this condition the pulse wave when it 

 traverses this portion of the vessel finds its walls completely relaxed, as it 

 were, and the force of the heart wave will consequently cause a greater dis- 

 tension of the arterial walls and a larger pulse wave in the recording ap- 

 paratus. If the outside pressure is increased beyond the amount of diastolic 

 pressure it will not only neutralize this latter, but will tend to overcome the 

 stiffness of the arterial wall. When the pulse wave passes through this stretch 

 it will be forced not only to distend the walls, but also to overcome the excess 



Fig. 190. Record (Erlanger) to show the maximum size of the recorded pulse 

 wave when the outside or extravascular pressure is equal to the internal diastolic pressure. 

 The artery is compressed first with a pressure above systolic, sufficient to obliterate the 

 lumen. As this pressure is lowered in steps of 5 mms. the recorded pulse wave increases in 

 size to a maximum and then again becomes smaller. The outside pressure with which the 

 maximum pulse is obtained measures the amount of the internal diastolic pressure (Marey's 

 principle) . 



of pressure on the outside. The movement of the walls with the pulse wave 

 will be less extensive in proportion to the excess of pressure on the outside. 

 If, therefore, one starts with an outside pressure sufficient to obliterate the 

 artery completely the recorded pulse wave will be small. As this pressure 

 is diminished, the pulse waves become larger up to a certain point and then 

 decrease again in size (see Fig. 190). The outside pressure at which this 

 maximum pulse is obtained measures, according to the principle stated above, 

 the diastolic pressure within the artery. That the principle is correct has 

 been shown by direct experiments upon the exposed artery of a dog, in which 

 the pressure was measured by the method outlined above and also directly 

 by a manometer connected with the interior of the artery.* In such experi- 

 ments upon man, however, one condition is present which detracts from the 

 absolute value of the results obtained, although, since it is substantially 

 a constant factor, it does not seriously interfere with relative results, that 

 is, with observations upon the variations of pressure under different condi- 

 tions. This source of error lies in the fact that in the living person the out- 

 side pressure can not be applied directly to the arteries, but only indirectly 

 through the intervening tissues. These tissues interpose a certain resistance 

 to the pressure exerted from without, and some of this pressure must be spent 



* Howell and Brush, " Proceedings of the Massachusetts Medical Society," 



