A text-book of physiology for medical students and physicians . tside of the artery its lumenwill 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 VELOCITY AXD PRESSURE OF BLOOD-FLOW. 493 when the vessel contains blood under no pressure and is kept patent only bythe stiffness of its walls (&). Under this condition the pulse wave when ittraverses this portion of the vessel finds its walls completely relaxed, as itwere,
A text-book of physiology for medical students and physicians . tside of the artery its lumenwill 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 VELOCITY AXD PRESSURE OF BLOOD-FLOW. 493 when the vessel contains blood under no pressure and is kept patent only bythe stiffness of its walls (&). Under this condition the pulse wave when ittraverses this portion of the vessel finds its walls completely relaxed, as itwere, and the force of the heart wave will consequently cause a greater dis-tention of the arterial walls and a larger pulse wave in the recording appa-ratus. If the outside pressure is increased beyond the amount of diastolicpressure it will not only neutralize this latter, but will tend to overcome thestiffness of the arterial wall. When the pulse wave passes through this stretchit will be forced not only to distend the walls, but also to overcome the excess. Fig. 204.—Erianger apparatus. The collar for the arm is not shown. The parts may beunderstood by reference to the schema given in Fig. 203. of pressure on the outside. The movement of the walls with the pulse wavewill be less extensive in proportion to the excess of pressure on the , therefore, one starts with an outside pressure sufficient to obliterate theartery completely the recorded pulse wave will be small. As this pressureis diminished, the pulse waves become larger up to a certain point and thendecrease again in size (see Fig. 202). The outside pressure at which thismaximum pulse is obtained measures, according to the principle stated above,the diastolic pressure within the artery. That the principle is correct hasbeen shown by direct experiments upon the exposed artery of a dog, in whichthe pressure was measured by the method outlined above and also directly 494 CIRCULATION OF BLOOD AND LYMPH. by a manome
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