496 CIRCULATION OF BLOOD AND LYMPH. 



pressure. (2) By using a mercury manometer provided with 

 maximum and minimum valves. Of the manometers that have 

 been devised to register accurately the quick changes in pressure 

 due to the heart beat, the one that has been most frequently used 

 is the membrane manometer of Hurt hie.* 



The principle made use of in the Hiirthle manometer is illustrated 

 by the diagram in Fig. 196. The instrument consists essentially of a small 

 box or tambour of very limited capacity; the top of the tambour is covered 

 with thin rubber dam and the cavity is filled with liquid and connected by 

 rigid tubing, also filled with liquid, with the interior of the artery or heart. 

 Variations in pressure in the artery are transmitted through the column of 

 liquid to the rubber membrane of the tambour, and the movements of this 

 latter are greatly magnified by a sensitive lever attached to it. The liquid 

 conduction and the small size of the tambour, which prevents any notice- 

 able outflow of liquid, combine to make a sensitive and very prompt recorder 

 of pressure changes. It is necessary to calibrate this instrument whenever 

 used in order to give absolute values to the records obtained. A specimen 

 of a blood-pressure record obtained with this instrument is shown in Fig. 198. 

 It will be noticed that the size of the heart- beat, relative to the distance from 

 the base line, is much greater than in the record obtained with the mercury 

 manometer, Fig. 193. 



Fig. 198. Blood-pressure record from a dog with a Hiirthle manometer. The size 

 of the heart beats is relatively much greater than with a mercury manometer. In this case 

 the systolic pressure is about 150 mms. Hg; the diastolic, 100 mms. ; and the heart beat 01 

 pulse pressure, 50 mms. 



The method that depends upon the use of maximum and minimum valves 

 may be understood by reference to Fig. 197. On the path between the artery 

 and the manometer one may place a maximum and a minimum valve so ar- 

 ranged that the blood-pressure and heart beat may be transmitted through 

 either valve. As is shown by the figure, if the connection is maintained 

 through the maximum valve for a certain time the highest pressure reached 

 during that period will be recorded, while, when the minimum valve is used 

 the lowest pressure reached will be indicated. 



Such valves, of course, act slowly and can not be used to determine the 

 maximum and minimum pressure in the artery during a single heart beat; 

 they record the highest and lowest point reached during a certain given 

 interval. 



Actual Data as to the Mean Pressure in Arteries, Veins, 

 and Capillaries. The mean value of the pressure in the aorta 

 has been determined for many mammals. It is found that the actual 

 * Archiv. f. d. gesammte Physiologic," 49, 45, 1891. 



