16 THE MECHANISM OF THE CIRCULATION. 



By means of a cannula a pressure record is taken in the left ventricle, the 

 cannula is then pulled out into the aorta, and another record taken. The two 

 readings thus obtained closely tally. As the intraventricular pressure rises 

 about 160 mm. Hg in one second, this instrument can record a variation of 

 pressure equal to 1600 mm. Hg per second, without either shooting on too far 

 or lagging behind. 



Hiirthle's 1 spring manometer consists of a small tambour, covered with a 

 rubber membrane only 5 '5 mm. in diameter. A button attached to the membrane 

 works against a steel spring, which acts as a counterpoise. The movement of 

 the spring is magnified and recorded by means of a lever provided with a writing- 

 style. Fluid is used as the transmission agent, and the 

 great principle of the instrument is the reduction of this 

 fluid to the smallest amount. The use of a practically 

 incompressible fluid, as a medium of transmission, is 

 open to one objection. Once set in motion, the fluid 

 column, extending from the ventricle to the manometer, 

 after the original force has ceased to act upon it, still 

 continues to move. The degree of this inertia is known 

 to be proportional to the mass and the square of velocity 

 of the fluid. In a properly constructed manometer, 

 the smallest mass of fluid practicable must enter for a 

 given increase of pressure. The practical limit is reached 

 somewhat before this mass becomes so small that its 

 movements are appreciably affected by the friction of 

 the lever, or, above all, by the friction of the writing 

 point against the smoked paper. 2 The Hiirthle tambour 

 is therefore made of the smallest dimensions, and the 

 swing of the instrument is further capable of being 

 damped down by the partial turning of a stopcock, 

 placed between the tambour and the artery, which 

 increases the friction of the fluid, and so diminishes its 

 velocity. This stopcock is turned until a point is ex- 

 perimentally found where the inertia is removed with 

 as little sacrifice as possible of the speed of the instru- 

 ment in following the changes of pressure. When the 

 stopcock is fully open, and the instrument undamped, 

 the Hiirthle manometer is found to record changes of 

 pressure equal to 200 mm. Hg in -02 second. This is 

 equivalent to 10,000 mm. Hg per second. 



Goltz and Gaule contrived another and valuable 

 instrument, the maximum and minimum manometer, by 



which can be recorded the highest or lowest pressure 



FIG. 8. GoltzandGaule's occ urring during a lengthened period of observation in 

 combined maximal, B, any chamber of the heart. 3 This is effected by 

 and minimal mano- placing valves between the intracardial cannula and 

 b^Htirth'le 8 ^ m dlfied ^ ie mercury manometers, and then, according to the 

 direction of the valves, the manometer becomes a maxi- 

 mal or minimal instrument. 



A simple form of valve can be constructed in any laboratory out of a tube 

 formed of thick glass, ground to a slant at one end, and brought to a 

 perfectly smooth surface. The orifice of the tube at this end is 1 mm. in 

 diameter. Over this is affixed a flap of oil-silk, which forms an efficient and 



1 Arch.f. d. ges. PhysioL, Bonn, 1888, Bd. xliii. S. 420. 



2 Porter, Journ. PhysioL, Cambridge and London, 1892, vol. xii. p. 518. 



3 Goltz and Gaule, Arch. f. d. ges. PhysioL, Bonn, 1878, Bd. xvii. S. 100. 



4 Hiirthle, ibid., Bonn, 1888, Bd. xliii. S. 438 ; Porter, Journ. PhysioL, Cambridge 

 and London, 1892, vol. xiii. p. 516. 



