244 PHYSIOLOGY CHAP. 



Setschenow, which prevents the mercury from making any rapid 

 movement. In the manometer represented in Fig. 68, p. 207, this 

 is easily effected by adjusting the screw placed at its lower end 

 to the required point. The mercury column is then practically 

 immobile between the highest and lowest points of the pulsatory 

 oscillations, and the apparatus merely records the average blood 

 pressure. 



In order to obtain as true a record as possible of even the 

 most rapid oscillations of blood pressure, the elastic manometer or 

 tonometer was invented, in which the mercury mass is replaced by 

 a spring or other elastic body, having but a small mass, and being, 

 therefore, more free from the errors due to inertia, and better 

 adapted to follow accurately the finer details of the pulsatory 

 oscillations of pressure. The history of the modifications and 

 gradual perfecting of the elastic manometer are of merely technical 

 interest. We must confine ourselves to mentioning the hollow 

 spring manometer of A. Fick (1864), constructed on the same 

 principle as Bourdon's metal manometer, employed in steam- 

 engines, and the metal manometer of Marey, which is constructed 

 on the principle of the aneroid barometer. In 1885 Fick invented 

 another flat spring manometer, which is simpler and more sensitive 

 than the preceding. The entire apparatus is reduced to a slender 

 tube ending in a small capsule, closed by a rubber membrane, 

 capable of small excursions which are transmitted to a flat steel 

 spring. By this method it is possible to reduce the movements of 

 the column of fluid in the tube connected with the artery to a 

 minimum, which facilitates the transmission of the more rapid 

 oscillations, and avoids the inconvenience of coagulated blood at 

 the point of the cannula. In order to magnify the oscillations of 

 pressure transmitted to the spring, and to record them on a 

 rotating drum, it is fitted with a long, light lever made from a 

 straw. Hiirthle perfected this manometer of Fick's by some 

 accessory contrivances, which made its application easier and more 

 certain, and confined the fluid between artery and manometer to 

 'the lowest possible minimum in order to transmit the variations 

 of pressure more rapidly and faithfully. Of course both Fick's 

 manometer and that of Hiirthle must be empirically graduated 

 by a mercury manometer in order to show absolute pressures. 



Still better than a steel spring, however, for obtaining true 

 curves of the pulsatory oscillations of pressure, is the tonometer 

 formed of elastic guttapercha, the simplest type of which is Marey 

 and Chauveau's sphygmoscope (Fig. 66, p. 205) in connection 

 with their writing tambour (Fig. 63, p. 201). In order to diminish 

 the mass of fluid communicating with the artery, Hiirthle reduced 

 Marey 's tambour to a small capsule, covered with a resistent 

 rubber membrane ; its excursions were magnified by a long and 

 very light lever, while the capsule, by omitting the sphygmoscope,. 



