ADVANCED DEMONSTRATIONS 389 



this cork into the small air-space which is left at the top of the finger- 

 stall. The other end of the wide tube is closed by a rubber cork. A 

 glass tube passes through this cork and is connected with a recording 

 tambour. The finger-stall acts as the spring. 



WyJ^^ 



FIG. 249. Arterial pressure recorded by a spring manometer. Effect of weak 

 excitation of the vagus during the period marked by the signal m. (Dubois.) 



By means of Hiirthle's differential manometer the relations of pressure 

 in any two ca.vities of the heart may be determined, and the exact 

 moments at which the valves shut and open found. In this instrument 



FIG. 250. HUrthle's differential manometer. 



two cannulae are brought into connection with tambours placed on 

 either side of the fulcrum of a lever. The lever works against a spring, 

 which in its turn sets a writing lever in motion. When the pressures 

 are equal the writing point is at zero. It rises or falls according as the 

 pressure in one cavity rises above or falls below that of the other. 



Connect by side tubes the mercurial manometer and the Hiirthle 

 manometer with the artery in the artificial schema. Take records with 

 each instrument on a moderately fast drum, and compare the results. 

 Connect by side tubes one side of the Hiirthle differential manometer 



