264 



CIRCULATION. 



Fig. 8. 



filled with mercury, with one arm bent at a right angle, so 

 that it can easily be connected with the artery. The press- 

 ure of the blood is indicated by a depression in the level of 

 the mercury on one side, and a corresponding elevation on 

 the other. 



This instrument is generally considered as possessing 



great advantages 

 over the long glass 

 tube; but for esti- 

 mating simply the 

 arterial pressure, it 

 is much less useful, 

 as it is more sensi- 

 tive to the impulse 

 of the heart. For 

 the study of the 

 cardiac pressure, it 

 has the disadvan- 

 tage, in the first 

 place, of consider- 

 able friction ; and 

 again, the weight 

 of the column of 

 mercury produces 

 an extent of oscilla- 

 tion by its mere im- 

 petus, greater than 

 that which would 

 actually represent 

 the force of the 

 heart. 1 



An important 

 improvement in 



Compensating Instrument of Marey. 



1 Ludwig devised a means of registering the oscillation in the hemody- 

 namometer of Poiseuille. He used a U tube of considerable size, and placed a 

 float on the surface of the mercury, to which a pencil was attached The point 



