THE GRAPHIC METHOD 



constant but shows incessant variations corresponding to the heart beats, respira- 

 tory movements, etc. These variations produce oscillations in the mercury col- 

 umn, which are recorded by placing on the free surface of the mercury a float (s) 

 to which is attached a light rod carrying at its upper end a writing point. The 

 writing point is adjusted so as to scratch a tracing on a lightly smoked, revolving 



FIG. 5. A membrane manometer, after Porter. 



drum. Fig. 4 represents a tracing (D) of the blood pressure in a rabbit recorded 

 in this way. (For further explanation of this experiment, see Chapter V.) 



Owing to the inertia of the mercury column, the actual variations of pres- 

 sure are not exactly reproduced by this method. They may be more faithfully 

 portrayed if the blood pressure can be brought to bear on an elastic membrane 



FIG. 6. A kymograph with "endless" paper, after Ludwig and Baltzar. 



or spring (elastic manometer). [Fig. 5 shows such a manometer. A small 

 chamber about 7 mm. in diameter is provided with two stopcocks, one of which 

 is connected with the artery the pressure in which is to be measured, while the 

 other opens to the atmospheric air. This chamber is closed above by an elastic 



