GENERAL PHYSIOLOGICAL METHOD 



When the muscle is thus caused to contract, it 

 lifts the lever and a " muscle curve," or myogram, 

 the proportions of which are determined by the 

 extent of the contraction and the speed of the 

 drum, is recorded. ED.] 



When it is desired to compare with one an- 

 other several successive contractions of the same 

 or of different muscles, the curves may be recorded 

 one above another by simply lowering the drum on 

 its supporting axis to different levels. Such a 

 series is shown in Fig. 2. 



Tracings made in this way are preserved for 

 future study by immersing the smoked paper for 

 a moment, after it has been cut loose from the 

 drum, in a solution of shellac in wood alcohol. 

 The alcohol evaporates quickly, leaving a perma- 

 nently hard varnish over the soot. 



The graphic method is adapted for recording 

 a great many other physiological phenomena. 

 The first use made of it on an extensive scale 

 was that of recording the blood pressure and its 

 variations (Ludwig, 1847). 



The blood pressure in an artery may be deter- 

 mined by tying a cannula into the central cut-end 

 of an artery and connecting it with a U-shaped 

 tube containing mercury (mercury manometer, 

 Fig. 3). When the connections are properly 

 made and the artery is undamped, the blood pres- 

 sure is brought to bear on the mercury column in the limb (a) of the tube, and the 

 column in the other limb (6) is forced upward. This difference, however, is never 



FIG. 3. The mercury manome- 

 ter, provided with a writing 

 point for recording the level 

 of the mercury in the limb 

 (6) of the tube. 



FIG. 4. Blood pressure curve taken from a rabbit. A . the line of no pressure ; T, the time 

 recorded in seconds; D, the pressure tracing. To be read from right to left. 



