THE STETHOORAPH 237 



that the actual movements of the mercury are not shown at all by 

 it. In order to overcome this difficulty, and to be able to record 

 accurately the movements of the mercury meniscus, the following 

 variation of the classical apparatus was contrived: 



A small tambour was joined to the distal end of the manometer 

 by a piece of pressure tubing and supplied with a very delicate 

 tracing lever which magnifies the movements of the membrane ten 

 to twenty times, as desired, the levers being variable in the con- 

 struction of the instrument. The surface of the tambour should not 

 be larger than 15 mm. in diameter. With this ratio between the two 

 surfaces and the levers multiplying ten to twenty times, the most 

 beautiful arterial tracing can be obtained, showing not only the 

 respiratory and percussion waves, but also the dicrotic wave clearly 

 superimposed on each cardiac wave. (See Fig. 52, page 92.) 



This apparatus as above described has one limitation, which may, 

 for certain kinds of work, seem to be a disadvantage. The pressure 

 tubing leading from the distal end of the manometer to the tambour 

 (Tb) is not attached to the manometer until after the rise of the 

 mercury in the distal tube following the removal of the clamp (C7). 

 After the tambour is attached to the manometer every movement of 

 the mercury is shown by corresponding movements of the tracing 

 point (t). Should there be a sudden rise or sudden fall of pressure, 

 it will be instantly and clearly shown by corresponding rise and fall 

 of the tracing. Should there be, however, a very gradual rise or a very 

 gradual fall, owing to physiological changes in tonus of the blood- 

 vessels, for example, or, perhaps, by the gradual action of some drug 

 on the animal under observation, this will not be shown by corre- 

 sponding rise and fall of the tracing. It will, however, be shown by 

 the stand of mercury in the distal limb of the manometer, and this 

 may be easily read off or noted from time to time. 



To offset this slight disadvantage one has the two advantages 

 above mentioned, namely, accuracy of the tracing of all the quicker 

 movements of the mercury and the assurance that one's apparatus 

 will always work. 



14. THORACIC CANNUL-ffi. 



A cannula for transmitting the air pressure from the pleural or 

 mediastinal or abdominal cavity may be easily constructed as follows : 

 Take a piece of ordinary soft and thin-walled glass tubing about 

 10 cm. in length and 3 cm. lumen. Grind one end diagonally sharp 

 as shown in Fig. 88, Can. 



15. THE STETHOGRAPH. 



In order to record graphically the movements of the chest one may 

 use various mechanical devices. The most simple device, and a most 



