174 MEASUREMENT OF RATE OF FLOW. [BOOK i. 



this lever projecting within the tube (and corresponding to the pendulum 

 of Vierordt's instrument) is moved on its fulcrum in the india rubber 

 ring by the current of blood passing through the tube, the greater the 

 velocity of the current, the larger being the excursion of the lever. 



FlG. 34. ELEMATACHOMETER OF CHAUVEAU AND LORTET. 



The movements of the short arm give rise to corresponding movements 

 in the opposite direction of the long arm outside the tube, and these, 

 by means of a marker attached to the end of the long arm, may be 

 directly inscribed on a recording surface. This instrument is best 

 adapted for observing changes in the velocity of the flow. For deter- 

 mining actual velocities it has to be experimentally graduated. 



The rapidity of the flow, and especially variations in the rapidity, may 

 also be studied in a more indirect manner by means of the following 

 method, called the ' plethysmographic method.' 



The principle of the plethysmograph is that changes in the volume 

 of a part or of an organ of the body, are measured by the displacement 

 of fluid in a chamber with rigid walls surrounding the part or organ. 

 A part of the body, the arm, for instance, is introduced into a cham- 

 ber with rigid walls, such as a large glass cylinder, which is filled 

 with fluid, the opening by which the arm is introduced being closed 

 with an india rubber ring or with plaster of Paris. The cavity of the 

 chamber is connected, at one spot, with a narrow glass tube, open at 

 the end, in which the fluid, after the introduction of the arm, stands at 

 a certain level. Any change in the volume of the arm manifests itself 

 by a change in the level of the fluid in the tube ; when the arm shrinks 

 the level falls, when the arm swells, the level rises. And by means of 

 a piston working in the tube, or by a float bearing a marker and 

 swimming on the top of the fluid, or by other contrivances, a graphic 

 record of the changes in the level of the fluid in the tube and so of the 

 changes in the volume of the arm may be obtained. Such an instru- 

 ment is called a plethysmograph ; and, as we shall see it may be applied 

 in various ways to various parts and organs of the body. 



