18 



PRACTICAL PHYSIOLOGY 



angles to the movement recorded. The magnification of the movement 

 of the muscle recorded by the lever is calculated by dividing the dis- 

 tance of the writing point from the axis by the distance from the axis 

 of the point of attachment of the thread from the tendon. The nearer 

 to the axis the muscle is attached the greater will be the magnification. 

 It is quite sufficient to magnify the movement of the muscle 5 times. 

 (b) The simple lever (Fig. 25) consists of two parts : a rigid femur- 

 clamp to hold the piece of femur, and a horizontal writing lever below 

 it to which the thread on the tendo-Achillis is tied. Care must be 

 taken that the femur-clamp and lever lie in the same plane, and that 



the muscle is tied to a point on the lever 

 vertically below the clamp. In this case 

 the movement of the writing point is in 

 the same plane as that of the movement 

 recorded. The magnification, as before, 

 is calculated by dividing the distance of 

 the writing point from the axis by the 

 distance of the point of attachment of 

 the muscle from the axis. 



The writing lever must be as light 

 as possible (see page 27, Chap. III.), but it 

 must be sufficiently rigid to prevent vibra- 

 tions being set up in it. For this purpose 

 writing levers are generally made of light 

 metal, glass, Japanese cane or straw. 



The actual writing point is made of 

 thin metal foil or moderately stiff paper 

 bent at its free end slightly over towards 



Fio. 26. Kymograph. .. , mi . - . L _ .. 



the drum. The writing point must lie 



as nearly as possible parallel to the recording surface, or, in other 

 words, at right angles to a radius of the drum. Further, the 

 bend near its end is necessary ; it acts as a weak spring and keeps 

 the writing point up against the recording surface in different positions 

 of the lever. For the end of the lever describes a curved line, and the 

 more it leaves the horizontal position the greater will be the distance 

 of the end of the straw from the recording surface. 



The Kymograph or recording drum (Fig. 26) consists essentially of 

 a stout brass cylinder which is made to revolve round a vertical axis 

 by either clockwork or string belting from a motor. It is necessary 

 to have some arrangement by which the speed of revolution can be 

 altered within wide limits ; this is obtained by various mechanical 

 devices in different patterns of drum, one of which is shown in Fig. 26. 



