90 GENERAL PROPERTIES OF LIVING TISSUES 



The bend makes the strip a weak spring, ena- 

 bling the point to remain in contact with the 

 drum throughout the excursion of the point on 

 the paper. The writing point should be as nearly 

 as possible parallel to the paper. Even in this 

 position, the distance of the end of the straw 

 from the paper is necessarily less when the lever 

 is horizontal than when raised by the contrac- 

 tion of the muscle, for the end of the lever 

 describes a curved line in a plane tangent to the 

 recording surface. Were it not for the spring 

 of the writing point, the latter would leave the 

 drum. To remain on the drum at the height of 

 the contraction, the point must at the beginning 

 of contraction press against the drum with much 

 more friction than is necessary simply for scratch- 

 ing through the layer of soot. Thus the distance 

 of the writing point from the axis is constantly 

 varying, and the magnification of the lever 

 is constantly changing. Within the limits ordi- 

 narily employed in physiology, the deformation 

 of the curve thereby produced is proportional to 

 the length of the arc through which the point 

 moves ; the curve should therefore be written 

 no larger than is necessary for clearness. 



When the smoked surface is at rest, and the 

 contracting muscle lifts the lever, the writing 

 point describes an arc ; when the muscle relaxes, 



