220 



PHYSIOLOGY 



the catch holding it in this position is released by the trigger, the spring, 

 which only acts for a short space, gives the frame and the glass plate a rapid 

 horizontal motion ; and the momentum carries the glass plate through the 

 rest of the distance, till stopped by the buffers. The velocity during this 

 time is nearly constant, as the friction of the guides is small. Two keys 

 are knocked over by pins on the frame and break electric circuits. The 

 relative positions at which the circuits are broken can be altered by a con- 

 venient adjustment. A tuning-fork vibrating about 100 per second fixed to the 

 base of the instrument marks the time ; its prongs are sprung apart by 



t' 



Cl 



FIG. 55. Blix apparatus for recording isometric and isotonic curves synchronically. 

 (Miss BUCHANAN.) p, the steel cylindrical support with jointed steel arm to 

 bear the isotonic lever I, which consists of a strip of bamboo with an aluminium 

 tip. t, the isometric lever, also of bamboo, except for a short metal part t', in 

 which are holes for fixing the muscle. The two wires from an induction coil are 

 brought, one to x , which is in connection with the support and hence with the metal 

 bar t', the other to y, which is insulated from the support but connected by a 

 copper wire with a thin piece of copper surrounding the isotonic lever at the point 

 where the muscle is attached to it. Cl, clamp for fixing the lower end of the 

 muscle when an isometric curve is to be taken. The axis of the isotonic lever is 

 at x, close to which is hung the weight of 50 grm. 



a block between their ends, and the same action which releases the glass 

 plate also frees the fork by removing the block and allows it to vibrate ; a 

 writing style then draws a sinuous line on the smoked surface of the moving 

 glass plate. A muscle lever with a scale-pan attached also forms part of the 

 instrument." 



The record obtained in either of these ways may, in consequence of instru- 

 mental inertia, be a very inaccurate reproduction of the true events occurring 

 in the muscle itself. When the muscle begins to contract it imparts a very rapid 

 movement to the lever, which therefore tends to overshoot the mark and deform 

 the curve. This source of error may be almost avoided by making the lever as 

 light as possible, and hanging the extending weight in close proximity to the 

 axle of the lever, as shown in Fig. 55. Since the energy of a moving mass is 



(mv^\ 

 = j, and the tension due to 



the weight as well as the velocity on contraction is directly proportional to the 

 distance of the weight from the axis, it follows that it is better to load the 



