PRACTICAL EXERCISES 815 



the nerve on electrodes connected through a short-circuiting key with 

 the secondary of an induction machine arranged as in Fig. 287. Put 

 the writing-point en the drum and set it off (fast speed). Open the 

 short-circuiting key till the nerve has been once stimulated, then close 

 it again. The curve obtained differs from a normal curve, in that 

 the period of descent (relaxation) is exceedingly prolonged. Now 

 connect the preparation from the ligatured limb with the lever, and 

 take a tracing of a single contraction. Put on a time-tracing with the 

 electrical tuning-fork (see Figs. 264, 265, p. ,755). 



ii. Measurement of the Latent Period of Muscular Contraction. 

 (i) For this the drum must travel at a faster speed than usual. It 

 is most convenient to use a drum rotated very rapidly by a cord 

 attached to a falling weight or by the recoil of a stretched rubber band 

 or spring. The arrangement for automatic stimulation described in 

 Experiment 6 (p. 813) may be employed. Or an electro-magnetic signal 

 may be connected in the primary circuit of the induction coil so that 

 when the primary is closed or opened the writing-point of the signal 

 moves. Arrange the writing-point of the signal on the drum in the 

 same vertical line as the writing-point of the muscle lever, and in the 

 same line place the writing-point of a vibrating electric tuning-fork. 

 The coil is adjusted for single opening shocks as in Experiment 5 

 (p. 811). Pith a frog, and make a muscle-nerve preparation. Arrange 

 it on the myograph plate. The muscle, or the nerve very near the 

 muscle, is to be excited by a single opening shock while the drum is 

 moving. When the curve has been traced, the latent period is got by 

 drawing a vertical line through the point at which the curve just begins 

 to rise from the abscissa line, and another through the signal mark. 

 The number of vibrations of the tuning-fork included between these 

 two verticals gives the latent period. 



Or (2) use the spring myograph (Fig. 251, p. 746), raising it oa blocks 

 of wood. Smoke the glass plate over a paraffin flame, or cover it with 

 paper, and smoke the paper. Connect the knock-over key of the myo- 

 graph with the primary circuit of an induction coil. Arrange a muscle- 

 nerve preparation on the myograph plate. Place electrodes below the 

 nerve as near the muscle as possible, and connect by a short-circuiting 

 key with the secondary. Bring the writing-point in contact with the 

 smoked surface of the spring myograph, so as to get the proper pressure. 

 See that the writing-point of the tuning-fork is in the right position for 

 tracing time. Then push up the plate so as to compress the spring, 

 till the rod connected with the frame which carries the plate is held by 

 the catch. 



With the short-circuiting key closed, press the release and allow an 

 abscissa line to be traced. Again shove back the frame till it is caught. 

 Push home the rod by means of which the prongs of the tuning-fork are 

 separated, and rotate it through 90. Close the knock-over key, open 

 the short-circuiting key, shoot the plate again, and a muscle-curve and 

 time-tracing will be recorded. Again close the short-circuiting key, 

 withdraw the writing-point of the tuning-fork, push back the plate, 

 close the trigger key, then open the short-circuiting key, and, holding 

 the travelling frame with the hand, allow it just to open the knock- 

 over and stimulate the nerve. The writing-point now records a vertical 

 line (or, rather, an arc of a circle), which marks on the tracing the 

 moment of stimulation. The latent period is obtained by drawing a 

 parallel line (or arc) through the point of the muscle-curve where it just 

 begins to diverge from the abscissa line. The value of the portion of the 

 time-tracing between these two lines can be readily determined, and 

 is the latent period. 



