;io A MANUAL OF PHYSIOLOGY 



myograph plate. Lay the nerve on electrodes connected through a 

 short-circuiting key with the secondary of an induction machine 

 arranged as in Fig. 261. Put the writing-point on 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 con- 

 traction. Put on a time-tracing with the electrical tuning-fork 

 (see Figs. 243, 245, pp. 653, 655). 



12. Measurement of the Latent Period of Muscular Contraction. 

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

 The arrangement for automatic stimulation described in Experi- 

 ment 6 (p. 706) 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. 706). 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. 228, p. 643), raising it on 

 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 myograph 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 



