NERVE 



511 



Expt. 79. Speed of the Nervous Impulse. (Apparatus: Kymograph, 

 etc. (myograph), glass nerve-plate, tuning-fork, electro-magnetic signal, 

 commutator, millimeter-rule.) Twist around each end of the dry glass 

 nerve-plate two fine wires 3 mm. apart, and connect their ends with the 

 four binding-posts of the commutator without its cross-wires. Connect 

 the rocker-posts with the secondary coil of the inductorium. Arrange 

 the myograph to write on the drum, the nerve-plate close to the top of the 

 gastrocnemius, the signal (and the key) in the primary circuit of the 

 inductorium writing directly and exactly under the pen of the myo- 

 graph 's lever. Hold the tuning-fork so that it will write under the signal. 

 Make a gastrocnemius preparation, having the nerve as long and normal 

 as possible. Lay the nerve on the glass nerve-holder over the two pairs 



FIG. 283 



Apparatus as arranged by students to show the speed of the nervous impulse in the 



frog's sciatic. 



of electrodes, one pair near the muscle, the other at the nerve's end. Rock 

 the commutator so that the shock will enter the nerve by the pair of 

 electrodes nearer the muscle. When the drum is spinning (raised by the 

 top screw from its friction-bearing) set the tuning-fork vibrating and 

 stimulate the nerves with a moderate break-shock (cutting out the make). 

 Now lower the drum to a fresh place. Shift the rocker so the current 

 may go in by the further pair of electrodes and make a similar curve with 

 a break-shock, making the time-record also. The time-interval between 

 stimulation and the beginning of the myogram (as shown by the tuning- 

 fork) will be less in the former curve than in the latter. Count the 

 difference in hundredths of a second. This is the time required for the 

 nervous impulse to pass between the two pairs of electrodes. Measure 

 this distance in millimeters and calculate the speed in meters per 

 second. 



