50 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



Stimulate with a tetanizing current for a few seconds and compare the 

 resulting kymograph curve with that of a short voluntary tetanus. 



16. Determination of Motor Points. — Let the left arm rest on the 

 indifferent electrode (copper plate covered with cotton soaked in Ringer) 

 and explore the surface of the left forearm with the stimulating electrode 

 (copper rod with tip covered with cotton soaked in Ringer) for motor 

 points (for long flexor of thumb — 2 points for flexions of digits — for 

 ulna nerve at elbow — deltoid muscle on shoulder, etc.) Mark skin 

 with ink. Make a chart. Use tetanizing current. Cf. Howell, 12th 

 Ed., Fig. 35; Tabulae Biologicae, 2, Plates XIV-XV, 1925. 



17. Polar Stimulation of Human Nerves. — Connect a pole changer 

 (rocking key) to six cells in series. The subject holds the indifferent 

 electrode, while the observer presses the stimulating electrode over the 

 motor point of the ulnar nerve at the elbow. Make the stimulating 

 electrode the cathode and close and open circuit. Reverse current by 

 means of current by adding dry cells repeating procedure each time. How 

 many cells are required in each of the four cases? This method is used in 

 the diagnosis of nerve injury (cf. Howell, 12th Ed., p. 106). The normal 

 sequence is cathode closing, anode closing, anode opening, cathode open- 

 ing (order of efficiency). 



18. Polar Changes of Irritability (Catelectrotonus). — Make a sciatic- 

 gastrocnemius preparation of frog with a long nerve (as far as spinal 

 cord). Connect non-polarizable boot electrodes to end posts of a pole- 

 changer with cross wires in circuit. Attach middle posts to 2 dry cells. 

 Do not spill ZnS04 on outside of electrodes. Place the nerve across 

 the boot electrodes. Arrange ordinary pt. electrodes to stimulate the 

 nerve between the boot electrodes and the muscle. One boot electrode 

 should be near the pt. electrodes. With drum rotating slowly stimulate 

 the nerve through the pt. electrodes with the weakest interrupted current 

 maintaining a slight tetanus (one dry cell). Now send for a moment 

 through the nerve an ascending galvanic current (with the anode of the 

 boot electrodes near the pt. electrodes). Break first the galvanic current; 

 then a moment later the tetanizing current. If necessary use a stronger 

 galvanic current. Repeat with a descending current (boot cathode next 

 the pt. electrodes). Do both poles of a galvanic current have the same 

 effect on irritability? Cf. Howell, 12th Ed., p. 101. Text p. 348. 



19. The Rate of Transmission of a Nerve Impulse. — Make a sciatic- 

 gastrocnemius preparation but split the vertebral column and retain the 

 entire length of the nerve. (Use a bull frog if available.) Connect 

 two pairs of electrodes to a rocking key. Lay the nerve on the two 

 electrodes, one near the muscle, and the other close to the piece of verte- 

 bral column. Take a simple twitch (with signal key and tuning fork) 

 when the rocking key is arranged to stimulate through the far electrode. 



