vi.] DIRECT EXCITATION 101 



is to exclaim on seeing these responses on the screen " What a 

 splendid cat ! " For they are, as you may have noticed, responses 

 to single break induction shocks. And notice also the delay, 

 about 2 seconds with an electrometer this time.* But we must 

 photograph this, which will be easily and expeditiously done on 

 the lecture-table by slipping in a recording instrument, so as to 

 receive the image of the mercury column. This is now done ; 

 the plate travels horizontally at a rate of about 3 mm. per 

 second ; the record is completed in 40 seconds; and in a few 

 minutes you will be able to compare it with that previously 

 taken by the recording galvanometer, and with your own 

 memory-image (Fig. 44). 



I repeat the two experiments to make sure that the effects 

 of indirect excitation are clear to you, and turn to the results 

 of direct excitation. I have no history to give you in this 

 connection, nor list of German names. You must be satisfied 

 with the story of the thing as given by the thing itself not a 

 complete story indeed, but a fragment, a word or two. 



62. Direct excitation.--^ pad of the cat's foot is cut off 

 and placed between electrodes on a bit of ebonite with a 

 central perforation, to ensure normal passage of the excita- 

 tion current. Excitation, compensation, etc., are applied as 

 you now well understand from previous lectures, in accordance 

 with the diagram now fully familiar to you (p. 152). I compensate 

 exactly, so that the galvanometer may be plugged and un- 

 plugged without disturbance (notice in passing that the current 

 to be compensated has been from surface to section, i.e., ingoing 

 through the skin ; it cannot therefore be current of injury, for 

 such current should be from section to surface, i.e., outgoing 

 through the skin) ; I excite the pad by a break induc- 

 tion shock in the ingoing direction, and on unplugging the 



* In another case a series of electrometer records came out : 

 Time post mortem . . 30 40 48 55 65 minutes. 

 Voltage of response . . 12 10 8 5 2 millivolts. 

 Period of latency . . 1.4 1.6 1.7 1.8 2.0 seconds. 

 There was no appreciable lost time with direct excitation, nor exces- 

 sive delay of transmission in the nerve itself; the delay was exclusively 

 " junctional," 



