66 THE SIGNS OF LIFE [LECT. 



sequent observers having principally studied the effects of direct 

 excitation on the mucous membranes (Biedermann), and on the 

 skin of the eel (Reid). Engelmann seems to have obtained a 

 negative variation of the current of rest i.e., presumably an out- 

 going effect but he took no account of direction of excitation. 

 Biedermann, in the mucous membranes of the tongue and of the 

 stomach, observed positive and negative variations of the current 

 of rest ; but in the case of the tongue two layers of mucosa are 

 under experiment, and in any case a mucosa is not the same 

 thing as the skin. 



Reid found that direct excitation of the eel's skin by induc- 

 tion shocks of either direction caused ingoing effects, sometimes 

 preceded by outgoing effects. I have made a large number of 

 experiments on this point with the same disposition of apparatus 

 as that described in a previous lecture in the case of the eyeball, 

 with results that have practically been invariable, i.e., with only 

 one or two exceptions in several hundred experiments. So that 

 I am practically certain that in the experiment you are about to 

 witness, direct excitation of the frog's skin by an outgoing or by 

 an ingoing induction shock will cause an outgoing skin response. 



{Experiment^ 



A piece of skin is set up between unpolarisable electrodes, 

 as in Fig. 26, and connected with a keyboard, galvanometer, 

 coil, and compensator, as in Fig. 19. We are, in fact, about to 

 test the skin for blaze-currents, just as we previously tested an 

 eyeball or a seed. Compensation is established (the galvano- 

 meter is shunted because a large effect is expected). With 

 the galvanometer plugged out of circuit, a break induction 

 shock is sent through the skin in the positive (outgoing) 

 direction (spot to your right), and immediately afterwards the 

 galvanometer plug is removed. A large deflection to your 

 right occurs ; and to show you what this large deflection with 

 the shunted galvanometer means as regards electro-motive 

 force of the excited skin, I will send current through the circuit 

 from T ^-Q of a volt ; the skin response has been two or three 

 times as great, i.e., its voltage has been 0.02 to 0.03 ; and this 

 is by no means a maximum value, I have seen it as much as 



