480 ELECTRO-PHYSIOLOGY CHAP. 



prolonged water-bath finally raises the E.M.F., in many cases, to 

 the same height as before the application of the salt solution " 

 (Engelmann). Stronger solutions of salt (0'4 - 0'8 %) act 

 still more strongly and energetically. These experiments on 

 the extraordinary effect of even very slight changes of concentra- 

 tion upon the magnitude of E.M.F. in the skin current cannot 

 obviously be referred to changes of conductivity, but undoubtedly 

 depend upon variations in the electromotive functions of the 

 active cells, which go hand in hand with changes in bulk of 

 water in the same. The skin of the frog is less sensitive than 

 that of the tongue to mechanical impacts (pressure, traction). 

 Still, after severe traction Engelmann found (I.e.) that the E.M.F. 

 fell in a few seconds from O'l Dan. to 0*006 Dan. Protracted 

 cold caused greater or less diminution of the entering normal 

 rest current, without, however, reversing it. At a temperature 

 of +4 C. Engelmann still observed an E.M.F. of 0'08 Dan. 

 Negative variations as a rule correspond with sudden positive 

 heat variations, their duration and magnitude growing with 

 increasing magnitude, duration, and spatial extension of the 

 rise of temperature. Among chemical agents C0 2 is emphatic- 

 ally a substance, the effect of which is to diminish the force of 

 the skin current " with extraordinary rapidity." In the space of 

 the first half minute Engelmann has often seen it fall to a sixth, 

 and less, of the original height. If the poisonous gas is removed 

 soon enough (by blowing in air or hydrogen) the E.M.F. may 

 return to its original proportions. So too, though in different 

 degrees, the action of anaesthetics like chloroform and ether, 

 which also produce marked negative variations even in minimum 

 doses. 



Want of oxygen, too, weakens the skin current after a long 

 period, and may, after 1-2 hours, reduce it to zero. With 

 renewal of air the E.M.F. increases more rapidly than it had 

 previously diminished, provided that oxygen had not been drawn 

 off for too long a period. 



The great variability of skin and mucosa currents, and their 

 extreme dependence on the most varying external influences, 

 lead us a priori to anticipate that the effects of artificial excita- 

 tion (whether direct or from the nerve) would, according to 

 circumstances, differ very considerably. Here again the frog's 

 tongue affords by far the most favourable conditions for experi- 



