320 



COMPARATIVE ELECTRO-PHYSIOLOGY 



In order next to show that epithelial cells in the animal 

 are relatively more excitable than epidermal, as we have 

 already found to be the case in vegetable tissues, I performed 

 the following experiment on the human lip. Here it was 

 important that the electrical connections should be main- 

 tained steady. A light spring- contact key was therefore 

 made, as seen in the lower part of fig. 195. The lower 

 contact of this spring-clip consisted of an amalgamated plate 

 of zinc leading to the lower electrode. Over this were tied 



four thicknesses of blotting- 

 paper soaked in zinc sulphate 

 solution. On this again were 

 placed three more thicknesses of 

 blotting-paper, soaked in normal 

 saline. The zinc plate which 

 formed the upper limb of the 

 clip, in connection with the 

 second electrode, was similarly 

 covered with separate layers of 

 blotting-paper, soaked in zinc 

 sulphate and normal saline re- 

 spectively. The protruded lower 

 lip was now placed in the clip, 

 as shown in the upper figure, in 

 such a way that the latter made 

 a gentle but secure contact. A 

 galvanometer and a source of 

 equi-alternating currents were 

 also placed in the circuit. Of 



UuCL 



FIG. 195. Experimental Arrange- 

 ment for Response of Human 

 Lip. 



Lower figure gives an enlarged view 

 of the spring-electrodes. 



the two electrodes, the upper was in connection with the 

 epithelial, and the lower with the epidermal surfaces. The 

 natural current was now found to flow in the tissue, as in the 

 corresponding cases of plant specimens, from the epidermal 

 to the epithelial. The perfect steadiness of the contact was 

 evidenced by the stillness of the deflected galvanometer spot 

 of light. On now applying the alternating excitatory 

 shock, the responsive current was found to be in the oppo- 



