uc 



: 



; 



CURRENT OF DEATH 167 



radial line. On the other hand, we obtain series of equi- 

 potential surfaces whose outlines closely follow those of the 

 boundaries of the different degrees of discolouration. 



I shall next proceed to give quantitative measurements. 

 The first point to be considered is that of the choice of a 

 definite electrical level, which is to be used as a standard. 

 If this point be selected in the living tissue, we shall find that 

 our standard of comparison is extremely variable, since the 

 tonic condition, on which its electrical level depends, is itself 

 subject to change. The only condition which cannot be 

 modified in any way is that of complete death. This may 

 be taken, then, as the standard level. The method of experi- 

 ent will thus consist in selecting a series of equidistant points, 

 abed, and so on, 5 mm. apart, along a radial line, passing 



;itwards from the central area, which is completely dead, to 

 e green tissue. The non-polarisable contacts E and E' are 

 first placed on a and b, then on b and c, c and d, and so forth. 

 The external circuit contains a high resistance, compared with 

 which any difference of resistance, as between any 5 mm. of 

 interposed tissue, becomes negligible. Hence, the successive 

 deflections of the galvanometer indicate the electro-motive 

 difference that exists between a and b, b and c, and so on. 



One difficulty which is experienced, in these measure- 

 ments of small electro-motive differences, lies in securing the 

 iso-electric condition of the non-polarisable electrodes them- 

 selves. Whatever precautions are taken in the construction of 

 these, a small electro-motive difference will sometimes be 

 found to exist between them. The existence of such a 

 difference is easily tested by bringing the kaolin ends of the 

 two electrodes in contact, or by dipping both of them close 

 together in a vessel of normal saline solution. Any electro- 

 motive difference of the electrodes, however small, will now 

 give rise to a large galvanometric deflection. 



I This difficulty may be overcome by first taking special 



precautions as to the purity of zinc rods and the chemicals 

 employed, and secondly, by keeping the electrodes for 

 a long time short-circuited, with their ends dipped in normal 



