Response of Nerve investigated with Electrometer. 305 



marked difference of potential is found to exist between the cross- 

 section and any point on the surface. 



A single excitation of the plexus rarely produces a displacement 

 of the image of the meniscus which is visible to the eye, but in the 

 photographic record such displacement is indicated by a definite 

 excursion of the character shown in fig/1 (iii). The form is a spike, 

 the limbs of which are more widely separated than those of the 

 uninjured fresh nerve fig. 1 (i). It indicates that the proximal 

 electrometer contact becomes first negative and then positive to the 

 distal one, but as the period at which the change of sign occurs is 

 later than in the fresh nerve the rate of transmission of the excitatory 

 process, of which the electrical change is an index, must be slower in 

 the kept than in the fresh nerve. 



If a new cross-section is made at the knee end and the distal 

 contact placed upon it, the resting demarcation change is at once 

 produced causing a marked downward displacement of the meniscus. 

 Each single excitation of the plexus now causes a change plainly 

 visible to the eye and the record shows as in fig. 1 (iv) that there is 

 an, excursion of considerable size, the initial rise being followed by 

 the prolonged tail, which is seen in the freshly injured nerve. 



The Alterations produced by the passage of a Polarising Current. 



It is well known that the passage of a galvanic current through 

 a portion of nerve profoundly modifies its electromotive conditions. 

 During the passage the regions outside the part traversed by the 

 polarising current are the seat of changes such that currents of 

 similar direction to the polarising one are present in the extrapolar 

 regions. 



It has been shown by Bernstein, Hermann and others that the 

 excitatory effects are modified under these conditions, and a consider- 

 able part of the present investigation has been devoted to the deter- 

 mination of the character of the change evoked by a single stimulus 

 under these circumstances. 



The experiments were arranged along the lines shown in fig. 2. 

 The excitation was restricted as before to the sciatic plexus, but an 

 additional pair of non-polarisable contacts was connected with a 

 circuit comprising a rheochord and two Daniell cells. These 

 polarising contacts were placed either between the exciting and the 

 electrometer ones as in arrangements A and B of fig. 2, or on the 

 distal side of the electrometer ones as in arrangements C and I). In 

 each case the polarising current may be reversed in direction and 

 four different modifications can be thus studied. It will be observed 

 that there is a fundamental difference as regards the position of the 

 electrometer contacts, and this is accompanied by corresponding 



z 2 



