ON THE MAMMALIAN NERVOUS SYSTEM. 305 



temperature, drying, &c., these influence the electrical resistance of the tissue sur- 

 rounding by altering the moisture with which it is infiltrated. The result is to cause 

 an alteration in the balance previously existing between the currents due to the tissue 

 and the compensator source. The character of this will be made evident by citing a 

 case. 



The sciatic nerve, prepared as above, and hanging free in air, was connected by its 

 cross section and surface with the galvanometer electrodes, the resting difference 

 between the contacts was compensated at first by a difference of '01 D. The nerve 

 was then moistened between the contacts, this necessitating a reduction of the 

 compensation to '0085 D. This alteration is obviously due to the fact that the 

 increased moisture has decreased the resistance in the surface of the nerve^ and 

 thereby increased the intensity of all currents which are present in that region. 

 These currents are, first, a portion of the balancing current, and secondly, the derivation 

 currents between the surface and cross section of the nerve. Whilst these two are 

 equally affected between the points of contact of the electrodes, the result as regards 

 the galvanometer circuit is very different. This will be rendered clear if we remember 

 that the tissue difference is usually that obtained by connecting two surface points of 

 a series of closed circuits, through which currents flow, and that the resistance of the 

 circuits varies inversely with the amount of tissue moisture, so that the amount of 

 spread, and thus the particular derivation on the surface, is diminished by the 

 increased moisture. The final result is that, as regards the galvanometer circuit, there 

 is now a current in the direction opposed to that due to the tissue difference, this 

 being due to the over compensation, and rendered more distinct by the lessened 

 resistance in the whole galvanometric circuit which the moistening involves. 



The effect of drying must be the reverse of that indicated. It became noticeable 

 in our prolonged experiments upon the Mammalian nerve and spinal cord, since it was 

 essential for our purpose to perform the experiments in a room at about 70 F. As 

 far as possible the errors due to this were obviated by the use of steaming sponges 

 placed under the structures, but a slow steady rise in the amount of the difference 

 during the first 5 or 10 minutes after the electrodes have been applied, and 

 maintained unaltered, is usually seen in the cord, vide infra, and although not a very 

 noticeable feature in nerve, may, when present, be partly attributed to this cause. 



As just indicated the temperature was kept tolerably constant, but, as HERMANN 

 and others have shown, the gradual rise in the temperature of the experimental room, 

 may cause a similar slow increase in the amount of the difference. This alteration 

 is, however, strictly of physiological origin, being related to a change in the molecular 

 condition which lies at the foundation of the difference. 



This brings us at once to the alterations which may be more especially ascribed to 

 physiological changes. 



(&.) Slow Physiological Changes not Obviously Excitatory. The alterations of this 

 kind in the resting difference due to physiological changes are iu the case of nerve 



MDCCCXCI. B. 2 R 



