ON THE MAMMALIAN NERVOUS SYSTEM. 297 



under observation from all contact with other tissues. This is not possible in the 

 case of the Mammalian central nervous system ; it is, however, quite feasible in the 

 case of the spinal cord to ensure an isolation which we find to be sufficient. This, as 

 has been detailed in the section dealing with the operative procedure, as regards its 

 essential feature consisted in always dividing the cord, freeing it for several centims. 

 from all its attachments, ligaturing the divided end, and suspending this portion in 

 air by means of the ligature (see Plate 29). The cord is thus only in connection with 

 the structures by its deep end, and any spread of electrical currents, &c., which have 

 their source in extraneous regions, can only occur in this portion of cord as " extrapolar 

 effects " ; further, the electrodes being placed one upon the extreme end (cross section), 

 the other only half to one centim. on the proximal side of this, such extrapolar deri- 

 vation effects, if present, must be still more diminished by the distance, 3 centims., 

 between the electrode contacts and the deep connections of the cord. 



The necessity of the above careful isolation has been impressed upon us by the ease 

 with which it is possible to introduce, errors even where this mode of connection is 

 carried out. If, for instance, either the proximal contact be allowed to slip down and 

 to approach too closely (within 1 centim. or less) to the deep structure, or if the 

 latter, by the presence of large quantities of liquid, &c., be brought into connection 

 with a portion of the isolated tract close to the proximal electrode, then, even with the 

 remaining electrode contact well isolated, electrical effects manifest themselves between 

 the contacts, which we have no doubt are really due to extrapolar spread from 

 changes in the deeper tissues. Thus, for instance, the anterior roots, even when 

 divided and one end isolated, present great difficulties, owing to the shortness of the 

 tract intervening between the contacts upon them and the cord from which they spring. 

 Electrical excitatory effects can be obtained on exciting the brain, which are apparently 

 situated in the anterior roots, but which are in this arrangement largely due to 

 electrical changes, situated in the cord, and occur as extrapolar derivations in the 

 anterior root. This is readily proved by using the method of control previously 

 referred to, viz. , cutting off the attachment of the root to the cord and then replacing 

 it in its old position, when, in spite of the want of structural and physiological con- 

 tinuity, the changes are still found to occur. 



We cannot lay too much stress on the necessity of careful isolation as far as prac- 

 ticable, and of careful investigation by both the methods indicated, and other similar 

 ones, of the extent to which observed electrical effects are due to mere physical 

 spread through the moist tissues of changes in other regions than the observed ones. 



It will be understood, therefore, that in the following experiments, when the cord 

 or sciatic nerve was observed, the length of the tissue afforded every facility for 

 placing the electrode connections at a safe distance from the point where each of these 

 structures came into relation with the general mass of the body, and in consequence 

 a position could be chosen in which all danger of extrapolar spread might with 

 ordinary care be guarded against. When, however, the roots were observed, since 



MDCCCXCI. B. 2 Q 



