29G MESSRS. F. GOTCH AND V. HORSLEY 



changes manifest themselves between two surface contacts is thus by itself no proof 

 that these electrical changes have their source in the tissue in the immediate neigh- 

 bourhood of the contacts. It is, however, easy to ascertain whether the opposite con- 

 clusion is or is not true, by slicing off the two portions of the surface to which the 

 electrodes have been attached, destroying their vitality, and replacing them in contact 

 with the subjacent tissues, so as to act as mere moist conductors. If under these cir- 

 cumstances electrical changes are observed in the electrode areas, then it is clear that 

 since all structural and physiological continuity is destroyed, these changes must be 

 due either to (l) physical properties of the observed region, which are in no way asso- 

 ciated with physiological vital changes ; or (2) to the physical spread of derived 

 currents from electrical changes of physiological origin in the deeper uninjured tissues. 



The further distinction between these two alternative causes is effected by observing 

 to what extent the electrical changes disappear in consequence of systemic death, 

 since this obviously will affect the second class but not the first. 



It is, in our opinion, essential in all exact experimental investigations carried out 

 upon a particular region of such a mass of conducting material, as the body of an 

 animal constitutes, to use a method which can by strict investigation carried out along 

 the above lines be shown to exclude such discrepancies. 



It may be pointed out that whilst we have selected the brain mass as a typical 

 instance, the same objections apply with equal or greater force to the investigations of 

 the spinal cord in situ. When it is remembered that the exposed cord lying in its 

 cavity is brought into immediate connection with a large mass of muscles, some unin- 

 jured, some cut for operative purposes, and with the whole contents of the abdominal 

 and thoracic cavities, it is not to be wondered at that the slightest movement of the 

 animal should cause very large electrical changes between any two portions of the 

 surface of the exposed cord, these being simply due to an alteration in the position of 

 muscles or other structures, all of which are the seat of electromotive change. Such 

 alterations must seriously influence the particular effect which may manifest itself 

 when any two portions of the exposed cord are connected with the galvanometer. 

 The force of these considerations is strengthened when it is borne in mind that 

 the particular effects which the electrical method is to gauge, are excitatory in 

 character. Of what value would any excitatory effect be, if, when it is evoked, 

 there are, as is almost always the case, not only marked excitatory electrical changes 

 in other organs, muscles, &c., but general movements and displacement of subjacent 

 parts as the result of the excitation ? 



Enough has now been said to point out the necessity for the utmost possible isola- 

 tion of the observed region, if it is desired to ascertain by the electrical method the 

 presence of electrical changes due to electromotive differences occurring in that region 

 only, and which may be therefore interpreted as indicating the presence therein of 

 excitatory conditions. 



The most complete method of isolation is obviously the removal of the structure 



