ON THE MAMMALIAN NERVOUS SYSTEM. 295 



Whilst one observer noted the electrical effect, the other noted the extent and 

 character of any muscular movements by which the animal responded to the stimulus. 



It need scarcely be said that the whole method involves the use of special pre- 

 cautions to avoid the introduction of fallacious and misleading effects, especially 

 when it is desired to obtain a succession of results which will admit of quantitative 

 comparison. 



These precautions are, in the opinion of the authors, of the utmost importance and 

 must be considered in some detail. 



B. Precautions in Connection with the Method. 



The special precautions may be grouped as follows : 



(1.) Those connected with the isolation of the particular region under observation. 



(2.) Those connected with the condition of the non-polarisable electrodes. 



(3.) Those connected with the condition of the animal. 



(4.) Those connected with the localisation, &c., of the excitation. 



(1.) The Isolation of the Particular Region Observed. 



It is, in our opinion, an essential condition for the accurate employment of any 

 method of localisation which relies upon the evidence of electrical changes in a given 

 region, that the region in question should be as far as possible isolated. In the 

 galvanometric experiments alluded to in the History (p. 279), as carried out upon the 

 cerebral hemispheres, such isolation was not affected ; a door was thus left open for 

 introduction of errors which it is not easy to control. 



We have repeatedly had occasion to observe that when a pair of non-polarisable 

 electrodes is placed upon the cord lying in situ, or upon the surface of the exposed 

 brain (see fig. 26, Chapter XL) electrical differences present themselves and influence 

 the galvanometer, this being evidently due to the fact that the parts with which the 

 contacts are made, since they form one directly continuous mass with the structures 

 around them, lie in the path of the derivations of currents, whose primary electro- 

 motive source is far removed from the electrodes. 



In this connection we may refer to the derivations of the electrical difference 

 between the different regions of the beating* heart which are present in the body. 



On connecting two points on the surface of the exposed brain by means of non- 

 polarisable electrodes with the galvanometer, any reflex movement of the scalp muscles 

 lying outside the exposed region was found to evoke electrical changes in the points 

 of contact, and if structures so far removed from the seat of observation can affect the 

 contacts, how much more easily will these be affected by sources of electromotive differ- 

 ences situated in the deeper fibres, &c., of the brain. The fact that electrical 



* WALLER, ' Phil. Trans.' 



