ON THE MAMMALIAN NERVOUS SYSTEM. 301 



positive electrical results following stimulation of a column, the posterior, for instance, 

 were to be attributed entirely to the localised stimulation of the structures upon which 

 the points of the electrodes rested. 



A further point is the special difficulty of maintaining the electrodes properly in 

 position, i.e., in contact with the cord, inasmuch as the spinal muscles on their 

 contraction shake the cord a little, although the spine be fixed. This, of course, 

 applies only to the end excited. Imperfect apposition of the electrodes was best 

 avoided by holding them in the hand, the latter being suitably supported. As is 

 described on p. 292, the duration of excitation was provided for by the revolving 

 mercurial key ; consequently, after the proper contacts had been made as above, each 

 observation was comparable with another as regards the degree of the excitation. 

 The important bearing this has on the quantitative value of the galvanometric readings 

 is sufficiently obvious. 



D. Nerve. In electrical excitation of the nerve, we were chiefly anxious at first to 

 avoid possibility of spread. Beginning with ordinary sheathed electrodes, into which 

 the sciatic nerve was laid, we very soon laid these aside for the more accurate and 

 simpler plan of applying platinum electrodes to the nerve raised in the air by a thread. 



The mode of exciting a nerve in relation to its cross and long diameter respectively, 

 has for some time been the object of research (HERMANN, BERNSTEIN, and others). 

 GAD,* in referring to the action of carbonic acid, and especially of alcohol, on the 

 excitability and conductivity of nerve fibres, raises this question again, and 

 strengthens the fact that transverse excitation is much more adequate (5-6 times) 

 than stimulation applied longitudinally. Apart from incidental polarisation it is clear 

 that the larger number of fibres lying in the principal axis between the poles chiefly 

 conditions the result. 



To produce a maximal effect we have, therefore, always applied the platinum 

 electrodes, so as to bend the nerve slightly between the terminal points. (See fig. 3.) 



Fi R . 3. 

 e 



n ; 



Inasmuch as we always sought to obtain, with as weak an excitation as possible, a 

 maximal effect for purposes of quantitative as well as qualitative comparison, it was 

 especially necessary for us to excite, without fail, all the fibres in the nerve. 



The usual method of simply placing the nerve on the electrodes, as, for instance, in 

 using sheathed or hook patterns, only enables a few fibres to be excited those 

 immediately in contact, or nearly so, with the metal. Thus, BEEVOR and HoRSLEyt 



* ' Arcliiv fiir Physiologie,' (DU Bois REYMbND). 1889, p. 350. 

 t ' Roy. Soc. Proc.,' 1888. 



