DETERMINATION OF DIFFERENTIAL EXCITABILITY 275 



were described in Chapter IX., as obtained by thermal and 

 mechanical stimulus. How perfect and how consistent, due 

 precautions being taken, these results may be rendered, will 

 be seen from the numerous records in this and the following 

 chapters. 



I have postponed till now the consideration of the mode 

 of application of these alternating shocks. The usual alter- 

 nating current from a Ruhmkorff's coil would be entirely 

 unsuitable for delicate and crucial experiments ; first, because 

 the excitatory values of the slow make- and quick break-shocks 

 are unequal ; and, secondly, because such currents leave their 

 residual polarisation effects. 



These defects I have been able, as stated in the last 

 chapter to avoid, by quick reversals of the primary current 

 which actuates an induction-coil. When the primary 



current is reversed from the plus to the minus direction, 



. 



we obtain an induction-current due to magnetic varia- 

 tions of lines of force, from, say, plus n to minus n. When 

 the primary current is re-reversed, from minus to plus* 

 we obtain an opposite induction-current, due to magnetic 

 variation, from minus n to plus n. It will be seen that if 

 these reversals of the primary current are made with equal 

 rapidity, the alternating induced currents will be equal 

 and opposite. The reversals are accomplished by means of 

 a Pohl's commutator, worked up and down by a crank, in 

 connection with an electric revolving motor (fig. 170). The 

 intensity of the induction-shock may be varied by sliding 

 the secondary nearer to, or further away from, the primary. 

 Having now described the general means of producing equi- 

 alternating electric shocks, it still remains to explain two 

 distinct methods of applying them, for the determination of 

 the differential excitability of the tissue. These may be 

 distinguished as : (i) the Method of the AFTER-EFFECT ; and 

 (2) the Method of DIRECT-EFFECT. According to the first 

 of these the method of the after-effect the tissue is excited 

 for a definite length of time, and the excitatory effect 

 observed, by connecting it immediately afterwards with the 



