EXCITATION AND INHIBITION 



justifiable to connect this fact with the rate of movement of some constituent 

 of the nerve system, somewhat as a push of a given strength, applied to a 

 resting heavy pendulum, will have a greater effect if the rate at which its energy 

 is imparted to the pendulum coincides with the vibration period of the latter. 



In the practical use of the condenser, it is important to remember that the 

 insulation is never perfect, so that if any delay occurs between the charge and 

 the subsequent discharge through nerve, the most effective part of the discharge, 

 namely the steepest fall of potential, will have been lost. For this reason, the 

 best arrangement of the circuit is that given in Fig. 102, ascribed by Hermann 

 (1906, p. 540) to Radakovid The condenser c is connected to a source of 

 adjustable potential through the nerve N. When the key K is closed, a chosen 

 fraction of the potential difference of the battery B is sent into the condenser 

 through the nerve. As long as the key remains closed, the full charge of the 

 condenser is kept up to the potential required, but the moment that the key 

 is opened, discharge takes place through the nerve and the part of the slide 

 wire between A and P. If 

 either the charge or the 

 discharge is not intended 

 to pass through the nerve, 

 a short circuit is made for 

 the time being between D 

 and K. 



Although the con- 

 denser is the most perfect 

 means of delivering accur- 

 ately measured stimuli to 

 a nerve, it requires some- 

 what complex apparatus 

 when a rapid series of 

 stimuli is required. For 

 ordinary use, the induced 

 currents produced in a coil 

 of fine wire, by the make 

 or break of a current in 

 another coil of larger wire 

 at an adjustable distance 

 from it, are substituted. 

 This arrangement, when 

 fitted with an automatic 

 interrupter, " Wagner's 

 hammer," is known as 

 "Du Bois Reymond's 



Coil," after its inventor. The currents induced by the make and break of a 

 current in the primary coil differ in their time course, owing to the fact that 

 the establishment of the current in the primary coil is retarded by self-induction, 

 which is naturally absent at break, since the circuit is no longer complete. The 

 break shock is therefore of a higher potential than the make shock. 



Further details of the various methods of electrical excitation will be found 

 in the article by Garten (1908). One or two facts may be mentioned here. The 

 current used to excite must obviously enter the nerve at one electrode, and leave 

 it at the other. It is always found that excitation takes place at the cathode 

 when the current is established and, if it has lasted for some time, at the anode 

 when it is broken. These facts can be made out best by the use of constant, 

 unidirection currents, which can be kept closed as long as desired. Of course, 

 when other than alternating currents are used, the electrodes must not be capable 

 of polarisation, 'ihe construction of non-polarisable electrodes will be found in 

 Garten's article (1908, pp. 333-339). A very convenient form is the modification 

 of Ostwald's calomel electrode described by Noyons (1909), or that of Philippson 

 (1912). No excitation occurs during the passage of a current as long as it remains 



FIG. 102. 



ELECTRICAL CIRCUIT FOR USE WITH CONDENSER 

 IN STIMULATING EXCITABLE TISSUES. 



(Hermann, 1906.) 



