5<5o 



CURRENT IN THE SPINAL CORD. 



Hering found that the negative variation of the nerve-current caused by tetanic stimulation 

 is followed by a positive variation, which occurs immediately after the former, i.e., it is 

 diphasic. It increases to a certain degree with the duration of the stimulation, as well as 

 with the strength of the stimulus, and with the drying of the nerve (Head). {Effect of Elcctro- 

 tonus, 335, I.). 



Negative Variation of the Spinal Cord. This is the same as in nerves generally. If a 

 current be conducted from the transverse and longitudinal surfaces of the upper part of the 

 medulla oblongata, we observe spontaneoiis, intermittent, 'negative, variations, perhaps due to 

 the intermittent excitement of the nerve-centres, more especially of the respiratory centre. 

 Similar variations are obtained reflexly by single stimuli applied to the sciatic nerve, while 

 strong stimulation by common salt or induction shocks inhibits them. 



Velocity. The process of negative variation is propagated at a measurable velocity along 

 the nerve, most rapidly at 15 to 25 C. (Stcincr), and at the same rate as the velocity of the 

 nervous impulse itself, about 27 to 28 metres per second. The duration of a single variation (of 

 which the process of negative variation is composed) is only 0'0005 to 0"0008 second, while the 

 wave-length in the nerve is calculated by Bernstein at 18 mm. '.-*'? 



Differential Rheotome. J. Bernstein estimated the velocity of the negative variation'iin a 

 nerve by means of a differential rheotome thus (fig. 406) : A long stretch of a nerve (N ?t) is 

 so arranged that at one end of it (N) its transverse and longitudinal surfaces are connected with 



Fig. 406. 

 Scheme of Bernstein's differential rheotome ; N n, nerve ; J, induction machine ; G, galvano- 

 meter, x, y, deflection of needle ; E, battery and primary circuit with C for opening it 

 at o ; c, for closing galvanometer circuit ; z z, electrodes in galvanometer circuit ; S, 

 motor. 



a galvanometer (G), while at the other end (n) are placed the electrodes of an induction 

 machine (J). A disc (B) rapidly rotating on its vertical axis (A) has an arrangement (C) at 

 one point of its circumference, by means of which the current of the primary circuit (E) is 

 rapidly opened and closed during each revolution. This causes, with each rotation of the disc, 

 an opening and a closing shock to be applied to the end of the nerve. At the diametrically 

 opposite part of the circumference is an arrangement (c) by which the galvanometer circuit is 

 closed and opened during each revolution. Thus, the stimulation and the closing of the 

 galvanometer circuit occur at the same moment. On rapidly rotating the disc, the galvano- 

 meter indicates a strong nerve-current, an excursion of the magnetic needle to y. At the 

 moment of stimulation, the negative variation has not yet reached the other end of the nerve. 

 If, however, the arrangement which closes the galvanometer circuit be so displaced (to o) along 

 the circumference, that the galvanometer circuit is closed somewhat later than the nerve is 

 stimulated, then the current is weakened by the negative variation (the needle passing back- 

 ward to a;). When we know the velocity of rotation of the disc, it is easy to calculate the rate 

 at which the impulse causing the negative variation passes along a given distance of nerve 

 from N to n. 



The negative variation is absent in degenerated nerves as soon as they lose their excita- 

 bility. 



