654 CURRENTS OF STIMULATED MUSCLES AND NERVES. 



on the longitudinal section to which the electrode is applied recedes from the 

 transverse section. The negative variation after chemical or mechanical irrita- 

 tion is observed especially in winter-frogs exposed to cold, also on application 

 of peripheral pressure-irritation to the skin, as well as in the fresh electrical nerve of 

 the torpedo. Non-medullated nerves exhibit negative variation, just as they 

 exhibit in general all electrophysiological phenomena in the same manner as 

 medullated nerves. 



The action of certain substances upon the isolated nerve of the frog gives rise 

 to changes in the appearance of the electrical phenomena. If the isolated nerve 

 is exposed to carbon dioxid, the negative variation remains in abeyance for a few 

 minutes, after which it occurs with increased intensity. Chloroform and ether 

 increase the electrical activity at first, later having an inhibiting effect. Potassium 

 bromid has an inhibiting effect. The influence of electrotonus upon negative 

 variation is discussed on p. 662. 



The galvanic relation of the still irritable spinal cord is in general the same 

 as that of the nerves. If longitudinal and transverse currents are established in 

 the upper portion of the medulla oblongata, spontaneous intermittent variations, 

 perhaps due to the intermittent stimulation of the centers situated in this locality, 

 and particularly of the respiratory center, are observed. Similar variations occur 

 also reflexly in response to individual electrical shocks applied to the sciatic nerve, 

 while strong irritation by means of sodium chlorid or induced currents inhibits 

 them. Also the surface of the cerebrum exhibits the development of currents, if the 

 centers situated within it, for example the psychosensorial, are irritated by stimu- 

 lation through the organs of special sense. 



6. The same phenomenon that is exhibited by the muscle, as de- 

 scribed in paragraph 3, is exhibited also by the nerve. The wave of 

 negativity can be best followed if its rapidity of propagation is dimin- 

 ished by the action of severe cold. In its progress the wave of negativity 

 does not decrease in extent, while it does so in the excised muscle. 



The process of negative variation is propagated through the nerve-fiber with 

 measurable rapidity, which is greatest at a temperature between 15 and 25 C. 

 This is the same as that of the propagation of the stimulus itself, and in the normal 

 average is from 27 to 28 meters a second. This rapidity exhibits the same 

 variations as the rapidity of propagation of the nerve-stimulus. The duration 

 of an individual variation, of many of which the process of negative variation is 

 constituted, is only from 0.0005 to 0.0008 second. The length of the waves in 

 the nerve is estimated at 18 mm. 



J. Bernstein has by means of the differential rheotome found in the following 

 manner the time required by the negative current-variation in the nerve to pro- 

 pagate itself from the point of irritation throughout the course of the nerve. A 

 long nerve (Fig. 230, Nil) is so arranged that at one of its extremities (N) trans- 

 verse section and surface are connected with the . electrodes of a galvanometer 

 (). At the other extremity (n) are the electrodes of an induction-coil (J). A 

 disc (B), rapidly rotated about its vertical axis (A) by means of a pulley (5), is 

 provided at one point of its periphery with a device (C) by means of which the 

 current of the primary circuit (E) is rapidly closed and again opened with each 

 revolution. In this way a stimulating closing and opening induction-shock is applied 

 to the extremity of the nerve with each revolution of the disc. At the diametric- 

 ally opposite side (r r) of the periphery of the disc is an arrangement (c) by means 

 of which the galvanometer-circuit is closed and opened with each revolution. 

 There thus take place at the same moment the stimulation and the closing of the 

 galvanometer-circuit. When the disc is rapidly rotated, the galvanometer indi- 

 cates the presence of a strong nerve-current, the magnetic needle being deflected 

 to the point y. At the moment when stimulation takes place, the negative varia- 

 tion has not yet advanced to the other extremity of the nerve. If, however, the 

 device that closes the galvanometer-circuit is so displaced at the periphery of the 

 disc (for example to o) that the galvanometer-circuit is closed somewhat later than 

 the nerve is stimulated, the current appears to be enfeebled by the negative varia- 

 tion, the needle being deflected only to x. If the rapidity with which the disc 

 revolves is known it will be readily found that the time corresponding to the dis- 

 placement of the closing must be equal to the rapidity with which the stimulus 

 causing the negative variation is propagated from the one extremity of the nerve 

 (n) to the other (N). 



