562 THEORIES OF NERVE- AND MUSCLE-CURRENTS. 



current, i.e., in the intrapolar area, and also in the part of the nerve outside the 

 electrodes, i.e., in the extrapolar area. This condition is called electrotonus 

 (du Bois-Reymond, 1843). 



The eleotrotonic current is strongest not far from the electrodes, and it may be twenty-five 

 times as strong as the nerve-current of rest ( 331, 5) ; it is greater 011 the anode than on the 

 cathode side ; it undergoes a negative variation like the resting nerve-current during tetanus ; 

 it occurs at once on closing the constant current, although it diminishes uninterruptedly at 

 the cathode (du Bois-Hcymond). On the contrary, between the electrodes, besides the polarising 

 current itself, there is no obvious electrotonic increase of the current to be observed (Hermann). 

 These phenomena take place only as long as the nerve is excitable. If the nerve be ligatured 

 in the projecting part in the galvanometer circuit, the phenomena cease in the ligatured part. 

 The above-described galvanic electrotonic changes of the extra-polar part are absent in non- 

 medullated nerve-fibres, whilst, on the contrary, the physiological electrotonus is present. 

 The physiological electrotonus of medullated nerves can be set aside by treating medullated 

 nerves with ether, whilst the physical phenomena remain (Bicdcrmann). 



The negative variation ( 332) occurs more rapidly than the electrotonic increase of the current, 

 so that the former is over before the electro-motive increase occurs. The velocity of the electro- 

 tonic change in the current is less than the rapidity of propagation of the excitement in the 

 nerves being only 8 to 10 metres per second (Tschirjcw y Bernstein). 



' ' The secondary contraction from a nerve " depends upon the electrotonic state. If the 

 si iatic nerve of a frog's nerve-muscle preparation be placed on an excised nerve, and if a con- 

 stant current be passed through the free end of the latter non-electrical stimuli being inactive 

 the muscles contract. This occurs because the electrotonising current in the excised nerve 

 stimulates the nerve lyiug on it. By rapidly closing and opening the current, we obtain 

 "secondary tetanus from a nerve" (p. 559). 



[Paradoxical Contraction. Exactly the same occurs when the current is applied 

 to one of the two branches into which the sciatic nerve of the frog divides. The 

 sciatic nerve of the frog divides at the lower end of the thigh into the peroneal and 

 tibial branches. If the sciatic nerve be divided above, and the peroneal branch be 

 also divided and stimulated with interrupted induction shocks, the muscles supplied 

 by the tibial branch will contract. There is no contraction of the muscle if the 

 peroneal nerve be ligatured.] 



Polarising After-Currents. When the constant current is opened, there are after-currents 

 depending upon internal polarisation ( 328). In living nerves, muscle, and electrical organs 

 this internal polarisation current, when a strong primary current of very short duration is 

 used, is always positive, i.e., has the same direction as the primary current. Prolonged dura- 

 tion of the primary current ultimately causes negative polarisation. Between these two is a stage 

 when there is no polarisation. Positive polarisation is especially strong in nerves when the 

 primary current has the direction of the impulse in the nerve ; in muscle, when the primary 

 current is directed from the point of entrance of the nerve into the muscle towards the end of 

 the muscle ( 334, II.). 



4. Muscle-Current during Electrotonus. The constant current also produces 

 an electrotonic condition in muscle ; a constant current in the same direction in- 

 creases the muscle-current, while one in an opposite direction weakens it, but the 

 action is relatively feeble. 



[Electrotonic Phenomena in Conductors. Mattcucci found that a metallic wire surrounded 

 by a moist conductor, when traversed by a galvanic current, exhibits currents possessing the 

 properties of electrotonic currents of nerves. He also found that the currents ceased if the 

 wire was of zinc, and the envelope a saturated solution of zinc sulphate. This shows that 

 these currents were due to polarisation between the core and the fluid. Hermann finds that 

 the currents only obtain when a polarisable core is present. A straw without joints, if filled 

 with a saturated solution of common salt, or the tentacles of a lobster when moistened with 

 saline solution, and traversed by a constant current, exhibit similar electrotonic currents 

 (Bering),] 



334. THEORIES OF MUSCLE- AND NERVE-CURRENTS. I. Molecular 



or pre-existence Theory. To explain the currents in muscle and nerve, du Bois- 

 Keymond proposed the so-called molecular theory. According to this theory, a 

 nerve- or muscle-fibre is composed of a series of small electro-motive molecules 

 arranged one behind the other, and surrounded by a conducting indifferent fluid. 

 The molecules are supposed to have a positive equatorial zone directed towards the 



