586 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



is the derived portion ; arid the altered condition of this part, which is pro- 

 duced by the extraneous current (this current having been experimentally 

 proved by M. Du Bois-Reymoud to exert no influence of its own on the gal- 

 vanometer), is termed the electrotonic state of the nerve. When the intensity 

 of the "nervous current" is increased, the nerve is said to be in the poifire 

 phase of this electrotouic state; and when it is diminished, the nerve is in the 

 negative phase of that state. By a proper arrangement, the same exciting 

 current may be made to produce the positive phase in one part of a nerve- 

 trunk, and the negative phase in another. Thus if the two extremities of a 

 nerve (Fig. 197, p and c) be so connected with two galvanometers that both 

 shall develop the "nervous current," and an intermediate portion be excited 

 by the transmission of an electric current in the direction z p, the nervous 

 current in the "derived" portion c will be increased in intensity, whilst that 

 in the portion _p will be diminished. Hence it may be inferred, that when 

 any portion of the length of a nerve is traversed by an electric current, be- 

 sides the usual electro-motive action of the nerve, a new electro-motive ac- 

 tion takes place in every point of the nerve, by a polarization of its electro- 

 motive elements, which action has the same direction as the exciting current 

 itself; and a current is thus produced in the "derived" portion, which is 

 added to the original " nervous current" at that end of the nerve at which the 

 direction of this new current and of the nervous current coincide (c), and is sub- 

 tracted at that end at which the directions are different (p*). The intensity 

 of the electrotonic condition is found to be materially affected by the dis- 

 tance at which the nerve is examined from the point where the "exciting 

 current" is applied, being always much greater near that point than at a 

 considerable distance from it. It is also powerfully influenced by the 

 strength of the exciting current, and by the length of the portion of nerve 

 through which that current passes, increasing in intensity with stronger cur- 

 rents, and diminishing in proportion to the length of nerve the current has 

 to traverse. These variations in the intensity of the "nervous current" con- 

 tinue as long as the "exciting current" lasts, and soon cease when the cir- 

 cuit of that current is broken ; a slight eufeeblement of the normal nerve 

 current being apparent for some time after both phases of electrotonus. It 

 is to the induction of the electrotonic state in the nerve supplying it, that 

 the contraction of a muscle is due, which ensues on the completion of the 

 circuit; and to the cessation of this state, that the muscular contraction is 

 due which is consequent upon the interruption of the circuit. Hence the 

 electrotouic changes in the condition of nerves may be observed without pre- 

 viously dividing them. When, on the other hand, a nerve is "tetauized" 

 by passing an interrupted and alternating current through a portion of it, 

 the effect is, as in the case of muscle, to produce a dhnhi ution in its own 

 proper current; the needles of both galvanometers, in the arrangement last 

 described, being deflected to the negative side, instead of one going back to 

 zero and the other having its positive deflection increased, as happens when 

 the "excited portion" is subjected to a continuous and uniform current. 

 The same negative variation of the nervous current has been demonstrated 

 by M. Du Bois-Reymond in nerves tetauized by other means, as by the use 

 of strychnia. And the phenomena both of the "electrotonic state" and of 

 the "negative variation," are precisely the same, whether motor or sensory 

 nerves be subjected to the experiment; thus making it appear that nerve- 

 force may be transmitted in either direction along each of these orders of 

 nerves. 



474. Electrotonus of Pfliiyer. Pfliiger has investigated with much care the 

 changes that occur in the excitability of a nerve in a state of clectrotouus, 

 or, in other words, through a portion of which an electrical current is passing. 





