DOUBLE CONDUCTION IN NERVES. 669 



electrotonus was marked. In tabetic patients reflex movements have also been 

 observed to be retarded, in the case of irritation by heat more than in that by cold. 



DOUBLE CONDUCTION IN NERVES. 



The property of living nerve by means of which it transmits an 

 impulse throughout its course is designated conductivity. All pro- 

 cedures that injure the nerve in its continuity, such as division, ligation, 

 crushing, chemical destruction, or that destroy its irritability at any 

 point, such as absolute deficiency of blood, certain poisons, for example 

 curare for the motor nerves, also marked anelectrotonus, abolish the 

 conductivity. The conduction takes place only through fibers directly 

 in communication, and it can never be transferred to an adjacent fiber 

 law of isolated conduction. By double conduction is understood the 

 ability of the nerve to transmit in both directions a stimulus applied 

 in its course. Naturally, as a result of the anatomical conditions in 

 the intact body the motor nerves are capable of conducting only in a 

 centrifugal direction and the sensory nerves only in a centripetal direc- 

 tion, but under suitable conditions it can be shown that each nerve is 

 capable, in the same way as an inanimate conductor, of conducting 

 in both directions. 



The evidence brought forward in support of the presence of double conduction 

 is as follows: If a nerve be irritated, alterations in its electrical properties appear 

 both in an upward and in a downward direction. If the posterior free extremity 

 of the electrical centrifugal nerve-fibers of the malapterurus be irritated, the 

 branches arising at a higher level are also set in irritation, so that the entire elec- 

 trical organ is discharged. If the lower third of the sartorius muscle in the frog 

 be divided longitudinally and one division be irritated mechanically, the irrita- 

 tion passes in the nerve-fibers thus separated at first upward to the point of 

 division and thence centrifugally in the unirritated muscular extremity whose 

 individual fibers now contract. The gracilis muscle is divided by a tendinous 

 line into two halves. The nerves to both are derived from a bifurcation of the 

 individual fibers in the nerve-trunk. Every irritation of the nerve for one por- 

 tion of the muscle causes contraction in both halves of the muscle. 



All of the earlier evidence in support of double conduction of nerves derived 

 from experiments on division and reunion will not bear rigid scrutiny. 



The intercentral association-fibers in the cerebrum must normally be 

 assumed to conduct in both directions. 



EMPLOYMENT OF ELECTRICITY FOR THERAPEUTIC PUR- 

 POSES. 



DEGENERATIVE REACTIONS OF MUSCLE AND NERVE. 



Electricity is much employed in medicine for therapeutic purposes. It may 

 be used in the form of the rapidly interrupted current of the induction-apparatus 

 (faradic current), or of the magneto-electrical machine, or of the extra-current 

 apparatus, or in the form of the constant current. The employment of electricity 

 is based upon its physical and physiological properties. 



In cases of paralysis the faradic current is applied by means of suitable wet 

 electrodes covered with sponge either to the muscle itself or to the point of entrance 

 of the motor nerve (Figs. 236, 237, 238, 239). The motor points on the face are 

 shown in Fig. 245, those of the neck in Fig. 244. In employing the faradic cur- 

 rent the object is, by means of artificially induced movements, to protect the 

 paralyzed muscle from secondary degeneration, which it would undergo as a 

 result of long-continued inactivity. If in addition to its motor nerves also the 

 trophic nerves of the paralyzed muscle are inactive, even long-continued faradiza- 



