126 THE PHYSIOLOGY OF NERVE 



the motor and sensory neurons, must have shown that the wave of 

 excitation is propagated along nerve-fibers in a particular direction, 

 namely from the receptor to the effector. Thus, afferent fibers con- 

 duct normally in a centripetal direction, and efferent fibers in a cen- 

 trifugal direction. This constitutes the so-called law of forward con- 

 duction. An entire nerve, on the other hand, need riot be purely 

 afferent or efferent in character, but may be composed of both types 

 of fibers. In the latter case, it is designated as a mixed nerve. Its 

 power of conduction, however, is not interfered with, because a spread- 

 ing of its impulses from fiber to fiber, is not possible under normal con- 

 ditions. Mixed nerves, therefore, may convey centripetal and centri- 

 fugal impulses at the same time. 



If the substance of a unicellular organism is stimulated, the wave 

 of excitation proceeds from the seat of the stimulation in all directions. 

 In a similar way, it may be noticed that the application of a stimulus 

 to the center of a single muscle-cell is followed a moment thereafter 

 by a contraction of its two ends. The results obtained with nerve- 

 fibers are practically the same, but naturally, this statement applies 

 only to nerves which are tested under experimental conditions. Thus, 

 the stimulation of a motor nerve manifests itself solely by a peripheral 

 reaction in spite of the fact that the wave of excitation is also propa- 

 gated in a centripetal direction. Quite similarly, the excitation of a 

 sensory nerve cannot betray itself by a reaction in the receptor, but 

 only by some central effect which in time may lead to a reflex motor 

 response. It is evident, therefore, that the law of forward conduction 

 may be changed by experimental means into a law of double conduc- 

 tion. The direction of the conduction, however, is not dependent up- 

 on differences in the substance of the nerve-fiber, but solely upon its 

 central and peripheral connections. The irreciprocity of conduction, 

 as we have previously seen, is wholly determined by the conditions 

 existing at the poles of the neuron. 



The fact that the nerve impulse may be propagated in both directions is most 

 clearly proven by the following experiment (Fig. 73) devised by DuBois-Rey- 

 mond. 1 Each end of a long stretch of nerve is connected with the poles of a gal- 

 vanometer. On stimulating the nerve about midpoint between these instruments, 

 it is noted that both needles are deflected. For the present this phenomenon 

 need not be explained further than to state that the passage of a nerve impulse 

 gives rise to an action current which betrays itself by a galvanometric negativity. 

 Inasmuch as this negative variation appeared at both ends of the nerve, it must be 

 concluded that the wave of excitation has progressed in this case in a central as 

 well as in a peripheral direction. It is also to be noted that this result may be 

 obtained not only with mixed nerves, but also with pure afferent or efferent nerves. 

 Gotch and Horsley 2 have modified the foregoing experiment by adjusting a galvan- 

 ometer to the distal end of one of the divided anterior roots of the sciatic nerve 

 of the cat, and by subjecting the distal trunk of this nerve to repeated stimulations. 

 The centrifugal conduction was manifested in this case by the contraction of the 

 leg muscles and the centripetal conduction by the deflection of the needle of the 



1 Thierische Elektrizitat, ii, 1849, 587. 



2 Philos. Transactions, 1891. 



