86 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



There is, however, an entirely different method of experimentation which 

 seems to prove that nerve-, like muscle-protoplasm, can conduct in both direc- 

 tion-. This method is based on the fact that though nerve-fibres rarely branch 

 in the peripheral nerve-trunks on their way to an organ, they may divide very 

 freely alter reaching it. Such branchings of fibres occur in muscle, and Kuehne 1 

 found that if one of these branches was stimulated, the irritation passed up 

 the branch to the nerve-fibre and then down the other branches to the muscle. 

 For example, he split the end of the sartorius muscle of a 

 frog by a longitudinal cut, and then found on exciting one 

 of the slips that the other contracted (see Fig. 32). Since 

 cross conduction between striated muscle-fibres does not 

 occur, no other explanation presents itself. Perhaps a still 

 more striking example is to be found in an experiment of 

 Babuchin 2 on the nerve of the electric organ of an electric 

 fish, the Malopterurus. The organ, consisting of many 

 thousand plates, is supplied by a single enormous nerve- 

 fibre which after entering the organ divides very freely so 

 Fig. 82.— Kuehne's ° f . . , . 



preparation of sarto- as to supply every plate. In this case mechanical stimu- 



f ho * ,1 " uble lation of the central end of one of the cut branches of the 



conduction in nerve. 



nerve, sufficed to cause an electric discharge of the whole 

 organ. The irritation must have passed backward along the irritated branch 

 until the main trunk was reached and then in the usual direction down the 

 other branches to the electric plates. 



Still another method is that which was employed by Du Bois-Reymond, 3 

 on the fibres of the spinal nerve-roots. When a nerve is excited to action it 

 undergoes a change in electrical condition, and this change progresses along 

 the fibre at the same rate and in same direction as the nerve-impulse. This 

 electrical change, though entirely different from the nerve-impulse itself, can 

 be taken as an indication of the direction of movement of the process of 

 conduction. Du Bois-Reymond found that if he stimulated the afferent fibres 

 of the posterior spinal nerve-roots of the sciatic nerve of the frog, a "nega- 

 tive variation current," as the current resulting from the change in the elec- 

 trical condition of the nerve is called, passed down the nerve in a direction 

 opposite to that which the normal impulse takes. Further, it was found 

 that if the sciatic oerve was excited, a negative variation current could be 

 detected in the anterior a- well as the posterior roots. Normally the irritation 

 only passes up the posterior roots and down the anterior, for normally the 

 sensory fibres of the posterior roots are excited only at the peripheral end and 

 the motor fibres of the anterior roots only at the central end. The experiment 

 showed both sensory and motor fibres to be capable of conducting in both 

 directions. Normally a nerve is stimulated only at one end, and therefore 

 conducts in only one direction. 



1 Archir fur Anatomie und Physiologie, 1859, S. 595. 



2 IbuL, is? 7, S 262. 



3 Tkierische Electncitdt, 1849, Bd. ii. S. 587. 



