DUPLEX TRANSMISSION IN THE NERVE. 



217 



fore the evident negative variation of the nerve-current 

 is a sign as yet the only known sign of the processes 

 which occur within the nerve during activity. This sign, 

 therefore, affords an opportunity of studying the ac- 

 tivity of the nerve itself independently of the muscle. 

 3. E. du Bois-Reymond made an important use of 

 this fact in order to determine the significant question, 

 whether the excitement in the nerve-fibre is propagated 

 only in one, or in both directions. If an uninjured 

 nerve trunk is irritated at any point in its course, two 



FK;. 59. TENSION IN NEUVES. 



actions are usually observable ; the muscles connected 

 with the nerve pulsate, and, at the same time, pain is 

 caused. The excitement has therefore been transmitted 

 from the irritated point both to the periphery and to 

 the centre, and it exercises an influence in both places. 

 Now, it may be shown that in such cases two differ- 

 ent kinds of nerves are present in the nerve-trunk 

 motor nerves, the irritation of which acts on the 

 muscle; and sensory nerves, the irritation of which 

 causes pain. In some places each of these kinds of 

 fibre occurs separately; and where this is the case, irri- 

 tation of the one results only in motion, irritation of 

 the other only in sensation. It is evident, therefore, 

 that the experiment in no way determines whether when 

 a motor nerve alone is irritated, the excitement is trans- 



