200 



PHYSIOLOGY 



CHAP. 



dichotomous branchings of the nerve-fibres occur principally at 

 the points at which the nerve enters the muscle, and in the 

 extramuscular part of the same nerve. This dichotomous division 

 of the nerve-fibres is brought about by the separation of the fibrils 

 of which, according to Schultze, the axis-cylinders are composed. 

 Hence the experiments of Kiihne not only yield a direct proof 

 of double conductivity, but they also imply that the isolated con- 

 duction which Johannes Miiller showed to be a property of the 

 axis-cylinder does not hold as between the fibrils of which each 

 axis-cylinder is composed. 



Kiihne employed the same method to demonstrate unequivocally 

 that the paralysing action of curare is localised in the end-plates 

 of the muscular nerves, and does not spread to the motor fibres 



Z 



N 



FIG. 130. Killing's experiment on frog's pectoral 

 muscle. -Y, iirrvt- which supplies the right 

 half (in) of the muscle ; the left half is cut 

 away leaving only the bridge Z, which con- 

 tains the part of the nerve that is mechanic- 

 ally stimulated. 



PIG. 131. Kiihne's experiment on the gracilis 

 muscle. N, nerve that gives off branches 

 to the two separate parts of the muscle 

 A' and L and to the bridge of muscle Z, 

 which is mechanically excited. 



(see Chapter I.). He employed the gracilis muscle of the frog, 

 which can be divided by a ligature into two portions, in only one 

 of which the poisoned blood circulates. The muscle thus treated 

 can be cut so that the nerve forms the only connection between 

 the curarised and non-curarised portions (Fig. 132). Under normal 

 conditions the mechanical stimulus applied at N, Z, or K pro- 

 duces a contraction of the entire muscle according to the law 

 of the backward conduction of excitation ; but in the curarised 

 muscle mechanical stimulation of the nerve at N and at k will 

 only cause contraction of the part K, i.e. the non-curarised portion 

 of the muscle, the same effect being produced by exciting the 

 branches / and I' of the curarised portion. This proves that the 

 nerve-fibres have not been paralysed by the curare, since con- 

 duction in a centripetal direction takes place in them, as under 

 normal conditions. 



It may be argued logically from the law of double conduction 

 that the motor and sensory nerves do not differ fundamentally in 



