180 GENERAL PROPERTIES OF LIVING TISSUES 



Irritability at Different Points of Same Nerve. r 



Determine the threshold value for the sciatic 

 nerve near the gastrocnemius muscle and about 

 two centimetres from the cut end of the nerve. 



The farther from the muscle the nerve is stim- 

 ulated, the lower will be the threshold value. It 

 has been suggested in explanation of this that 

 the nerve impulse gathers force as it passes 

 along the nerve, and is the more powerful the 

 longer the nerve which it traverses (avalanche 

 theory). It has also been suggested that the 

 nearer to the nutrient cell of origin the stim- 

 ulus is applied, the greater the effect. The true 

 explanation lies in the fact that the irritability 

 of the nerve is raised in the neighborhood of the 

 cross-section by the passage of the demarcation 

 current through that portion, as explained on 

 page 296. Tigerstedt has shown with mechani- 

 cal stimuli that the uninjured nerve has equal 

 irritability throughout. 



The Excitation Wave remains in the Muscle or 

 Nerve Fibre in which it starts. In order to 

 limit the stimulus to one or two fibres, the 

 method of unipolar stimulation may be adopted. 



Fasten in one post of the secondary coil of 

 the inductorium arranged for tetanizing currents 

 a wire soldered to a blunt needle. The needle, 

 except near the free end, and the lower part of 



