iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 221 



energy, since they induce pain in sensory nerves and muscular 

 twitches from motor nerves. Slight pressure or traction may 

 temporarily increase the excitability of nerve, but it is sometimes 

 possible, by slow but continuous mechanical action, to destroy con- 

 ductivity and excitability in a nerve without any perceptible 

 previous excitation. Paralysis of the brachial plexus has been 

 noted clinically as resulting from the constant use of crutches, 

 and paralysis of the recurrent laryngeal nerve is often due to its 

 compression by an aneurism. 



Physiologists have devised various means of applying 



c 



FIG. 142. Induction coil. (Du Bois-Reymond.) 



mechanical action, which has the great advantage of being 

 perfectly easy to localise, as a nerve stimulus. The simplest 

 method is that of rapid section of the nerve with scissors. For 

 the quick repetition of mechanical stimuli, Du Bois-Reymond used 

 a little toothed wheel that compressed successive portions of the 

 nerve. Heidenhain employed a small hammer arranged so that it 

 always tapped a fresh bit of the nerve. A similar tetanomotor 

 was employed by Wundt and perfected by Tigerstedt, which acted 

 for a given time upon the same point of the nerve. Langendorff 

 substituted a vibrating tuning-fork for the hammer. Finally von 

 Uexkiill (1895) invented a rigid hammer which tapped the nerve 

 as it lay over a very soft pad, so that it was possible to stimulate 

 the same point for a long time without injuring the nerve ; this 



