ELEMENTARY EXPERIMENTAL PHYSIOLOGY 47 



is turned by hand about half an inch. The strength of the current 

 is slowly increased until a small contraction follows each break-shock ; 

 this is the minimal stimulus. The distance in centimetres of the 

 secondary from the primary coil is noted upon the drum. The make- 

 shock is weaker than the break, so that it is necessary to use only 

 the one or the other in this experiment. 



The intensity of the current is still further increased until the most 

 powerful contraction of the muscle, as indicated by the height of the 

 nearly vertical lines upon the drum, is obtained ; the stimulus is now 

 maximal. Any further increase in the strength of the stimulus is 

 not accompanied by a bigger contraction; a supra-maximal stimulus 

 only produces a maximal contraction, and is liable to damage the nerve. 



It may be, as Gotch has suggested, that the difference between 

 maximal and minimal stimulation depends upon the number of the 

 constituent fibres of the nerve stimulated. A weak electric current 

 may affect only a few fibres, and therefore the result will be only a 

 slight contraction, due to the excitation of those muscle-fibres alone 

 which are supplied by the nerve-fibres. 



It will be found that the excitability of the nerve changes, so that 

 with the same strength of stimulus there will not be the same minimal 

 point. A loss of excitability readily occurs if the nerve be allowed 

 to dry, but during this process there may be irregular fluctuations 

 in the excitability of the nerve above and below the normal. 



Mechanical Stimulation of the nerve can be shown by pinching the 

 nerve with a pair of forceps ; the muscle contracts, showing that a 

 nervous impulse was produced. Such a method of stimulation injures 

 the nerve, but by means of simple arrangements a nerve can be 

 stimulated mechanically without damage. A light hammer worked 

 by an electro-magnet may be used to tap the nerve, or small drops 

 of mercury from a funnel may be allowed to fall upon the nerve. 

 Such methods are useful in experiments in which an electrical stimulus 

 might introduce a source of fallacy, but for ordinary experiments 

 they are undesirable, since there is a difficulty in maintaining a 

 constant strength of stimulus, and there is a danger of damage to the 

 nerve. 



Thermal Stimulation is next shown by the application of a hot wire 

 to the nerve. The muscle contracts. The damaged portion of the 

 nerve is cut away, and to the end of the living nerve is applied a 

 crystal of common salt ; the muscle soon shows irregular twitches due 

 to the chemical stimulation of its nerve. The last form of stimulus is 

 obviously limited to special experiments, for the stimulus is not easily 

 graduated and damages the nerve. 



