iv GENERAL PHYSIOLOGY OF NERYOUS SYSTEM 253 



comparison with the very variable latent period for the frog. It 

 is also a striking fact that when strong currents are used con- 

 tractions not only appear at make and break of the current, but 

 there is frequently a tonic contraction or galvanotonus during the 

 whole time the current is passing. 



The law of contraction in man is of great practical importance 

 in differentiating between normal and morbid states of the nerves, 

 as with the latter the above reactions may be deficient or absent, 

 owing to depression or abolition of excitability and conductivity. 



The so-called reaction of degeneration is clinically of great 

 interest. It occurs -when the muscle and nerve degenerate, 

 either from pathological processes in the trophic centres, or 

 because the connections of the latter with the muscles have been 

 interrupted (neuritis, compression and injury of the nerves). 



Two principal forms of the reaction of degeneration can be 

 distinguished : Erb's reaction, and reaction at a distance. 



(a) Erb's, reaction is characterised by a primary phase of 

 increased galvanic excitability, with loss of direct and indirect 

 faradic excitability. Later on, galvanic excitability, too, disappears 

 in the nerve, and the contractions obtained on exciting the muscle 

 directly become slow, prolonged, and irregular, and are most 

 marked on closure at the anode or positive pole; with the 

 advance of the degenerative process stronger and stronger 

 currents are required to excite the muscle, till finally all trace of 

 electrical excitability disappears. 



(&) Reaction at a Distance. The reaction to which Ghilarducci 

 (1895) gave this name is constantly exhibited under the same 

 pathological conditions as Erb's reaction. To demonstrate it the 

 large electrode (indifferent electrode) is placed as for Erb's reaction 

 on the sternum or nape of the neck ; but instead of applying the 

 exploring electrode to the surface of the muscle as for Erb's 

 reaction, it is placed below it at a distance so much the greater 

 from the peripheral extremity, as the tendon of the muscle to be 

 explored is shorter and the .patient more delicate (e.g. to examine 

 the deltoid in children of less than a year old the exciting 

 electrode is placed upon the back of the hand). ' 



" Eeaction at a distance " is distinguished from " Erb's reaction " 

 by the following characteristics : 



(a) The muscular contractions constantly predominate at the 

 closure of the negative pole ; 



(/3) They are manifested with currents three to four times 

 weaker than those required to make the muscle contract with 

 direct excitation ; 



(y) They persist long (three to four years) after every trace 

 of electrical excitability, as tested by classical methods, has 

 disappeared. 



Reaction at a distance is thus of far greater importance than 



