54 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



beneath the physical anode, than in the region of the physiological anode, 

 beneath the physical kathode. 



These differences in the strength of the irritation process developed in these 

 different regions is well shown by examining the reaction of nerves to cur- 

 rents of gradually increasing strength. The effect of the opening and closing 

 irritation is seen to be as follows : 



Weak currents. Medium currents. Strong currents. 



K< i KCC KCC 



ACC ACC 



AOC AOC 



KOC 



The natural order, therefore, would be KCC, ACC, AOC, KOC. Some- 

 time-, however, AOC is stronger than ACC; this happens when on account 

 of the relation of the surrounding tissues to the nerve the density of the cur- 

 rent at the physiological anode is great as compared with the density at the 

 physiological kathode. Bordier 1 tested the strength of battery current neces- 

 sary to awaken minimal sensations by unipolar excitations, and found the 

 effect to be greatest by KC, then AC, then At ) ; and that it was least by Kl) — 

 i.e., sensory behave like motor nerves. 



In testing the effect of the battery current on the nerves and muscles of 

 man, it is customary to use one small and one large electrode (Fig. 6, d, <',/). 

 The small electrode is placed over the part to be stimulated, while the large 

 electrode is put over some distant portion of the body. This arrangement 

 causes the current to be condensed, and hence efficient, when it enters or 

 leaves the small exciting electrode, and to be diffused, and hence ineffective, 

 at the large indifferent electrode. For example, the indifferent electrode 

 may be placed on the sternum or over the back of the neck, while the excit- 

 ing electrode may be put over the ulnar nerve at the elbow. The two poles 

 may be connected with the battery, a pole-changer, rheostat, milliamperemeter, 

 and exciting-key being introduced in the circuit. The pole-changer permits 

 the exciting pole to be made A or K at the wish of the operator, the rheostat 

 allows the strength of current to be raised gradually, and the milliampere- 

 meter shows the strength of the current employed. With this arrangement 

 the renetion of the nerve can be readily tested. 



When the currents employed are strong, it occasionally happens in the 

 case of meu thai not only are the make and break followed by the usual rapid 

 contractions of short duration, but during the closure of the current there is 

 a continued contraction — galvanotonous, as it is sometimes called. This is 

 especially seen under certain pathological conditions. 



When the nerve or muscle is diseased we may have the above order 

 changed, and A( !< ! obtained with weaker currents than KOC, and KOC than 

 AOC (Babinski) 2 . This is known as the reaction of degeneration. Under 



1 Bordier: Archives de Physiologie normaleet Pathohgique, 1897, ]>i> 543-553. 



2 I5;ii>in.ski : Comples rendus de la Sociele de Bioloyie, 1899, p. o4o. 



