150 THE PHYSIOLOGY OF NERVE 



the impulse into the latter. The breaking contractions are absent, 

 because both anodic stimulations are as yet too weak. As the strength 

 of the current is increased to medium (M) , the breaking contractions 

 also appear, because even the anodic stimulations have now attained 

 a strength sufficient to generate nerve impulses. The making con- 

 tractions, however, continue to be larger than the breaking, because 

 the cathodic stimuli are more powerful than the anodic. With medium 

 currents no direct blocking effects are obtained, although the anodic- 

 making and cathodic-breaking depressions are now more powerful 

 than during the passage of weak currents. 



With a strong ascending current (S) , no reaction is obtained on the 

 making, because the anode is situated next to the muscle. The nerve 

 impulse generated at this time at the cathode cannot reach the end- 

 organ, because the strongly depressed and non-conductile anodic region 

 intervenes. On the break of this current, however, the impulse can 

 reach the motor organ without hindrance, because the now stimulating 

 anode is situated near the muscle and the depressed cathodic area far 

 ajway from it. With a strong descending current a contraction is 

 obtained only on the making, because the stimulating cathode is 

 now situated near the muscle and the depressed anode far away from 

 it. On the breaking of this current, however, the impulse developed 

 in the anodal region cannot reach the muscle, because the non-conduc- 

 tile cathode is interposed between it and the end-organ. 



The Law of Contraction of Normal Human Nerve and Muscle. — 

 Pfliiger's Law as such cannot be applied to human muscle and nerve, 

 because the conditions here met with are entirely different from those 

 presented by excised muscle. Living human muscle is covered by 

 skin, adipose tissue, fascia and connective-tissue envelopes, and the 

 nerves are generally so deeply placed that they are not accessible to 

 stimulation by means of two widely separated electrodes. For this 

 reason, their excitation is usually effected with a single electrode ad- 

 justed as follows: The battery consists of about 25 to 30 cells which 

 may be quickly joined in series so as to be able to increase the strength 

 of the current with the least possible loss in time. In this circuit is 

 inserted a pole changer by means of which the polarity of the elec- 

 trodes may be reversed at any moment. One of the electrodes con- 

 sists of a broad metal plate wrapped in a bolster of cotton. The latter 

 is moistened with saline solution to reduce the resistance of the skin. 

 The other electrode is pointed and is equipped with a key which may 

 be closed and opened at will. If a current of a certain voltage is per- 

 mitted to pass through two electrodes of this construction, it will be 

 found that the excitation invariably arises at the pointed one, because 

 the current attains here its greatest density and striking force. At 

 the broad nietal plate, on the other hand, it is able to scatter more 

 widely through the tissues without actually acquiring a high stimu- 

 lating intensity. For this reason, the former is designated as the 

 stimulating and the latter as the indifferent electrode. 



