THE LAW OF CONTRACTION. 569 



Eckhard observed that, on opening an ascending medium current applied to the hypoglossal 

 nerve of a rabbit, one-half of the tongue exhibited a trembling movement instead of a contrac- 

 tion, while on closing a descending current, the same result occurred ( 297, 3). According to 

 Pfl tiger, the molecules of the passive nerve are in a certain state of medium mobility. la 

 cathelectrotonus the mobility of the molecules is increased, in anelectrotonus diminished. 



B. The law for inhibitory nerves is similar. Moleschott, v. Bezold, and 

 Donders have found similar results for the vagus, with this difference, that, instead 

 of the contraction of a muscle, there is inhibition of the heart. 



C. For sensory nerves also the result is the same, but we must remember that 

 the perceptive organ lies at the central end of the nerve, while in a motor nerve it 

 is at the periphery (muscle). Pfliiger studied the effect of closing and opening 

 a current on sensory nerves by observing the reflex movement which resulted. 

 Weak currents cause only closing contractions ; medium currents both opening and 

 closing contractions; strong descending currents only opening contractions; and 

 ascending only closing contractions. Weak currents applied to the human skin 

 cause a sensation with both directions of the current only at closing ; strong descend- 

 ing currents a sensation only at opening ; strong ascending currents a sensation 

 only at closing (Marianini, Matteucci). When the current is closed, there is prickly 

 feeling, which increases with the strength of the current (Volta). Analogous 

 phenomena have been observed in the sense organs (sensations of light and sound) 

 by Volta and Bitter. 



D. In muscle, the law of contraction is proved thus by fixing one end of the 

 muscle, keeping it tense, so that it cannot shorten, and opening and closing the 

 current at this end. The end of the muscle, which is free to move, shows the same 

 law of contraction as if the motor nerve were stimulated (v. Bezold). On closing 

 the current, the contraction begins at the cathode ; on opening, at the anode 

 {Engelmann). E. Hering and Biedermann showed more clearly that both the 

 closing and opening contractions are purely polar effects ; when a weak current 

 applied to a muscle is closed, the first effect is a small contraction limited to the 

 cathodic surface of the muscle. Increase of the current causes increased contraction 

 which extends to the anode, but which is weaker there than at the cathode ; at the 

 same time, the muscle remains contracted during the time the current is closed. 

 On opening, the contraction begins at the anode; even after opening, the muscle 

 for a time may remain contracted, which ceases on closing the current in the same 

 direction. 



By killing the end of a muscle in various ways, the excitability is diminished near this part. 

 Hence, at such a place the polar action is feeble (van Loon and Engelmann, Biedermann). 

 Touching a part with extract of flesh, potash, or alcohol diminishes locally the polar action, 

 while soda salts and veratrin increase it (Biedermann). 



Closing Continued Contraction. The moderate continued contraction, which is sometimes 

 observed in a muscle while the current is closed (fig. 329, O), depends upon the abnormal pro- 

 longation of the closing contraction at the cathode when a strong stimulus is used, or during 

 the stage of dying, or in cooled winter frogs ; sometimes the opening of the current is 

 accompanied by a similar contraction proceeding from the anode (Biedermann). This tetanus 

 is also due to the summation of a series of simple contractions ( 298, III.). By acting on a 

 muscle with a 2 per cent, saline solution containing sodic carbonate, the duration of the con- 

 traction is increased considerably, and occasionally the muscle shortens rhythmically ( 296) 

 (Biedermann). 



If the whole muscle is placed in the circuit, the closing contraction is strongest 

 with both directions of the current ; during the time the current is closed, a con- 

 tinued contraction is strongest when the current is ascending ( Wundt). 



Inhibitory Action. The constant current, when applied to a muscle in a con- 

 dition of continued and sustained contraction, has exactly the opposite effect to 

 that on a relaxed muscle. If a constant current be applied by means of non- 

 polarisable electrodes to a muscle in a state of continued contraction, e.g., after 

 poisoning with veratrin or through the contracted ventricle, when the current is 

 closed, there is a relaxation beginning at the anode and extending to the other 



