DEVELOPMENT AND DISAPPEARANCE OF ELECTROTONUS. 665 



through the hypoglossal nerve, twitching of one-half of the tongue (instead of 

 contraction) on opening the circuit of an ascending current and a similar mani- 

 festation on closing the circuit of a descending current. 



Pfliiger has represented the contraction-law diagrammatic ally. According to 

 him the molecules of the resting nerve are in a state of a certain moderate degree 

 of mobility. In katelectrotonus the mobility of the molecules is increased, while 

 in anelectrotonus it is diminished. Accordingly, stimulation is produced if the 

 nerve-molecules pass from the state of moderate mobility into that of free mobility, 

 or if they pass from a state of difficult mobility into one of moderate mobility 

 (of rest) . 



Analogous phenomena, such as are yielded by the contraction-law for the 

 motor nerves, can also be established for the inhibitory nerves. Moleschott, 

 v. Bezold, and Bonders have examined the cardiac branches of the vagus in this 

 connection. The results correspond entirely with those obtained with motor 

 nerves, except naturally that inhibition of the heart's action occurs in this in- 

 stance, instead of the contraction that takes place on stimulation of a motor nerve. 



The sensory nerves likewise react in a similar manner, although it must be 

 borne in mind that the reacting organ in this instance is situated at the central 

 extremity of the nerve-tract, while in the case of the motor nerve it is situated 

 at the peripheral extremity, in the muscle. Pfliiger studied the influence 

 of closure and opening on sensory nerves by observing the resulting reflex con- 

 traction. Feeble currents caused contraction only on closure; currents of mod- 

 erate strength contraction on both closure and opening; strong descending currents 

 contraction only on opening, and ascending currents contraction only on closing. 

 Applied to the skin of man feeble currents give rise to sensation only on closing 

 with the current passing in either direction; while strong descending currents 

 give rise to sensation only on opening, and strong ascending currents only on 

 closure. During the closure of the circuit there is a prickling, burning sensation, 

 which increases with the strength of the current. The phenomena (sensations 

 of light and of sound) observed in the nerves of special sense, are analogous to 

 the foregoing. 



In the muscles the contraction-law is tested by keeping one extremity stretched, 

 so that it cannot shorten, and closing and opening the circuit in this situation. 

 The movable extremity then exhibits the same law of contraction as if the motor 

 nerve were stimulated. On closing the contraction begins at the kathode, on 

 opening at the anode. 



E. Hering and Biedermann demonstrated more thoroughly in this connection 

 that contractions on closing and opening are purely polar effects. They found 

 that when a feeble current is passed through the muscle, the first result that ap- 

 pears is a small contraction confined to the kathodal half of the muscle. 

 Increase in the strength of the current causes greater contraction, which extends 

 to the anode, but is feebler at this point than at the kathode. At the same time 

 the muscle remains in a state of permanent contraction during the period of 

 closure. On opening, the contraction takes place from the situation of the anode. 

 Also after opening the muscle may remain for some time in a state of contrac- 

 tion, which ceases on closing the current passing in the same direction. The 

 law of polar effects manifests itself also in the unstriated muscle of the ex- 

 cised uterus and intestine kept warm; also in the isolated ventricle of the frog, 

 as well as in the musculocutaneous tube of worms and holothurians. 



In some animals apparent deviations from the foregoing law of Pfluger occur, 

 but these are only apparent. They are caused by the actions of ions induced 

 in part by the internal and in part by the external polarization. In the proto- 

 plasmic current in chars Hermann observed with each stimulation attended with 

 a wave of negativity sudden arrest of the movement analogous to a muscular 

 contraction. In this instance there is thus a law of arrest instead of a law of 

 contraction. 



Destruction of the extremity of a muscle by various procedures gives rise to 

 diminution of irritability in the neighborhood of the portion destroyed. There- 

 fore, the polar effects in such a situation are but feeble. Also moistening such a 

 point with meat-infusion, potassium hydroxid, or alcohol diminishes the polar 

 effects locally, while sodium-salts and veratrin increase them. 



Under certain circumstances not only permanent irritation, but also contrac- 

 tion, may appear at both extremities of a muscle on passing a current longi- 

 tudinally through it, for example after destruction of one of its extremities, or in 

 case of peripheral muscular paralysis in man, during closure as well as after open- 

 ing of a galvanic current. 



