564 ACTION-CURRENTS. 



Demarcation-Current. Every injury of a muscle or nerve causes at the point of injury 

 (demarcation surface) a dying substance, which behaves negatively to the positive intact substance. 

 The current thus produced is called by Hermann the "demarcation-current." If individual 

 parts of a muscle be moistened with potash salts or muscle-juice, they become negatively electrical ; 

 if these substances be removed these parts cease to be negative (Bicdermann). 



It appears that all living protoplasmic substance has a special property, whereby injury of a 

 part of it makes it, when dying, negative, while the intact parts remain positively electrical. 

 Thus, all transverse sections of living parts of plants are negative to their surface (Buff) ; and the 

 same occurs in animal parts, e.g., glands and bones. Engelmann made the remarkable observa- 

 tion that the heart and smooth muscle again lose the negative condition of their transverse 

 section, when the muscle-cells are completely dead, as far as the cement-substance of the nearest 

 cells ; in nerves, when the divided portion dies, as far as the first node of Ranvier. When all 

 these organs are again completely streamless, then the absolutely dead substance behaves 

 essentially as an indifferent moist conductor. Muscles divided subcutaueously and healed do 

 not exhibit a negative reaction of the surface of their section. 



All these considerations go to show that the pre-existence of a current in living 

 uninjured tissues can no longer be maintained. 



Theoretical. Griinhagen and L. Hermann explain the electrotonic currents as being due to 

 internal polarisation in the nerve-fibre between the conducting core of the nerve and the enclos- 

 ing sheaths. Matteucci found that, when a wire is surrounded with a moist conductor, and the 

 covering placed in connection with the electrodes of a constant current, currents similar to the 

 electrotonic currents in nerves, and due to polarisation, are developed. If either the wire or 

 the moist covering be interrupted at any part, then the polarisation current does not extend 

 beyond the rupture (p. 562). The polarisation developed on the surface of the wire by its 

 transition-resistance causes the conducted current to extend much beyond the electrodes. 



Muscles and nerves consist of fibres surrounded by indifferent conductors. As soon as a con- 

 stant current is closed, on their surface, internal polarisation is developed, which produces the 

 electrotonic variation ; it disappears again on opening or breaking the current. Polarisation is 

 detected by the fact that, in living nerve, the galvanic resistance to conduction across a fibre is 

 about five times, and in muscles about seven times greater than in the longitudinal direction. 



Action-Current*. The term "action-current" is applied by L. Hermann to the currents 

 obtained during the activity of a muscle or nerve. When a single stimulation-wave (contrac- 

 tion) passes along muscular fibres, which are connected at two points with a galvanometer, then 

 that point through which the wave is just passing is negative to the other. Occasionally, in 

 excised muscles, local contractions occur, and these points are negative to the other passive 

 parts of the muscle (Bicdermann). In order, therefore, to explain the currents obtained from 

 a frog's leg during tetanus, we must assume that the end of the fibre which is negative partici- 

 pates less in the excitement than the middle of the fibre. But this is the case only in dying or 

 fatigued muscles. 



According to 336, D, the direct application of a constant current to a muscle causes con- 

 traction first at the cathode, when the current is closed, and when it is opened, at the anode. 

 This is explained by assuming that, during the closing contraction, the muscle is negative at 

 the cathode, while with the opening contraction the negative condition is at the anode. 



If a muscle be thrown into contraction by stimulating its nerve, then the wave of excitement 

 travels from the entrance of the nerve to both ends of the muscle, which also behave negatively 

 to the passive parts of the muscle. According to the point at which the nerve enters the muscle, 

 the ascending or descending wave of excitement will reach the end (origin or insertion) of the 

 muscle sooner than the other. On placing such a muscle in the galvanometer circuit, then at 

 first that end of the muscle will be negative which lies nearest to the point of entrance of the 

 nerve [e.g., the upper end of the gastrocnemius), and afterwards the lower end. Thus, there 

 appears rapidly after each other, at first a descending, and then an ascending, current in the 

 galvanometer circuit, of course reversed within the muscle itself (Sig. Mayer) ( 332, 4). 



The same occurs in the muscles of the human fore-arm. When these were caused to contract 

 through their nerves, at first the point of entrance of the nerve (10 cm. above the elbow-joint) 

 was negative, and then followed the ends of the muscles when the contraction -wave, with a 

 velocity of 10 to 13 metres per second, reached them (L. Hermann) ( 399, 1). 



If a completely uninjured, streamless muscle be made to contract directly and in toto, then 

 neither during a single contraction, nor in tetanus, is there a current, because the whole of the 

 muscle passes at the same moment into a condition of contraction. 



Nerve-Currents. Hermann also supposes that the contents of dying or active nerves behave 

 negatively to the passive normal portions. 



Imbibition Currents. When water flows through capillary spaces, this is accompanied by 

 an electrical movement in the same direction (Quincke, Zollner). Similarly, the forward move- 

 ment of water in the capillary interspaces of non-living parts (pores of a porcelain plate) is also 

 connected with electrical movements, which have the same direction as the current of water. 



