PHYSIOLOGY OF NERVE TISSUE. 85 



a second, for sensitive impressions 40 feet, and for tactile im- 

 pressions 140 feet a second. 



Electric Currents in Muscles and Nerves. If a muscle or nerve 

 be divided and non-polarizable electrodes be placed upon the natural 

 longitudinal surface at the equator, and upon the transverse section, 

 electric currents are observed with the aid of a delicate galvanometer. 

 The direction of the current is always from the positive equatorial 

 surface to the negative transverse surface. The strength of the cur- 

 rent increases or diminishes according as the positive electrode is 

 moved toward or from the equator. When the electrodes are placed 

 on the two transverse ends of a nerve, an axial current will be 

 observed the direction of which is opposite to that of the normal 

 impulse of the nerve. 



The electromotive force of the strongest nerve-current has been 

 estimated to be equal to the 0.026 of a Daniell battery ; the force 

 of the current of the frog muscle, about 0.05 to 0.08 of a Daniell. 



Negative Variation of Currents in Muscles and Nerves. If a 



muscle or nerve be thrown into a condition of tetanus, it will be 

 observed that the currents undergo a diminution of negative variation, 

 a change which passes along the nerve in the form of a wave and 

 with a velocity equal to the rate of transmission of the nerve im- 

 pulse. The wave-length of a single negative variation has been 

 estimated to be eighteen millimeters, the period of its duration being 

 from 0.0005 to 0.0008 of a second. 



It is asserted by Hermann that perfectly fresh, uninjured muscles 

 and nerves are devoid of currents, and that the currents observed 

 are the result of molecular death at the point of section, this point 

 becoming negative to the equatorial point. He applies the term 

 " action currents " to the currents obtained when a muscle is thrown 

 into a state of activity. 



Electrotonus. The passage of a direct galvanic current through a 

 portion of a nerve excites in the parts beyond the electrodes a con- 

 dition of electric tension, or electrotonus, during which the excita- 

 bility of the nerve is decreased near the anode or positive pole, and 

 increased near the cathode or negative pole ; the increase of excita- 

 bility in the catelectrotonic area that nearest the muscle being 

 manifested by a more marked contraction of the muscle than the 

 normal when the nerve is irritated in this region. The passage of 

 an inverse galvanic current excites the same condition of electrotonus ; 



