100 HUMAN PHYSIOLOGY. 



cal 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 current increases or dimin- 

 ishes 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 whose direction is opposite to that 

 of the normal impulse in 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 or 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 impulse. The wave length of a single 

 negative variation has been estimated to be 18 millimetres; 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 a molecular death at the point of section, this point becoming negative 

 to the equatorial point. He applies the term " action currents " to the cur- 

 rents obtained when a muscle is thrown into a state of activity. 



Electrical Properties of Nerves. When a galvanic current is made 

 to flow along a motor nerve from the centre to the periphery, from the 

 positive to the negative pole, it is known as the direct, descending or centri- 

 fugal current. When it is made to flow in the reverse direction it is known 

 as the inverse, ascending or centripetal current. 



The passage of a direct current enfeebles the excitability of a nerve; the 

 passage of the inverse current increases it. The excitability of a nerve may 

 be exhausted by the repeated applications of electricity ; when thus 

 exhausted it may be restored by repose, or by the passage of the inverse 

 current if the nerve has been exhausted by the direct current or vice versa. 



During the actual passage of a feeble constant current in either direction 

 neither pain nor muscular contraction is ordinarily manifested ; if the current 

 be very intense the nerve may be disorganized and its excitability destroyed. 



Electrotonus. The passage of a direct galvanic current through a por- 

 tion of a nerve excites in the parts beyond the electrodes a condition of 

 electric tension or electrotonus, during which the excitability of the nerve 

 is decreased near the anode or positive pole, and increased near the kathode 



