PROPERTIES AND FUNCTIONS OF NERVES. 101 



or negative pole; the increase of excitability in the katelectrotonic 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; and the diminution of excitability near the anode, the anelec- 

 trotonic area, that now nearest the muscle, being manifested by a less 

 marked contraction than the normal when the nerve is stimulated in this 

 region. Between the electrodes is a neutral point where the katelectrotonic 

 area emerges into the anelectrotonic area. If the current be a strong one, 

 the neutral point approaches the kathode; if weak, it approaches the 

 anode. 



When a nervous impulse passes along a nerve, the only appreciable effect 

 is a change in its electrical condition, there being no change in its tempera- 

 ture, chemical composition or physical condition. The natural nerve cur- 

 rents, which are always present in a living nerve as a result of its nutritive 

 activity, in great part disappear during the passage of an impulse, under- 

 going a negative variation. 



Law of Contraction. If a feeble galvanic current be applied to a recent 

 and excitable nerve, contraction is produced in the muscles only upon the 

 making of the circuit with both the direct and inverse currents. 



If the current be moderate in intensity, the contraction is produced in the 

 muscle both upon the making and breaking of the circuit, with both the 

 direct and inverse currents. 



If the current be intense, contraction is produced only when the circuit 

 is made with the direct current, and only when it is broken with the inverse 

 current. 



The Reaction of Degeneration. Two different applications of elec- 

 tricity are used in electro-physiology and electro-therapeutics the constant 

 or galvanic, and the interrupted or faradic currents. Injured and paralyzed 

 muscles and nerves react differently to these two kinds of stimuli, and the 

 facts are of the greatest importance in the diagnosis and therapeutics of the 

 precedent lesions. The principal difference of behavior relates to the 

 reaction of degeneration a condition produced by paralysis of any kind. 

 It is characterized by a diminished or abolished excitability of the muscles 

 to the faradic current, while there is at the same time an increased excita- 

 bility to the galvanic current. The synchronous diminished excitability of 

 the nerves is the same for either current. The term partial reaction of 

 degeneration is used when there is a normal reaction of the nerves, but the 

 muscles show the degenerative reaction. This condition is a characteristic 

 of progressive muscular atrophy. 



