222 HUMAN PHYSIOLOGY 



whole phenomenon has been called physical electrotonus in 

 distinction from physiological electrotonus (change of irritability). 

 The electrical resistance of the nerve in the direction of the 

 fibre is 2\ million, in the transverse direction 12^ million, times 

 as great as that of mercury. 



Induced currents stimulate at the kathode only, hence act 

 as weak currents in respect to the law of contraction. 



The uninjured motor nerves in the human body seem to 

 follow other laws of contraction than the excised nerves of 

 a nerve-muscle preparation. When one electrode is placed 

 on the skin above a nerve to be investigated and the other 

 on some indifferent part of the body (back, neck) remote 

 from the first electrode, a make contraction follows the 

 feeblest but yet effective current when the electrode placed 

 on the nerve is the kathode (kathode-make contraction). 

 A little stronger current produces anode-make and anode- 

 break contraction (when the electrode on the nerve is the 

 anode), and, in a very strong current, also the kathode- 

 break contraction. This apparent deviation from the law 

 of contraction is due to the nature of the spreading of the 

 current in the human body. The nerve, in this case, is 

 traversed by the branching currents in diagonal and trans- 

 verse directions, not merely longitudinally as in the excised 

 nerve. 



(i) Irritability and conductivity depend also upon normal 

 vital conditions. Not only do excised nerves gradually 

 lose their irritability and conductivity, but also nerves which 

 have lost (by cutting, disease) their normal connection with 

 the nerve cells. A nerve thus severed dies, the axis-cylinder 

 and the medullary sheath disappearing and connective tissue 

 being deposited. Sometimes regeneration of the nerve trunk 

 still connected with the cell takes place. 



A change in the irritability of the nerve by fatigue has 

 not yet been definitely proven. 



Nothing is known concerning the chemical composition of the 

 real nerve-substance. No processes of metabolism have ever been 

 demonstrated in the stimulated or unstimulated nerve. The 

 metabolism is, at any rate, even in stimulated nerves, very slight, 



