GENERAL PROPERTIES OF THE NERVES. 601 



If a sufficiently powerful constant current be passed through a nerve, disorganization 

 of its tissue takes place, and the nerve finally loses its excitability, as it does when 

 bruised, ligatured, or when its structure is destroyed in any other way. It was thought 

 by Galvani, and the idea has been adopted by Matteucci, Guerard, and Longet, that a 

 current directed exactly across a nerve, so as to pass at right angles to its fibres, does 

 not give rise to muscular contraction. This view is now accepted by most modern 

 experimenters. 



All who have experimented upon the action of galvanism upon the mixed nerves 

 have noted the fact alluded to above, that the phenomena of contraction are manifested 

 only on closing or on opening the circuit. Take, for example, a frog's leg prepared 

 with the nerve attached; place one pole of a galvanic apparatus on the nerve and 

 then make the connection, including a portion of the nerve in the circuit ; with the 

 feeblest current, contraction occurs only on closing the circuit. This takes place only 

 when the current follows the direction of the nerve (descending), and there is no con- 

 traction either on closing or opening the circuit with the ascending current. With what 

 is called the " weak " current (Pfliiger), contraction occurs only on closing the circuit, 

 for currents of either direction. With the " moderate" current, contraction occurs both 

 on closing and on opening the circuit, for currents of both directions. With the 

 "strong" current, contraction occurs only on closing the circuit with the descending 

 current and only on opening the circuit with the ascending current. The above con- 

 stitute what is called Pfltiger's " law of contraction." After a time the nervous irrita- 

 bility becomes somewhat enfeebled by exposure of the parts. The phenomena then ob- 

 served belong to the conditions involved in the process of " dying " of the nerve. In the 

 later stages of this condition, the phenomena may be formularized as follows: 



If the sciatic nerve attached to the leg of a frog, prepared in the usual way for such 

 experiments, be subjected to a feeble galvanic current, there is a time when muscular 

 contraction takes place only at the instant when the circuit is closed, no contraction 

 occurring when the circuit is opened ; and this occurs only with the descending current; 

 viz., when the current flows toward the periphery, the positive pole being above and the 

 negative below. If the poles be reversed, so that the galvanic current flows from the 

 periphery toward the centres (the ascending current), contraction of the muscles occurs 

 only when the circuit is opened and none takes place when the circuit is closed. These 

 phenomena are distinct after the irritability of the parts has become somewhat dimin- 

 ished by exposure or by electric stimulation of the nerve. 



FIG. 190. Frog* s legs prepared so as to show the contrasted action of the descending and the ascending cur- 

 rent. (Matteucci). 



A very simple experiment made by Matteucci strikingly illustrates the contrasted 

 action of the descending and the ascending currents. The posterior extremities of a frog 

 are prepared so as to leave the nerves of the two sides connected together by a portion 



