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, Gu6rard, 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 manifest- 

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

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

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

 mildest currents, contraction occurs only on closing the circuit, independently of the 

 direction of the current ; with currents of medium strength (Pfluger), contractions 

 occur both at closing and opening the circuit, for currents of either direction ; with 

 strong currents, contraction occurs only at the closing of the direct current and the 

 opening of the inverse current. After a time, however, the nervous irritability becomes 

 somewhat enfeebled by exposure of the parts. The phenomena then observed 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 made, no contraction 

 occurring when the circuit is broken ; and this occurs only with the direct 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 inverse current), contraction of the muscles occurs 

 only when the circuit is broken 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, but they may occur in per- 

 fectly fresh parts, when the galvanic current is very strong. Usually, when the ner- 

 vous irritability is at its height, contractions occur both on closing and breaking the cir- 

 cuit ; but they are more powerful on closing the circuit, for the direct current, and on 

 breaking the circuit, for the inverse current. This fact has been noted by all modern 

 experimenters. 



FIG. 190. Frog's legs prepared so 



to show the contrasted action of the direct and the inverse cut rent. 

 (Matteucci.) 



A very simple experiment made by Matteucci strikingly illustrates the contrasted 

 action of the direct and the inverse current. The posterior extremities of a frog are pre- 

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



