

GENERAL PROPERTIES OF THE NERVES. 603 



induced in a series of legs, the nerve of one being in contact with the muscles of another. 

 This illustrates the great delicacy of the galvanoscopic frog's leg, as it will indicate a 

 current due to a single muscular contraction, which does not affect an ordinary galva- 

 nometer. It is conclusively proven that the "induced contraction," as just described, is 

 not due to an actual propagation of the galvanic current, but to a stimulus attending the 

 muscular contraction itself, by the fact that the same phenomena occur when the first 

 muscular contraction is induced by mechanical or chemical excitation of the nerve. 



Galvanic Current from the Exterior to the Cut Surface of a Nerve. Before we study 

 certain phenomena presented in nerves of which a portion is subjected to the action of 

 a constant galvanic current, it is important to note the fact that there exists in the 

 nerves, as in the muscles, a galvanic current from the exterior to their cut surface. This 

 fact has been noted by all who have investigated the subject of electro-physiology. It 

 has been roughly estimated by Matteucci that the nerve-current has from one-eighth 

 to one-tenth the intensity of the muscular current. The existence of the nerve-current 

 has, as far as we know, no more physiological significance than the analogous fact ob- 

 served in the muscular tissue. It is presented in nerves removed from the body and has 

 no relation to their functional activity, whether in normal action or excited by artificial 

 stimulation. 



Effects of a Constant Galvanic Current upon the Nervous Irritability. Aside from 

 the disorganizing effect upon the nerves of a powerful constant current, which is due 

 solely to decomposition of their substance, a feeble current has been found to exert an 

 important influence upon the nervous irritability, according to the direction in which the 

 current is passed. The law in accordance with which this influence is exerted is stated 

 by Matteucci as follows : 



"A continued electric current passed through a mixed nerve, the crural or the lum- 

 bar, for example, modifies the excitability of the nerve in a very different manner, accord- 

 ing to its direction. The excitability is enfeebled by the passage of the direct current, 

 and, on the contrary, it is preserved and augmented, at least within certain limits, by the 

 inverse current. The time necessary in order that the current shall produce this modifi- 

 cation is proportionate to the degree of excitability of the nerve and in inverse ratio to 

 the intensity of the current. After the breaking of the circuit, the modification of the 

 nerve tends to cease at a period that is short in proportion as the excitability of the nerve 

 is great and the intensity of the current is feeble. This proposition explains the differ- 

 ence in the electro-physiological effects of the continued current according to its direc- 

 tion, and the well-known phenomenon of voltaic alternations." 



This law has been carefully studied and formularized, as above, by Matteucci, but its 

 discovery is attributed by physiological writers to Pfaff. After a time, varying with the 

 excitability of the nerve and the intensity of the current, the direct current will destroy 

 the nervous irritability, but this may be restored by repose, or more quickly by the pas- 

 sage of an inverse current. If the inverse current be passed first for a few seconds, a 

 contraction follows the breaking of the circuit; and this contraction, within certain 

 limits, is more vigorous the longer the current is passed. At the same time, the pro- 

 longed passage of the inverse current increases the excitability of the nerve for any kind 

 of stimulus. When the inverse current has been passed through the nerves for several 

 hours, breaking of the circuit is followed by very violent contraction and a tetanic con- 

 dition of the muscles, enduring for several seconds. 



Electrotonus, Anelectrotonus, and Catelectrotonus. 



Many years ago, Du Bois-Reymond discovered the curious and interesting fact that, 

 when a constant galvanic current is passed through a portion of a freshly-prepared nerve, 

 those parts of the nerve not included between the poles are brought into a peculiar con- 



