590 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



of electrical currents applied to nerves in producing muscular contraction 

 is liable to many variations, and that numerous circumstances must be 

 taken into consideration before an explanation can be given of any partic- 

 ular phenomenon. Thus it has long been well known that though contrac- 

 tion of a muscle usually occurs at the moment of opening and closing a 

 current of electricity transmitted through its motor nerve, yet that this is 

 not constant, the contraction failing sometimes at the moment of closing 

 and sometimes at the moment of opening the current. The singularity of 

 this phenomenon has attracted the attention of many observers ; and the 

 names of Hitter and Nobili, Matteucci, Du Bois-Reymond, Eckhard, and 

 Pfluger deserve especial mention for the light which they have thrown upon 

 this "obscure and difficult department of physiology. From their observa- 

 tions and from those of many other experimenters, it is now 

 ascertained that the effects produced upon the muscle are de- 

 pendent first, upon the direction of the electrical current 

 passed through the nerve, whether centrifugal i. e., from 

 the origin to the periphery of the nerve (Fig. 203), or centrip- 

 etal i.e., from the periphery towards the origin (Fig. 204); 

 secondly, upon the state of excitability of the nerve ; and 

 thirdly, upon the strength of the current. Hitter and Nobili, 

 in whose observations the strength of the current was disre- 

 garded, endeavored to show that a definite succession of re- 

 sults followed the application of the electrical stimulus, ac- 

 cording to the direction and the excitability of the nerve. 

 Ritter, operating on Frogs, the excitability of whose nerves 

 departed slowly after death or excision, was able to distinguish six stages ; 

 whilst Nobili, operating in the warmer climate of Italy, could distinguish 

 only four. The following tabular arrangement taken from Funke will show 

 the relations of the contraction of the muscle to the degree of excitability 

 remaining in the nerve, and to the direction of the current : 



RITTER AND NOBILI'S LAW OF CONTRACTION. 



C indicates closure of the current ; O indicates opening of the current, or breaking 



contact. 



FIG. 203. FIG. 204. 



-r 



