ix ELECTRICAL EXCITATION OF NERVE 



121 



with ascending direction, an expiratory slowing, or arrest, of 

 respiration, although a frog's leg included iu the circuit was 

 not once made to contract (Fig. 172). 



The persistent excitation of secretory nerves by the constant 

 current has been established on the frog's tongue, by the changes 



& 



Fio. 172. Respiratory curves (rabbit). Gradual shunting of ascending current into vagus. 1 Dan. 

 (S) Closure, (Oe) opening of current. The rheochord slider was shifted from a to b. (LangendorfT 

 and Oldag.) 



of the mucosa current during excitation of the glosso-pharyngeal 

 (Biedermann, 8). In the cardiac vagus Griitzner confirms the 

 earlier conclusions of v. Bezold (Unters. ilber die Innerv. des 

 Hcrzens, Leipzig, 1863, p. 72), since with a current of twelve 

 pincus-cells the make and break only are effective, as shown by 

 the following curve (Fig. 173). 



Another proof that current excites not merely at the moment 

 when it begins (or ceases), or during variations of density, but 

 throughout its entire passage, is seen in the fact that at a given 

 uniform strength of cur- 



f\ I c. 1 



rent a closure twitch only *' 



appears when the duration 

 of current has outlasted 

 a certain time (A. Fick). 

 This fact is already familiar to us in muscle more particularly 

 when non-striated where it is easily determined. Its proof is 

 more difficult in nerve, because the time -values involved are 

 exceedingly small. While, e.g., in smooth molluscan adductor- 

 muscle the maximum effect of a given strength of current is not 

 reached even with a duration of ^^ sec., the same, according 

 to Konig (10), is obtained in excitation of the nerve after 

 0'017 -O'0 18 sec. Under all conditions, therefore, we must 

 allow for the fact that very brief closures of current produce no 



FlG - 



