48 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



When human striated muscle undergoes degeneration as a result of an in- 

 jury to its nerve, the degenerating muscle comes to resemble normal unstriated 

 muscle in its reactions to electricity, responding feebly to induced currents, at 

 a time when irritability to mechanical stimuli and to direct battery currents 

 is even increased. This is used by clinicians as a means of diagnosis of the 

 condition of the nerve and muscle. 



From what has been said it is evident that the rule laid down by Du Bois- 

 lieymond (see p. 32) must be modi tied in so far that there is for each tissue 

 a limit to the rate at which a change of intensity of the electric current acts 

 as an irritant. 



(e) Effect of the Angle at which the Current Enters and Leaves the Muscle 

 and Nerve. — The angle at which the current acts on the muscle-fibre has 

 been found to have a bearing upon its power to stimulate. Leicher 1 succeeded 

 in obtaining definite experimental evidence that when the current is so sent 

 through a muscle as to cross it at right angles to its fibres it has no irritating 

 effect, and that its power to stimulate increases as the angle at which the 

 threads of current strike the muscle-fibres decreases, being greatest when the 

 current passes longitudinally through the fibres. 



Similarly, it was found by Albrecht and Meyer 2 that the irritating effect 

 of the electric current is most active when it flows longitudinally through the 

 nerve, and that it is altogether absent when it flows transversely through it. 

 This view is doubted by some observers, who would attribute the difference 

 observed to differences in the electrical resistance. It is true that the resist- 

 ance to cross transmission is greater than to longitudinal transmission, but it 

 is not likely that this difference suffices to explain the lack of response to cur- 

 rents applied at right angles to the nerve-axis. 



Relative Efficacy of the above Conditions upon the Irritating Power of the 

 Electric Current. — When a current is applied to an irritable part of a nerve 

 or muscle at an angle suitable to excitation, the stimulating effect of the current 

 depends upon the rate at which its intensity is changed, the strength and 

 density of the current, i. e. its intensity, and the duration of the current. 



Fick 3 gives the following schema (Fig. 24) for the different ways in which 

 the intensity of the electric current may be varied, and compares the effects 

 of these different methods of application of the current. It must be re- 

 membered that a decrease of intensity acts no less than an increase to produce 

 excitation. In the above schema the abscissa represents the time, and the 

 ordinates the strength, of the current. Suppose the rise of intensity has a 

 form such as is represented in a, Figure 24 — that is, that the strength of the 

 current increases to a considerable height, but very slowly. Such a rate of 

 change, even though the rise of intensity were continued until the strength of 

 current was very great, would have no exciting effect upon a nerve and might 



1 Uhtersfuehimgen aus dem phytioloffischen Institut der Unircrsitat Halle, Heft i. S. 5. 



'-' /'lliiyer J s Archiv, 1880, Bd. xxi. S. 462. 



s Beitrage zur vergleichendc Phn/siologie du- irritublen Substanzcu, Braunschweig, 1863. 



