58 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and, if warmed, to rapid variations. Under pathological conditions the reac- 

 tion of nerve and muscle to electric currents may become blunted, and, as the 

 tissue degenerates, its power to respond to rapid changes of the electric current 

 is lessened. If a nerve be cut, the part which is separated from the influence 

 of the nerve-cells degenerates. The irritability at first increases and then 

 very rapidly decreases, in from three to four days being wholly lost. As the 

 nerve regenerates, the irritability is recovered very gradually, and the power 

 to respond to the relatively prolonged action of mechanical stimuli is regained 

 sooner than the ability to reply to changes as rapid as those of induced cur- 

 rents. Howell and Huber observed that regenerating nerve-fibres when they 

 have reached the stage resembling embryonic fibres, i. e. are strands of proto- 

 plasm without axis-cylinders, fail to respond to induction currents, though they 

 can be excited by mechanical stimuli. It was found that it is not until the 

 axis-cylinder has grown down into the regenerating fibres that the nerve is 

 capable of responding to induction shocks. 



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- 

 Eeymond (see p. 47) must be modified 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. An extreme illustration of this may be found in the astonish- 

 ing fact lately published by Tessla, that although in general alternating dynamo 

 currents are very deadly, a current of even high voltage may be passed through 

 the human body with impunity, provided that the rate of alternations be suf- 

 ficiently rapid. 



(e) Effect of ike 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 J 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 



> Untermchungen aus dem physiologischen Institut der Universitai Halle, Heft i. p. 5. 

 2 PftugeSs Archiv, 1880, Bd. xxi. p. 462. 



