POLAR EX C IT A TION IN MAN. 5 1 5 



obviously cause an initial relaxation, succeeded by a return to tonicity, 

 i.e. an augmentation. A further experiment makes it clear that such 

 bipolar interference effects are not the causative factors. The abductor 

 muscle free from tonus cannot relax in response to the excitation 

 of its inhibitory nerve fibres, and any subsequent tonicity cannot there- 

 fore be due to anodic interference with such cathodic excitation of 

 inhibitory fibres. Such a muscle shows no alteration at the in- 

 stant the current is made, but the tonicity still develops during the 

 closure. It must therefore be due to prolonged excitation of the 

 augmentor nerves, the muscular effect of which is retarded in its onset. 

 This retardation is undoubtedly due to the impairment of conductivity, 

 since it becomes more pronounced as the stimulating current is increased 

 in intensity. 



The nerves to the muscles of the crayfish claw, owing to their 

 different degrees of excitability, and to the distinctive characters of the 

 muscular response evoked by their excitation, thus furnish an admirable 

 object for demonstrating the possibility of polar excitation being a 

 prolonged state. 



The results do much to confirm the truth of Biedermann's views as 

 to the twofold character of polar excitation in the medullated nerves of 

 voluntary muscle. 1 These are pre-eminently susceptible under normal 

 circumstances to the onset or cessation of current flow, but they can 

 be so altered as to respond, in addition, to the prolonged polar 

 change following current flow. The nerves of some other structures 

 respond normally to both the initial and the prolonged changes. E. du 

 Bois-Eeymond's law of excitation, which places rapidity of variation in 

 current intensity as the effective determinant of exciting efficiency, 

 applies only to the production of the initial excitations, and is not 

 therefore of general application. The efferent nerve appears to be 

 differentiated, so that its susceptibility to the form of stimulus shall be 

 related to the activity of the peripheral responding mechanism ; if a 

 continuous response is that most appropriate for the discharge of its 

 functions, the nerve is susceptible to the continuous form of excita- 

 tion. 



It is in consequence of such varying susceptibility that certain 

 nerves are incapable of electrical stimulation by currents of short 

 duration, unless these are rapidly repeated. Thus the vasomotor 

 nerves, whose stimulation in the cervical sympathetic causes constriction 

 of the vessels in the rabbit's ear, are not easily excited by a single 

 induced current, though readily excited by a series of such currents; 

 the secretory nerves, such as the glosso-pharyngeal, the excitation 

 of which causes secretory and corresponding electromotive changes in 

 the frog's tongue, are readily stimulated by the closure and opening 

 of galvanic currents of some duration, but are only slightly excited, if at 

 all, by induced currents. 2 



Polar excitation in man. — The polar excitation of a nerve is com- 

 plicated when this is surrounded by a mass of conducting tissue. In all 

 the experiments previously referred to, care is taken to insure that the 

 nerve between and on each side of the poles is free ; but when, as in the 

 excitation of nerves in man, one electrode is applied to the skin over the 

 course of a nerve, it is clear that, though this may represent truly 



1 Biedermann, " Elektrophysiologie," 1895, S. 596, 597. 



2 Biedermann, Arch./, d. ges. Physiol, Bonn, 1893, Bd. liv. S. 241. 



