236 ELECTRO-PHYSIOLOGY CHAP. 



a short-circuit," it cannot be doubted that these expiratory effects 

 are due to excitation of the expiratory fibres of the cervical 

 vagus, by some variation in the intrinsic nerve current. Obviously, 

 when the nerve is raised from, or dropped on to, the wound in the 

 neck, there must be complication from the currents of the injured 

 muscles. Knoll also refers the brief expiratory effect which 

 usually appears after simple division or constriction of the vagi 

 in situ, to excitation of the nerve by its own current, and this 

 effect must certainly be regarded as the analogue of tetanus from 

 division of the sciatic nerve in a cooled frog. It should be 

 noticed that the peripheral end of the vagus cannot be stimulated 

 to action, i.e. retardation of the cardiac beat, by its own current. 



The marked E.M.F. of the non-medullated olfactory nerve of 

 the pike explains the fact that its current readily and invariably 

 excites frog nerves. If looped over the end of a glass rod, it 

 may be dropped on to any point of a frog's nerve, forming a fine 

 pair of electrodes, and never fails to produce vigorous twitches 

 in the leg if contact is made at the transverse and longitudinal 

 sections (Kiihne, 11, p. 97). Kiihne even succeeded in exciting 

 curarised frogs' sartorii by the demarcation current of the pike's 

 olfactorius. 



Much interest, especially in regard to the theory of the 

 break twitch, attaches to the phenomena of interference 

 between the nerve current and an artificial current, when the 

 exciting electrodes are applied to the proximity of a cross-section, 

 or to any point along the nerve that happens to be electrically 

 active. Pfluger pointed out that the excitability of a tract of 

 nerve must be positively affected by its own current when a 

 transverse section is made, or a lateral branch of the nerve 

 amputated above the tract, since the demarcation current throws 

 that part of the nerve into katelectrotonus. If a leading -off 

 circuit connects the transverse section, or a proximal point on the 

 longitudinal surface, with any other point of the latter, current 

 passes through the part of the nerve between the contacts from 

 transverse to longitudinal section. Seeing that the separate axis 

 cylinders are, like 'the entire nerve -trunk, surrounded by in- 

 different conducting sheaths, there must (apart from the special 

 conditions in medullated nerve) be lines of current within the 

 sheaths in the same direction, making their exit at different 

 points of the surface of the single fibres, as of the entire nerve, 



