PHYSIOLOGY OF NERVE :;:* 



nerve. The reflex contraction can be called out by stimulating an 

 afferent nerve by the induced current, or even by the impulses 

 reaching the cord as a result of touching or pinching the animal. 

 The E.M.F. of the negative variation is about '00025 volt, as 

 determined by the capillary electrometer ( 12 ). 



It is possible to take a step further and examine the negative 

 variation in nerves which are conducting the impulses which 

 naturally traverse them without the complication of any disturb- 

 ing agency. Two nerves have been employed for experiments of 

 this kind, the phrenic and the vagus. Both these conduct im- 

 pulses which are repeated at each act of respiration, and so it is 

 possible to arrange the rather elaborate apparatus required with 

 some degree of certainty of obtaining the phenomenon often 

 enough to be readily investigated. Reid and Macdonald ( 13 ) ex- 

 amined the phrenic ; Lewandowsky ( 14 ), Alcock and Seemann ( 12 ), 

 and Einthoven ( 15 ) the vagus. The latter is by far the easier object 

 to study, and allowing for the increasing delicacy of the instruments 

 employed, the results agree very closely as far as the nervous 

 phenomena are concerned. 



Lewandowsky, using the ordinary galvanometer, found that one 

 negative variation occurred every time the lungs were artificially 

 blown out. Alcock and Seemann, with the capillary electrometer, 

 observed the same phenomenon. They found in addition that a 

 negative variation occurred at each natural inspiration, and that 

 an effect was also observed when the air was sucked out of the 

 lungs. Einthoven, using his string galvanometer, by far the most 

 sensitive instrument yet invented, 1 was able to add certain very 

 interesting details which are worthy of careful consideration. 



Einthoven's results will be understood from an examination of 

 Fig. 4. The upper line is the shadow of the " string " (a fibre 



1 This galvanometer consists of a fibre of silvered quartz, about 2 'OM in diameter, 

 suspended in a powerful magnetic field (20,000 C.G.S. units). The fibre is illuminated 

 by the. light of the electric arc, and the shadow of the fibre, magnified 600 diameters, 

 is thrown on a photographic plate. The instrument is thus a ' ' moving coil " 

 galvanometer, with the coil reduced to its simplest expression. When a current 

 of electricity passes down the fibre, it is deflected, and 10~ 8 amperes give a 

 measurable deflection. 



The instrument therefore measures current not voltage ; but if the resistance of 

 the circuit is known, the voltage can be determined very simply by Ohm's law. In 

 nearly all electro-physiological problems it is the E.M.F. that is of interest, as the 

 resistance and consequently the current is varied by circumstances not bearing 

 on the experiment. 



