496 ELECTRO-PHYSIOLOGY 



tract. A special contrivance enabled him to alter the " period of closure " 

 i.e. the time for which the polarising current was sent through the polaris- 

 able object from '00 1-20 sees. The same contrivance effected closure of 

 the galvanometer circuit, after breaking the battery circuit at a minimal and 

 constant interval. 



The resulting secondary electromotive effects in the muscle are essentially 

 dependent on the density and duration of the primary current, and are very 

 confused, owing to the constant interference of negative and positive effects. 

 "With a current density below that of 2 Groves, and with a very brief 

 closure, no polarisation is, as a rule, perceptible on the galvanometer. The 

 first traces obtained with 1 Dan. and 1 sec. closure are negative. The first 

 positive traces appear with 2 Groves, and about 0'3 sec. closure." 



With increasing period of closure, du Bois found that positive polarisation 

 quickly reaches its maximum, and then declines more slowly, and passes 

 over into negative polarisation, which again rises to a maximum. He fixes 

 the " critical point " of closure as that at which positive passes into negative 

 polarisation. The ma*ximal positive polarisation in these experiments was 

 at a closure of 0*075 sec. with 20 Groves (!) ; the maximal negative 

 polarisation at ten minutes' closure of 1 Grove. Brief impacts of current 

 (induction shocks) invariably produce positive polarisation only. 



Both positive and negative polarisation are very persistent, and some- 

 times outlast the opening of the polarising current for twenty minutes or 

 more. If the current is broken at the " critical point," du Bois not 

 infrequently observed a diphasic effect, usually in the direction of first 

 negative and then positive polarisation. This is due to the fact that, while 

 both polarisations are simultaneously present from the moment of closure, 

 they increase in a different degree, "negative polarisation rising more in 

 proportion with the time of closure, while positive polarisation rises quickly 

 at first, and then more slowly." 



Du Bois-Reymond further concluded from experiments in which the 

 upper and lower half of regularly - constructed muscles were traversed 

 alternately by the current, and tested for polarisation, that " strong positive 

 polarisation is exhibited in the upper half with ascending, in the lower half 

 with descending direction of current." 



Dead muscles still exhibit traces of negative internal polarisability, that 

 are completely abolished only by boiling ; positive polarisation, on the 

 contrary, is exclusively characteristic of living muscle. 



Du Bois-Reymond concluded, " not that electromotive forces homodro- 

 mous with the primary current are generated by the positively polarisable 

 tissues, but that the carriers of pre-existing electromotive forces (electromotive 

 molecules) are homodromously adjusted with the primary current." 



How little these results really support the molecular theory, is obvious 

 from the later investigations of Hering and Hermann (68, 69). 



In the first place, Hering proves conclusively that there can be no 

 question of internal positive or negative polarisation in du Bois-Reymond's 

 sense, since the actual seat of the electromotive changes induced by the 

 exciting current is at those points of the contractile substance by which 

 the current enters or leaves the muscle (the physiological poles) : so that the 

 close relation between these phenomena and the polar action of the current 

 is unmistakable. 



