522 SCIENCE PROGRESS. 



the winter pigeon, but there is no keratinisation in the latter 

 case, and in the former the change certainly does not favour 

 the electromotive force. Again, though Hermann ascribes 

 the electromotive force of the action current to gland activity, 

 his theory does not explain matters fully till a contractile 

 sheath is added to the gland ; yet the tongue glands of the 

 frog or the skin of an eel give good enough action currents 

 without muscle. 



The " myogenic theory" of Engelmann is also for the 

 last reason invalid, let alone the fact that in cases where 

 muscle is present it rests upon the rejected " pre-existing 

 theory " of muscle current. 



We must with Biedermann consider the normal "rest 

 current " of a gland — and it is normal, and not due to 

 "injury" — as the electrical expression of a continuous 

 chemical activity in its cells, and that on excitation we 

 deal, not with a new electromotive force originating from 

 other sources, but simply with a change in the action of the 

 same structures, which are the origin of electromotive force 

 also in the condition called, by contrast, that of " rest". 



As regards the admittedly diphasic nature of the 

 " current of action," as we have seen, two theories may 

 be entertained ; that of Bayliss and Bradford, deduced from 

 experiments with the mammalian salivary glands, and that 

 of Biedermann, built upon facts obtained with mucus- 

 secreting glands and membranes. 



According to the former theory an outgoing " action 

 current " is the result of flow of fluid in some part of the 

 secreting mechanism (physical and indirect) ; ingoing cur- 

 rent is associated with the elaboration by the protoplasm of 

 secretion stuffs (physiological and direct). 



According to the latter theory, both outgoing and ingo- 

 ing "action currents" are the direct result of cell activity, 

 i.e., both physiological, but the two currents are paired with 

 the two hypothetical sides of cell life, dissimilation and 

 assimilation. If, however, we follow Biedermann into his 

 further speculation as to the ultimate nature of the two sets 

 of processes related to the ceil currents we find he reaches 

 the opposite conclusion to Bayliss and Bradford, and gives 



