158 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



this material cannot be minimized since many of our current concepts are 

 largely dependent on this work. It is to be hoped that the studies of Cole and 

 Moore (4) on the squid giant axon in situ will lead to a better surviving prepara- 

 tion which can be subjected to an adequate range of experimental conditions 

 and measurement. 



A preparation has been needed which is stable for long periods for a more 

 rigorous test of contemporary hypotheses under strictly steady state conditions. 

 This requirement is met by the sciatic of the tropical toad, Bufo marinus, which 

 can be deprived of the peripheral connective tissue sheath that ordinarily 

 controls entry and exit of ions (31, 49, 51) and which does not undergo a 

 measureable physical, chemical or functional change for days in ordinary frog 

 Ringer's (48). In the respects mentioned, it is far superior to bullfrog nerve, 

 which cannot be desheathed without undergoing drastic alterations (47). The 

 usual limitations of a multifibered preparation are present, namely, those 

 requiring special methods to evaluate the actual protoplasmic contents and 

 intracellular potentials from measurements of whole nerve; but with recently 

 developed techniques these are not insuperable, particularly in the light of 

 recent findings with single vertebrate and invertebrate fibers (20, 55, 56). Some 

 of the progress in this direction will be discussed later. 



For purposes of this review, information obtained from preparations of 

 different types will be drawn upon. Since certain aspects have been much more 

 fully developed for one type of fiber or nerve than another, a number of specific 

 observations may lack the generality to be desired. An effort will be made to 

 designate specific forms when observations have been so restricted; caution 

 must be exercised not to generalize freely until corroborative data become 

 available for other systems. Not until such data are at hand can the special be 

 distinguished with any degree of certainty from the general. 



One more comment is necessary before turning to the major business of this 

 report. For simplicity, and to a large extent of necessity because of lack of 

 evidence as to their functional role (e.g. 39), the detailed structures of nerve 

 and its fibers will be largely ignored. Considerable progress has been made in 

 microscopic and submicroscopic anatomy (15), which can be expected to be of 

 increasing value to physiologists as the probability of artifacts is minimized by 

 use of techniques such as the freeze-dry method. But attempts at correlation 

 with physiological findings can at present be regarded merely as tentative. 

 The term 'membrane' continues to be a useful cover for our ignorance and to 

 serve as a boundary to which are usually ascribed the mechanics of the phenom- 

 ena we observe. The convenience of such thinking should not deceive us into 

 believing that the several sheathing elements so characteristic of all individual 

 nerve fibers — myelin, Schwann and connective tissue sheaths — or that the 

 peculiarities of protoplasmic structure and organelles play no part in our ob- 

 servations. The relationships which have already been established between 



