964 



SCIENCE 



[N. S. Vol. XXXVII. No. 965 



acteristics and arrangement of the solid col- 

 loidal material. 



However simply organized a cell may 

 seem, there are certain elements of struc- 

 ture which appear always to be present, and 

 to play a fundamentally important role in 

 stimulation and in other life-processes. 

 These are the membranes. Most, if not all, 

 living cells are delimited from the medium 

 in which they live by thin semi-permeable 

 colloidal surface-films, the so-called plasma- 

 membranes. Similar semi-permeable parti- 

 tions are often found in the cell-interior, 

 e. g., about nuclei, vacuoles, chromato- 

 phores, and other structures. They appear 

 to be formed of the same colloids as the 

 other protoplasmic structures, namely, pro- 

 teins and lipoids. These colloids, like many 

 other organic substances, have, when dis- 

 solved in water, a marked influence in low- 

 ering the surface-tension of the solvent. 

 Any substance thus acting tends, by the 

 operation of Gibbs's principle, to collect or 

 condense on the free surfaces; if the sub- 

 stance is colloidal in nature it may there 

 pass out of solution and form a solid sur- 

 face-film or membrane; and it is probably 

 under conditions essentially like these that 

 the cell-membranes are formed. Artificial 

 membranes similar in many of their prop- 

 erties to the plasma or nuclear membranes 

 of cells may be formed in protein solutions 

 about droplets of chloroform, mercury or 

 other water-immiscible substances. Now 

 the plasma-membranes of irritable cells un- 

 doubtedly play a fundamentally important 

 part in stimulation, as will be seen below, 

 so that it will be necessary to consider first 

 some of the essential properties of these 

 membranes before passing to the considera- 

 tion of the stimulation-process itself and its 

 modification by anesthetics. 



The plasma-membranes are typically 

 semi-permeable structures — so much so that 

 living cells form in many cases the most 



convenient and rapidly acting osmometers 

 that we possess. If we place living cells, 

 like plant-cells or blood corpuscles, in so- 

 lutions of sugars, neutral salts and various 

 other substances not in themselves immedi- 

 ately injui'ious to the cells, osmotic effects 

 result from which the osmotic pressure of 

 the solution relatively to that of the cell- 

 contents can be estimated with great accu- 

 racy — as the researches of de Vries, Over- 

 ton, Hedin and many others have shown. 

 Two provisos are necessary in making use 

 of living cells as osmometers : first, the dis- 

 solved substance must not by its own ac- 

 tion impair the semi-permeability of the 

 membrane, and second, it must not appre- 

 ciably penetrate the membrane during the 

 time occupied by the experiment. The 

 plasma-membranes are in fact semi-perme- 

 able only in relation to certain classes of 

 substances ; towards others they show them- 

 selves freely permeable, and the character 

 of these substances is important, because 

 indication is thus afforded of the chemical 

 nature of the materials composing the mem- 

 branes. This is a matter of fundamental 

 importance in the theory of anesthesia. 

 Let us take for example a tissue composed 

 of typical irritable cells, such as a frog's 

 voluntary muscle. In studying the osmotic 

 properties of this tissue, Overton found 

 many years ago that the cells behaved in 

 solutions of certain substances as if they 

 were enclosed by strictly semi-permeable 

 membranes; the chief of such substances 

 are sugars, neutral salts, polyatomic alco- 

 hols like mannite, and amino-acids like 

 glycocoU; but toward a large series of 

 mainly organic substances, including alco- 

 hols, esters, aldehydes, hydrocarbon-deriva- 

 tives and others, the membranes behaved 

 as if freely permeable. Thus in an m/8 

 (0.7 per cent.) solution of NaCl the muscle 

 retains its weight unaltered, neither absorb- 

 ing nor losing water; similarly in solutions 



