THE PERMEABILITY OF MEMBRANES 141 



due to the diminution of permeability towards muscariue, brought about by 

 atropine. 



The Nerve Synapse. According to the view advocated by Sherrington (1906, 

 p. 16) the communication of a nerve impulse to the cell body of another neurone 

 takes place across a membrane, the " synaptic membrane." It is, therefore, owing 

 to changes in the permeability of this membrane that impulses are allowed to 

 pass or not. Whatever may be the actual chemical substance that diffuses 

 through, or whether only a physical process is involved, there is every probability 

 that the ions of dissociated salts play a large part in the transmission of nerve 

 processes, so that it is a matter of importance to see what kind of action may 

 be looked for. 



We have as yet very little direct evidence on the question, but there are some observa- 

 tions which are of interest. Locke (1894) found that immersion of the sartorius muscle 

 of the frog in 0'7 per cent, sodium chloride had the effect of preventing the muscle from 

 contracting when the nerve to it was excited, although its direct excitability was not abolished 

 and the effect was not produced if the nerve alone were immersed. Addition of traces of 

 calcium salt to the solution restored the normal state. According to Overton (1904, p. 280), 

 reflex excitability is lost in the absence of calcium from the central nervous system and one is 

 obviously reminded of the action of calcium salts on colloids. Whether the synaptic 

 membrane requires to be more or less semi-permeable, in the osmotic sense, in order to permit 

 the excitatory process to pass, cannot be answered until we know more as to the nature of 

 this process. 



Certain facts to be described below with respect to reciprocal innervation in 

 reflex action are made more explicable if we could imagine a membrane permeable 

 to certain ions in one direction only. There is some evidence that the skin of 

 the frog is permeable to sodium ions from without in, but not from within out. 

 The most satisfactory evidence seems to be that the skin acts as a rectifier for 

 alternating currents, that is, it allows the one part of the period, in which the 

 current is flowing in one direction, to pass through more easily than that in 

 which the current flows in the opposite direction (Bayliss, 1908, p. 235). 



This result would also be obtained, as Hober justly points out (1911, p. 493), if the cell 

 membrane on the inner side of the skin were permeable to one only of the ions of the salt. I 

 found, in fact, that similar phenomena are shown by a system consisting of a solution of Congo- 

 red inside a parchment paper membrane, which is permeable to the sodium ion of the salt only. 

 Since a current can only pass continuously when a quantity of positive ions can pass to the 

 negative pole equal to the negative ions passing to the positive pole, it follows that, if the 

 positive pole is outside the membrane, which is impermeable to the negative ions, these can 

 never get to the positive electrode outside at all ; while, if this electrode is on the same side of 

 the membrane as the anions, so that they can reach it without hindrance, the current will 

 pass readily, because the cations can pass through the membrane. 



It does not seem necessary, therefore, to assume an irreciprocal permeability, 

 which is ditncult to conceive. In any case, it would only be possible in the 

 case of a living membrane, to which energy was being supplied by cell activity. 

 Otherwise, there would be a spontaneous difference of potential kept up between 

 the two sides of the membrane and the possibility of a perpetual motion machine. 



Fertilisation of the Egg Cell. In this process, it has been shown by M'Clendon 

 (1910, p. 256) that the membrane becomes considerably more permeable to 

 electrolytes, evidenced by the increase of electrical conductivity of a mass of 

 eggs of the sea urchin on fertilisation. There is other evidence of increased 

 permeability in the escape of pigment observed by Lillie, who regards the 

 essential element in the artificial segmentation under the influence of certain 

 salts as an increase in the permeability of the cell membrane. 



Gray (1913), also, found diminution of electrical resistance in Echinus eggs 

 in the process of fertilisation, followed by return to or towards the normal. 



The Permeability of the Blood Vessels. It is plain that all substances necessary 

 for the nutrition of cells and all those produced by the cells, so far as they 

 pass into the blood stream, have to pass through the wall of the capillaries (except, 

 perhaps, in the case of the liver Schafer, 1902). Some of these substances are 

 in the colloidal state, and therefore, unless the cells are permeable to colloids, 

 which does not seem probable, these colloids must escape between the cells, by a 

 process like that of filtration. This question will come up for discussion later, 

 but it may be remarked here that, as far as the blood proteins are concerned, 



