134 THE CELL 



to prevent polarisation of the electrodes (Part II.). The electrodes 

 which are used to lead the current from the cell (or group of cells) 

 to the galvanometer are subject to polarisation as explained in 

 Chap. X. The products of electrolytic decomposition of the cell 

 substance are transported to the electrodes and accumulated there. 

 The deposition of these products at the two poles, in course of 

 time, alters the nature of the electrodes. The cathode, for instance, 

 because of the accumulation of positive ions on it, becomes more 

 and more anodal. This produces an electric tension that causes 

 a current, the so-called polarisation-current, to flow in the opposite 

 direction to the original one. As this current grows in strength 

 it reduces the value of the tissue-current and after a short time 

 completely obliterates it. (See also Muscle, Chap. XIII., and 

 Nerve, Chap. XVII.) 



(/) Electrical effects are produced in living cells by suitable 

 stimulation. If a cell is injured the injured part becomes electro- 

 positive to the rest. This phenomenon, apart from any other 

 conditions, would be quite sufficient to justify the postulation of a 

 cell membrane. Consider the cell as a mass of protoplasm in an 

 envelope of matter which is permeable to the negative ion but 

 not to the positive ion of a dissociated electrolyte. This will 

 cause a difference of potential on the two sides of the membrane. 

 Inside will be a preponderance of negative ions while outside will 

 be an equal preponderance of positive ions. Current of Injury. 

 If now we could connect the inside of the membrane with the 

 outside, there would be a flow of current till the difference of 

 potential had been equalised. Current would flow to the pierced, 

 injured (or inside) part from the outside. That is, the injured 

 part would be similar to the zinc pole of a zinc-copper galvanic 

 couple. There the flow of current is from the zinc to the copper 

 inside the battery. The zinc is therefore said to be electro- 

 positive. The current, however, flows from the copper to the 

 zinc, outside the battery. The zinc is, therefore, said to form the 

 negative pole. Current of action. When alterations of tension 

 or stress take place in a cell they are accompanied by alterations 

 in electric potential. The part under stress becomes as if injured, 

 i.e. electropositive or zincy to the normal or unstressed part. 

 This may be due to an increase in the permeability of the membrane 

 at the stressed part, so that the positive ion gains access to the 

 cell. The seat of stress does not, however, remain at its point of 

 origin but passes as a wave of increased stress over the whole cell, 

 followed immediately by an electropositive wave. 



