FOOD DIGESTION AND RESPIRATION 83 



But the most interesting phenomenon showing a change of per- 

 meability is the electrical change which takes place in activity. 

 We may spend a short time on this, since the explanation applies 

 to muscle and nerve as well as to secreting glands. 



We saw above that a membrane may be impermeable to an 

 electrolyte because one of the ions of this electrolyte cannot pass 

 through, although the membrane may be easily permeable to the 

 other ion. This opposite ion goes out only as far as the attraction 

 of the oppositely charged ion inside the membrane will allow it. 

 We have here the Helmholtz double layer in one of its forms. A 

 simple illustration may enable this important conception to be 

 grasped. Imagine two large pastures separated by a fence, and 

 that the spaces between the bars of this fence are wide enough to 

 allow lambs to get through, but too narrow for their mothers. 



Membrane. , Procoplasm. 



-f-, -^ -4- -4- -f- -I- 



(A) (B) 



FIG. 3. Diagram of the electrical state of the resting cell (A) and 

 that of an active cell (B). 



Introduce into one of these pastures a flock of sheep, each ewe with 

 one lamb. In the course of their wanderings they will arrive at the 

 fence. The propensity of the lambs to wander further will take 

 them through the fence, but the ewes must remain behind. How- 

 ever, the attractive forces of their mothers, especially that of food, 

 will prevent the lambs from being in any number far from the fence 

 at any time. Similarly, the presence of the lambs in the adjoining 

 field will prevent the ewes from wandering far from the fence. It 

 may be said that the thickness of the layer would be somewhat 

 great, but if we imagine molecules magnified to the size of sheep 

 the proportions would not greatly differ from the molecular one. 



Let us suppose that the membrane of a particular cell is 

 permeable to the cations of some salt contained within the cell, 

 impermeable to the anions of this salt. The cell will be surrounded 

 by an electrically positive sheath, the other component of the 

 double layer with a negative charge being on the inside of the 

 membrane. Fig. 3 shows the arrangement at A. If we connect 



