CILIARY ACTIVITY 25 



defined by the Doiman membrane equilibrium, are intimately 

 affected by the acidity of the medium with which they are in 

 contact. 



Gray has outlined an hypothesis according to which the 

 chemical events of the contraction cycle in muscle and ciliated 

 cell are closely analogous. It may be considered under three 

 headings : («) the possible production of an acid substance 

 during the contractile process ; (b) the relation of oxygen 

 consumption to the events of activity and recovery ; (c) the 

 nature of the substances used up in the transformation of 

 chemical into kinetic energy by the cell. 



The production of acid may be taken first. The only 

 method at present available for obtaining any evidence on this 

 question is derived from studying the relation of ciliary activity 

 to the ionic constituents of its surroundings. Ciliary motion 

 of Mytilus gill-filaments can be preserved for many hours in a 

 Van 't Hoff solution containing chlorides of Na, K, Ca; and Mg 

 in the same proportions as sea- water at a pH. about 7*8. On 

 addition of acid the cilia on the gill of Mytilus cease to move 

 when the hydrogen ion concentration of the solution reaches 

 a limiting value on the acid side of neutrality. What is 

 especially interesting is the way in which this stoppage is 

 brought about ; the cilia come to rest in an acid solution by 

 a gradual slowing of the rate, without reduction in the ampHtude 

 of the beat, till finally movement is arrested at the end of the 

 effective stroke, i.e. in the relaxed condition. This fact, while 

 suggesting that the acid does not exert its effect by damaging 

 the contractile fibrils of the cilium itself — since the amplitude 

 is not directly affected — at first sight points to the conclusion 

 that the effect is a surface one, concerned only with the rate at 

 which the excitation state is generated. 



That the action is not a surface one, however, Gray has 

 proved by several lines of experimentation. The first depends 

 upon the fact that the weak organic lipoid-soluble acids 

 penetrate the lipoid membrane of the cell more readily than do 

 strong acids. Similarly, weak bases like ammonium hydrate 

 are more penetrative than strong bases like sodium hydroxide. 

 If the cessation of ciliary motion in acid medium were a surface 



