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Mr. J. Gray. 



The alteration in the length or tension of a fibre exposed to acid depends 

 upon the concentration of salts present (Fischer), or on the ease with which 

 water can be drawn from the surrounding fluid. Hence, when ciliated cells 

 are exposed to a solution whose osmotic pressure is capable of withdrawing 

 a considerable amount of water from the cells, the amount of tension, set up 

 by a normal amount of acid at the surface of the fibres, will be reduced : 

 consequently, in solutions of high osmotic pressure, the amplitude of the 

 beat is affected, and the cilia stop when the amount of free water is zero. 



The above conception of the ciliary mechanism has two advantages : (i) it 

 does not endow the cilium with any hypothetical structure ; (ii) it brings the 

 mechanism into line with what is known of other contractile tissues. There 

 is, however, one corollary to the hypothesis which applies equally to cilia and 

 to muscle cells. If the liberation of an acid at the surface of the ciliary 

 fibrils enables the cilium to store potential energy and perform the recovery 

 stroke — then, when ciliary activity ceases in the presence of an experimental 

 acid, we must assume that the latter acid does not come into contact with 

 the contractile fibrils, since the cilia come to rest with the fibrils relaxed. 

 When, however, stronger concentrations of acid are used, the cilia stop 

 partly (in some cases almost completely) contracted, and occupy a position 

 near the end of the normal recovery stroke (see fig. 7). We may well 



Fig. 7. — Diagram illustrating the effect of acid on the terminal cilia of Mytihts. 

 (a) With acid just strong enough to stop cilia ; note all the cilia stop at the end of 

 the effective beat, (b) Acid of considerably greater strength ; note the cilia come 

 to rest between the two phases of the recovery stroke, (c) and (d) Positions 

 at beginning and end of the normal effective stroke. 



suppose that in this case the experimental acid has reached the contractile 

 fibres. Precisely the same phenomena are found in the heart: weak 



