Mr Or ay, The Mechanism of Ciliary Movement 355 



ciliiim will fly forward: in doing so the potential energy will be 

 expended in the form of work done on the water. 



There are, however, certain cilia {e.g. in Ctenopliores) which 

 differ from the cilia of Mijtilus in that they can be stopped in the 

 position normally occupied at the end of the recoyery beat. We 

 must regard such cilia as possessing potential energy when at rest, 

 in the same way as a stretched spring which is held back by a 

 mechanical catch. Such cilia appear to resemble muscle fibres very 

 closely. 



Now we may enquire the origin of the potential energy stored 

 in the cihum during the recovery stroke. It must be derived from 

 the chemical energy either in the cihum itself or in some other part 

 of the cell. The problem, therefore, narrows itself to a determination 

 of the possible methods whereby chemical energy can be converted 

 into kinetic energy by means of a fibrous and elastic mechanism. 

 At this point the similarity of the ciliary apparatus to a muscle 

 fibre becomes obvious, and it is convenient to summarize the 

 mechanism of the muscle fibre as analysed by A. V. Hill*. 



When at rest a muscle fibre may represent a stretched elastic 

 body which possesses potential energy stored as tension energy 

 in some substance A (which may be inactive connective tissue 

 fibres). Within the muscle, however, are certain fibres B which are 

 capable of developing a tension when in contact with some chemical 

 substance which is liberated at the time of excitation. We can 

 regard this chemical substance as lactic acid, so that the muscle 

 fibres are capable of developing a tension (just as a piece of catgut 

 develops a tension) when in contact with lactic acid — by the 

 absorption of water. In the resting muscle the svibstance A is kept 

 stretched possibly by the osmotic properties of the liquid in its 

 interstices. When stimulated, lactic acid is set free from some 

 carbohydrate compound, the fibres B take up the energy thus set 

 free and develop a tension by taking up water from the interstices 

 of the substance A. Consequently the muscle contracts and utilises 

 the energy in both A and B. After shortening the lactic acid 

 diffuses away, i.e., is removed from the fibres B. They therefore 

 give up their water which passes back into the interstices of A, and 

 the muscle lengthens. It is clear that there is no a priori objection 

 to applying such a hypothesis to the cilia of Ctenophores — but as 

 far as these cilia are concerned it is a hypothesis and nothing more. 



In the case of the cilia of Mytilus we can analyse the system as 

 follows. At the end of the effective stroke a tension is set up in the 

 fibres by the liberation of some chemical substance (which may be 

 an acid) from the interior of the cell. The equilibrium between this 

 tension and the elasticity of the cilium draws the latter to the 

 position occupied at the end of the recovery stroke, and in doing so 



* A. V. Hill and W. Hartre-, Phil. Trans. Roy. Soc. vol. 210 b, 1920, p. 153. 



