354 Mr Gray, The Mechanism of Ciliary Movement 



an elastic rod which moves forward from a pivot at its base, and which 

 exposes the maximum amount of surface to the water. During the 

 recovery stroke (Fig. 3), the movement is slower, and the shape of 

 the cihum is different. The cihum is drawn back as a Hmp string or 

 piece of unstretched rubber in which a stress is set up which starts 

 at the base and is transmitted to the free end. The path followed 

 by the cihum is essentially the same as that of a fishing hue during 

 the backward movement of the cast, whereas during the forward 

 effective beat the cihum resembles a stretched spring which is 

 suddenly released. The essential difference is that during the for- 

 ward effective stroke the cihum is expending energy in the form 



Forward or Effective Beat 



Fig. 2. 

 -» Backward or Recovery Beat 



Fig. 3. 



of work, whereas during the recovery stroke the cihum is storing 

 potential energy and is performing a minimum of external work 

 during its change of position. 



The simplest conception of such a mechanism is to regard the 

 cihum at the end of its effective stroke as a relaxed elastic body 

 The recovery stroke is brought about bythe setting up of a tension 

 m this body : the tension begins at the base of the cihum and passes 

 on to Its apex. The direction of the tension is approximately along 

 the hne which the cihum occupies at the end of the recovery stroke 

 In other words the position at the end of the recovery stroke 

 represents the equilibrium between the force which is distorting 

 the cihum and its own elasticity. If the force apphed be removed 

 the potential energy stored in the cihum will be released, and the 



