THE MECHANISM OF BEAT 153 



long by 165 A in diameter, apparently in the form of a spiral 

 with 25 turns, while in the shortened condition it is 350 A long 

 by 250 A in diameter, and in end-view it now has the appearance 

 of a wheel with 15 cogs. Biochemical evidence suggests that the 

 sheath contains about 200 molecules of about 50,000 molecular 

 weight, so that the shortening of the sheath could result from a 

 change from 25 turns of eight units per gyre to about 14 turns of 15 

 units per gyre. Note that this structure has a similar diameter 

 to that of a ciliary fibril, and that a change in length actually 

 involves the sliding of one gyre relative to the next. 



Several electron micrographs have shown the appearance of a 

 spiral structure in the peripheral fibrils of cilia (p. 19), and it 

 seems reasonable to suggest that the contraction of ciliary fibrils 

 could be the result of a shortening and widening of this spiral. 

 The amount of shortening required is small in cilia compared 

 with that found in isotonic contraction of muscle, e.g. the maximum 

 shortening in the fibrils furthest from the axis of bending in both 

 cilia (Bradfield, 1955) and sperm tails (Gray, 1955) is of the order 

 of 5 per cent, while fibrils nearer to the axis of bending need 

 shorten by less than this in order to produce the observed amount 

 of bending. Fibrils on the outside of the bent region are assumed 

 to be stretched by a corresponding amount, but this depends on 

 the site of the compression elements in the ciliary shaft. The 

 tension that must be exerted in each fibril of the cilium (2 -4 x 10~^ 

 dyn) to produce the bending couple calculated by Harris (1961) 

 is remarkably close to the force exerted by a single filament of 

 striated muscle in isometric contraction (3 '3 x 10~^ dyn) (Hanson 

 and Huxley, 1955). However, the ciliary fibril doublets, and even 

 the subfibrils, are much larger than either of the muscle filaments 

 that these authors describe, and in fact the wall of the ciliary 

 fibril may be thicker than the thinner filaments of striated muscle. 

 The figure calculated by Harris takes account only of the external 

 resistance to movement of the ciliurn. There is no evidence that 

 the two subfibrils of any particular peripheral fibril of a cilium 

 can move relative to each other ; indeed, it appears that they both 

 share a common wall along the line that divides them. 



Several attempts have been made to explain the way in which 

 the observed bending patterns of cilia and flagella are produced 

 by the shortening of the peripheral fibrils; the ideas put forward 



