154 



MOVEMENT OF CILIA AND FLAGELLA 



by Bradfield (1955), Gray (1955) and Sleigh (1960) all follow 

 similar lines. The model used by Gray to demonstrate his theory 

 is shown in Fig. 41a; it represents a short length of a flagellum 

 in which the central rod R is the compression element which 

 prevents shortening, while the peripheral fibrils are capable of 

 shortening by contraction. In Fig. 41b the position of the fibrils 

 is shown in a transverse section of a flagellum in relation to the 

 axes X-Y and W-V. Fibrillar contractions are assumed to bend 

 the flagellum about the axis X-Y, but remain balanced about the 

 axis W-V. If the tension in fibrils to the right (in the figures) of 



b. 



Fig. 41. (a) A model representing a short segment of a 

 flagellum (see text) (from Gray, 1955). (b) The arrangement 

 of fibrils in a cilium or flagellum (seen looking towards the base) 

 in relation to the axis of bending X-Y and the plane of beat 



PF-F(from Sleigh, 1960). 



X-Y is greater than that to the left, the jlagellum will bend to 

 the right and fibrils on the left will be stretched. A bend to 

 the left will similarly result from a greater tension on the left-hand 

 side. It is interesting that if all of the fibrils of the model were 

 to contract simultaneously, and each exerted the same force, the 

 distribution of fibres is such that the resultant bending moment 

 about the line X- Y would be zero. 



A complete cycle of flagellar bending, moving first to the right 

 and then to the left, could result from a series of contractions in the 

 peripheral fibrils in the order indicated by the arrows in Fig. 41b. 



