THE FUNCTIONS OF THE PARTS OF CILIA 151 



may also function sometimes in the conduction of co-ordinative 

 impulses; the basal bodies must take part in both of these 

 functions. The excitation of contractions in the ciliary fibrils 

 almost certainly starts in the basal body, although contraction of 

 glycerine extracted models in ATP may take place after de- 

 tachment of the basal body. 



It is interesting that a " basal foot " is found on fibrils 5 and 6 

 of the basal bodies of Anodonta cilia, where the effective stroke 

 is known to be directed towards this pair of fibrils ; this foot may 

 be concerned in excitation, and thin fibrils which could function 

 in conduction are seen attached to the foot in some electron 

 micrographs (PL lib). The same cilia show a functional 

 adaptation of the root structure. Since the effective stroke of 

 the cilia, caused by contractions in fibrils 5 and 6, cause them to 

 swing around a fulcrum at the base of the cilium, maximum strain 

 will have to be taken by the roots attached to the bases of 

 fibrils 9, 1 and 2. It is found that a large root is attached to the 

 bases of these three fibrils in the basal body, and smaller roots 

 from fibrils 5 and 6 join the main root at a lower level. When 

 the main root splits into two parts, these diverge in a plane at 

 right angles to the plane of beat, and can take equal strain during 

 the effective stroke. 



4. The Mechanism of Beat 



Three distinct processes are involved in the beating activity 

 of a cilium, viz. (1) the excitation of contraction, (2) the contraction 

 itself, and (3) the propagation of the contraction along the length 

 of the ciliary shaft. For normal regular beating of a cilium all 

 three are required. The study of glycerine- extracted models of 

 flagella and sperm has indicated that the contractile elements will 

 show regular alternations of contraction and relaxation even after 

 the normal structure has been at least partly disrupted, so that 

 some automaticity seems to be built into the contraction machinery. 

 These models do not show propagation of the contraction. The 

 rate of beat of cilia may be modified by many factors, some of 

 which no doubt have a direct effect on the contractile mechanism, 

 while others may act on the excitation mechanism ; the very exist- 

 ence of ciliary control in metachronism and reversal shows that the 

 ciliary contraction machinery is not entirely automatic in its action. 



