THE MECHANISM OF BEAT 163 



while in magnesium chloride the propagation rate is speeded up 

 by about the same amount as the frequency. If both of these 

 factors affect the rate of shortening of the ciliary fibrils, either by 

 increasing external resistance or decreasing internal resistance 

 to movement, and if the propagation is mechanical, these are the 

 results that we should expect. Additional evidence for the 

 dependence of propagation on contraction is provided by the fact 

 that the rate of propagation usually increases as the wave of 

 flexure moves towards the tip, where the resistance to movement 

 is lessened. 



Some mention of the excitation of contraction has already been 

 made above. It is known that glycerine-extracted models will 

 contract alternately on the two sides in the presence of ATP, 

 and that cilia can be mechanically co-ordinated in very viscous 

 media. In both cases a shortening of the fibrils, either following 

 stretching or because of viscous drag, could be responsible for 

 exciting the new contraction of the cilia. While some cilia may 

 normally be excited to beat by mechanical means, many cilia 

 may be co-ordintaed metachronally by some internal mechanism 

 which is most likely to act by excitation of the cilia (see p. 184). 

 We are ignorant of the mechanism of excitation. It is tempting 

 to suggest that it is connected in some way with the provision 

 of suitable conditions for the breakdown of ATP to release the 

 energy for contraction, and that this could result either by some 

 structural change (shortening ?) or by some chemical change, the 

 latter perhaps concerned with some change in ionic concentrations. 

 Of the drugs which increase the rate of transmission of metachronal 

 waves, both serotonin and veratrine also increase the rate of beat 

 (i.e. the rate of excitation) considerably, and the latter drug is 

 thought to act by an effect on the ionic balance across the cell 

 membrane. It is difficult to visualize any means by which the 

 excitation may be restricted so that only one fibril is excited 

 initially, unless it be assumed that this fibril has the lowest 

 threshold, or that other fibrils are refractory. The '* basal foot " 

 on the ciliary basal body of Anodonta may provide a specific site 

 of excitation in this case, but such structures are not widely 

 known. Another problem concerns the way in which reversal 

 of beat fits into this picture, unless the changes that cause reversal 

 specifically favour the excitation of other fibrils in the ring. 



M 



