THE MECHANISM OF BEAT 161 



of a wave in an elastic rod. In terms of biological processes, the 

 transmission is also too slow to be compared with nerve conduction, 

 but bears closer comparison with the transmission of the wave 

 of contraction in muscle fibres. Harris suggested that the rate 

 of conduction of the contraction in muscle fibres has a maximum 

 value of about 1 mm/sec, and he quotes examples from muscles 

 as diverse as the striated wing muscle of insects (where the unit 

 is the sarcomere), the body wall muscle of Ascaris, the stomach 

 wall muscle of the cat, the slow adductor and the byssus retractor 

 muscles of Mytilus. If either muscles or cilia are under restraint 

 the conduction may be slower than this. It seems then as if there is 

 an intrinsic conduction mechanism in these contractile structures. 



If we consider the propgataion of a contraction wave along a 

 single fibril, a contraction in one region will be expected to 

 stretch an adjacent uncontracted region, and it would seem 

 reasonable to follow Gray (1955) in believing that the release of 

 tensile energy follows mechanical stretching. This stretching 

 could uncover active ATP-ase sites, which could cause the release 

 of energy for contraction in the same manner as has been suggested 

 for muscle. In the whole cilium, however, the fibrils on the 

 opposite side of the shaft are stretched at least as much as the 

 adjacent parts of the same fibril, yet they do not contract. This 

 may be because fibrils on the convex side of the bend in the shaft 

 have not completed their contraction cycle and are still refractory. 

 Harris has suggested that this idea should be abandoned in favour 

 of the assumption that it is shortening which excites contraction, 

 while stretching inhibits it. There is some evidence that 

 shortening of muscle may cause activation, e.g. in the rhythmic 

 contraction of the tymbal muscle of the cicada (Pringle, 1954), 

 although most workers have assumed that stretching precedes 

 contraction. Observations on the effects of high viscosity of the 

 medium on Stentor membranelles gives evidence in favour of the 

 shortening theory, for a cilium may be excited to beBt by 

 mechanical drag communicated through the viscous medium. 

 In this way the bending of one cilium assists that of its neighbour, 

 and it appears that contraction is excited by the mechanical 

 initiation of a slight bend at the base of the neighbouring cilium 

 before the normal metachronal excitation is complete. The 

 normal bending cycle is not shortened in the viscous medium, 



