24 



COMPARATIVE PHYSIOLOGY 



Forward or Effective Beat 



Backward or Recovery Beat 



or recovery stroke. The form of the beat suggests that it is 

 during the rapid effective stroke that the cilium performs v^ork 

 on the surrounding medium. At the conclusion of the forward 

 movement, it can be seen that the ciHum, which at the beginning 

 of the effective stroke is a more or less rigid rod moving forward 

 on a pivot at its base, becomes limp ; a stress is set up w^hich 

 starts at its base and is transmitted thence to its free end. For 

 the purpose of forming a working hypothesis, it may be assumed, 

 as suggested by Gray, '' that the energy which is expended by 

 the cilium is stored as tension energy." We are entitled to 



surmise that this energy has 

 its origin in some chemical 

 compound either in the cilium 

 itself or in the cell to which 

 it is attached. The problem 

 to be faced is the elucidation 

 of the sequence of events by 

 which chemical energy is con- 

 verted into kinetic energy ; 

 or — as implied above — the 

 chemical processes by which 

 the state of tension in the 

 cilium is relieved. 

 We shall here accept Gray's hypothesis as a basis for 

 discussion. The results of the foregoing survey of muscular 

 contraction have led us to conclude that the physical changes 

 which result in contraction are associated with the production 

 of lactic acid from carbohydrate without intake of oxygen, and 

 that oxygen is employed in the recovery process to restore the 

 mechanism to its original condition. Though ciliated 

 epithelium is structurally very different from muscle, there are 

 two sets of considerations which suggest the possibility that a 

 similar sequence of chemical phenomena might be found to 

 underlie the changes of physical state, which in both cases 

 result in liberating contractile energy. One is that lactic 

 acid is an obligatory intermediary in the breakdown of carbo- 

 hydrates in all animal tissues. The other is that the physical 

 properties of proteins, being probably dependent on conditions 



Fig. 8. — Diagram of ciliary motion 

 (after Gray). 



