134 RHYTHM 



found that the posterior vacuole varied from 15 sees, to 

 30 sees, with an average of 22-5 sees., whereas the anterior 

 vacuole contracted fairly constantly about every 35 sees. 

 The rate of pulsation varies with the amount of salt in the 

 surrounding water. 



The protoplasm circulating in the living plant-cell is again 

 rhythmic. It passes up the sides and down the middle in a 

 definite, cyclic flow. If it bears the nucleus with it, the 

 nucleus is at one moment of time at one end of the cell 

 and a little later at the other end. It passes from end to end 

 rhythmically. 



Amongst the most prominent examples of rhythm in parts 

 of cells are the cilia. They move independently of nervous 

 impulse. The stimulus that induces them to beat comes from 

 the protoplasm of the cell, and if this be completely removed 

 the cilia cease to act. The cilia of the infusoria bustling about 

 in a puddle, or those of the cells lining our breathing tubes, 

 beat in unison and at a definite rate for each individual. Thus 

 the beat of the first cilium is followed by the beat of the second, 

 third, and so on. No single cilium ever contracts out of order, 

 and it does not make its appropriate movement till the pre- 

 ceding cilium of the row has moved. It bends just after its 

 neighbour has stopped bending. The first one, as it were, 

 gives the signal to the others. Like the ''stroke " in an "eight," 

 it sets the pace. If it ceases to beat, all the others cease to beat ; 

 if it starts beating again, the others follow suit. The work 

 they perform is said to be much less than that of muscular 

 movement, and it has been determined that a Paramoecium 

 0-25 mm. in length would be able to lift about nine times the 

 weight of its own body by its ciliary action. 



The flagella of antherozoids or of spermatozoa also beat 

 rhythmically. Their vibrations can be broken up and made 

 irregular by external factors, but in normal health they beat 

 in unison. 



Rhythm in Cells 



Rhythm in cells is perhaps best shown amongst the uni- 

 cellular plants and animals. They are born ; they grow up ; 

 they reproduce ; and sooner or later the great majority of them 



