HYDROSTATIC PRESSURE 



87 



150 atm (about 2000 Ib/in^), but there was a strong acceleration of 

 beat at about 200 atm, and the cilia finally stopped at 600 atm. 



The effect of sudden changes of pressure on the frequency of 

 beat of lateral cilia of excised gill filaments of Mytilus was 

 studied by Pease and Kitching (1939). A sudden increase of 

 pressure caused an immediate increase in frequency and a slow 

 return to normal, while a sudden decrease of pressure caused an 

 immediate decrease in frequency and a slow return to normal 

 (Fig. 21). Within the range 1000 to 5000 or 6000 Ib/in^, the larger 

 the change in pressure to which the cilia were subjected, the 

 larger the frequency change. Above 5000 or 6000 Ib/in^ the 

 ciliary frequency fell rapidly away and permanent injury was 



Fig, 21. The effect of sudden changes in hydrostatic pressure 

 on the frequency of beat of lateral cilia of Mytilus gills (from 



Pease and Kitching, 1939). 



caused. Two effects of pressure on ciliary frequency are seen 

 here; firstly, a deleterious effect of high pressures above about 

 5000 Ib/in^, which is common to other physiological processes, 

 and secondly, a temporary change in the rate of beat occurring 

 whenever the pressure is changed, which is comparable with a 

 similar effect on cardiac rhythm (Edwards and Cattell, 1928, 

 1930). 



Rather different results have been obtained in studies on the 

 effect of increased hydrostatic pressure on the frequency of beat 



