18 



VERTEBRATE RESPIRATION 



into and out of the system and across the respiratory epithelium 

 would be intermittent. 



The division of the respiratory chambers by gill resistances 

 was not envisaged in this mechanism so that the hydrostatic 



Mouth 



movement opening 



Operculum l,. 



movement expancding 



Mouth '' 



valves open 

 Opercular 

 valves open 



+ 10cm. 

 H2O" 



Pressures in 

 respiratory 



cavities 



-1-Ocm. 

 H2O' 



Differential 

 pressure 



1-0 

 cm. 

 Buccal 



cavity 



positive 



1 second 



Fig. 5. 



Movements of the mouth and operculum together with the pressure 

 changes recorded in the buccal and opercular cavities of a trout. The 

 dashed lines refer to the buccal pump and the continuous lines to the 

 opercular pump. The differential pressure shows the greater pressure 

 in the buccal cavity throughout most of the cycle and is striped during 

 the phase when the water flow is produced mainly by the opercular 

 suction pump. (After Hughes, G. M. and Shelton, G.: /. exp. Biol. 

 Vol. 35,1958). 



pressure recorded in the buccal and opercular cavities would be 

 almost identical throughout the whole cycle. In fact there are 

 marked differences between them (fig. 5). It is now known that 

 the mechanism involved is best considered as a double pump 



